UNIVERSITY OF CALIFORNIA COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA FACTORS INFLUENCING THE YIELD, COMPOSITION, AND QUALITY OF RAISINS H. E. JACOB BULLETIN 683 January, 1944 UNIVERSITY OF CALIFORNIA BERKELEY, CALIFORNIA CONTENTS PAGE Kinds of raisins 3 Factors of quality in raisins 6 Influence of grape maturity and drying method on the yield and composition of raisins 11 Sources and preparation of the fruit for the drying tests 12 Methods of drying 12 Time required for drying 15 Effects on yield of raisins 18 Effects on composition of the raisins 21 Eelation between grape maturity and the quality of the raisins 32 Relation of the weight per unit volume to the acid and insoluble-solids content of natural Thompson Seedless raisins 36 Discussion 40 Proper time for harvesting raisin grapes 42 Summary 43 Acknowledgments 44 FACTORS INFLUENCING THE YIELD, COMPOSITION, AND QUALITY OF RAISINS 1 H. E. JACOB 9 Although raisins have long been a staple food, their quality has fluctuated between wide limits, partly through variability in the grapes utilized and partly through lack of the knowledge needed to secure a uniform product. In periods of stress the standard for food products may have to be lowered some- what, but every effort should still be made to obtain the best grade consistent with maximum production. The information in this bulletin should be helpful in obtaining the most and the best raisins possible from the grapes available. KINDS OF RAISINS Our word raisin is a contraction of the French raisin sec, meaning "dry grape." Any dried grape, therefore, has some right to the name raisin. By usage, however, the term has been limited mainly to a few varieties. Three of these — Thompson Seedless, Black Corinth, and Muscat of Alexandria — pro- duce nearly all the raisins of international trade. The grape called Thompson Seedless in California is known as Sultana in Australia and South Africa ; White Kishmish or Oval Kishmish (or Kechmish) in Turkey, Persia, and other countries of western Asia; or, sometimes, Sultanina. The Black Corinth is also called Zante Currant, Currant, or simply Zante in English-speaking countries, and Staphis (meaning raisin) in Greece ; the name Panariti is occa- sionally used. The Muscat of Alexandria has various synonyms; perhaps the most common are Moscatel gordo bianco (Spain), Gordo Blanco (Australia), White Hanepoot (South Africa), and Zibibbo (Italy) ; it is a favorite in all warm grape-producing countries. Other varieties are of local importance, such as the Sultana of California (the Round Kishmish of Turkey and Persia) ; Rosaki and Dattier in Greece, also in Turkey and elsewhere in Asia Minor ; Monukka ; and Cape Currant. Certain wine and table grapes are occasionally dried, but should then be called "dried grapes" to distinguish them from the raisins of commerce. In addition to the grape variety, the trade names applied to raisins may signify the method of drying (natural, golden-bleached, sulfur-bleached, lexia) ; the principal place of origin (Vostizza, Patras, Pyrgos, Smyrna, Mal- aga, Valencia) ; the condition in which the raisins are offered for sale (layers, loose, seeded) ; the size grades (4 crown, 3 crown, 2 crown, and so forth) ; and the quality grades (extra standard, standard, substandard; extra fancy, fancy, choice). Often the trade terms have different significance in different regions or with different kinds of raisins. The crown grades, for example, used in California to indicate only the size of Muscat raisins, when applied to the sultana and lexia raisins of Australia indicate also color and quality. The Greek terms Vostizza, Patras, and Pyrgos, while primarily names of produc- ing regions, are also suggestive or indicative of quality. 1 Keceived for publication January 7, 1943. 2 Associate in Viticulture and Associate Viticulturist in the Experiment Station. [3] 4 University of California — Experiment Station Thompson Seedless. — Raisins from the Thompson Seedless grape are dried in the natural condition, without dipping treatment, mainly in California, Iran (Persia), and Turkestan, U.S.S.R. In California they are called Thomp- son Seedless or naturals to distinguish them from dipped raisins. Sometimes the trade knows them simply as seedless. They are dark — grayish-black or grayish-brown — with the natural bloom largely intact, rather tough-skinned but meaty, of characteristic oxidized flavor, dry on the surface with no sticki- ness or oiliness, and, when properly dried, have but little tendency to cake in storage. For dessert purposes — eating out of hand — they are often preferred. For cooking, their dark color, tough skins, and strong flavor may not always be desirable ; and other lighter-colored, more tender, and less oxidized types seem to be preferred in all but the American markets. In California, in late prewar years, about 85 per cent of the total raisin production has been from Thompson Seedless grapes, and 85 to 90 per cent of these raisins have been of the natural type. Sultana. — In the United States the name Sultana is applied to a round- berried, nearly seedless grape — the Round Kishmish of Asia Minor — and to its natural sun-dried raisins. Although these raisins resemble natural Thompson Seedless, they are usually considered inferior, being less meaty and more acid, with occasional semihard seeds. They are very different from the light-colored, tender-textured sultana raisins of Australia and South Africa. In Australia, South Africa, and other British countries the name sultana is applied to the light-colored, tender raisins made by various processes other than natural sun-drying, from the variety called Thompson Seedless in Cali- fornia. Most commonly the grapes are dipped into a solution containing potas- sium carbonate, with an emulsion of olive oil. 3 They may be dried in direct sunlight as in Turkey, Greece, Persia, and Turkestan, or on covered wire- shelved racks as in Australia and South Africa. In color the raisins vary from greenish yellow to medium or dark brown. Uniform yellow or light amber is preferred and is most often obtained from whitish-yellow or yellow grapes that are rapidly dried, with no exposure to dew and rain, and shaded during the latter part of the drying period. Dark or, especially, mixed or uneven colors are objectionable. In some regions, sulfur dioxide treatment of the grapes or raisins is occasionally used to brighten the color. The raisins are tender- skinned and when used by bakers will slice with the product, instead of hanging on to the slicing knife and being dragged through, as often happens with the tougher-skinned natural Thompson Seedless. In baking they have less tendency to caramelize than the natural raisins. They are attractive in light-colored cakes and puddings. The surface is often slightly oily, and the flavor is characteristically nutlike. For eating out of hand, many persons con- sider them less palatable than the naturals. The product from Australia and South Africa is always sold as sultana. Similar raisins from Greece, Turkey, Persia, and Turkestan are usually re- ferred to as sultanas or the sultana type in English-speaking countries, but in the region of production other names are applied. Occasionally other raisins dried by the same process may be listed as sultana, and the term appears to be 8 Mrak, E. M., and J. D. Long. Methods and equipment for the sun-drying of fruits. California Agr. Exp. Sta. Cir. 350:1-09. November, 1941. Bul. 683] Yield, Composition, and Quality of Raisins 5 sometimes used loosely to include the light-colored golden-bleached and sulfur- bleached product from California. Golden-bleached. — In California between 15,000 and 30,000 tons of light- colored (bleached) raisins are produced annually, mostly by the golden-bleach process. First, Thompson Seedless grapes are sorted to remove clusters of damaged, underripe, or overripe fruit. Next they are dipped in a solution of 0.2 to 0.5 per cent sodium hydroxide (caustic soda, lye) at 190° to 200° F, until the skins show slight checking when the grapes are cooled and washed in a rinse or spray of cold water. They are then exposed for 2 to 4 hours to the fumes of burning sulfur in a sulfur house. 4 The quantity of sulfur varies from 2 to 4 pounds per ton of grapes. After sulfuring comes dehydration. 5 The de- hydraters are of the tunnel type with forced air flow and are heated by natural gas or oil. There may or may not be partial, controlled recirculation of the air. Drying is done between 140° and 165°. The raisins are brilliant lemon yellow to golden yellow, moderately tender, and sometimes a little sticky. Their sulfur dioxide taste is objectionable to some consumers for eating out of hand, but entirely disappears when they are baked or cooked. The presence of the sulfur dioxide during drying and storage is necessary to preserve the yellow color. In international trade the golden-bleached raisins of California compete successfully with Australian-type sultanas from other regions, despite the probable culinary superiority of the latter. Where dehydrater fuel is cheap the golden-bleached can be produced at a relatively low cost. Sulfur-bleached. — Grapes for making the sulfur -bleached raisins are treated much like the golden-bleached up to the start of drying. The hot dip, however, is often held at a temperature 4 or 5 degrees lower. In drying, the grapes intended for the sulfur-bleached raisins are exposed, on trays, to direct sunlight for 3 or 4 hours in very hot, dry weather ; 2 or 3 days in cool, cloudy weather. Then the trays are stacked, and the drying is completed in shade. The resultant raisins appear waxy and are cream to faintly reddish yellow, accord- ing to the length of exposure to direct sunlight before stacking ; otherwise they resemble the golden-bleached product. The cost of production is essentially the same as for golden-bleached, but the market is more limited. A few thousand tons are produced in California each year. Soda-dipped, or Soda-bleached. — Occasionally, in California and elsewhere, some hot-dipped but not sulfured Thompson Seedless grapes are sun-dried. The dip used may be hot lye, as for the golden-bleached, or a mixture of sodium hydroxide and sodium carbonate. The commercial raisins thus made are usually sold as soda-dipped or soda-bleached. When dried rapidly they are light-amber to medium brown, moderately tender, mild-flavored, sometimes slightly sticky on the surface. The main advantage over natural sun-drying is the shorter drying time — about half as long. Oil-dipped. — Thompson Seedless grapes for the so-called oil-dipped raisins are treated in one or the other of two dips, which differ in the composition and temperature of the solutions. One solution is cold (100° F or less), containing * Long, J. D., E. M. Mrak, and C. D. Fisher. Investigations in the sulfuring of fruits for drving. California Agr. Exp. Sta. Bul. 636:1-56. 1940. 5 Nichols, P. F., and A. W. Christie. Dehydration of grapes. California Agr. Exp. Sta. Bul. 500:1-31. 1930. (Out of print.) 6 University of California — Experiment Station about 4 per cent sodium carbonate and a little caustic soda with a thin film of olive oil floating on the surface. For the other dip, a hot (170° or higher) but weaker solution of sodium carbonate is used, also with a film of oil. After dipping, the grapes are dried on trays in direct sunlight. The raisins are medium to dark brown, fairly tender, slightly oily but not sticky. Black Corinth. — Kaisins of this variety are all dried in direct sunlight or in shade without pretreatment and are called currants or Zante currants in all English-speaking countries. Although Greece and Australia are the principal producers, small quantities are made in California and South Africa. The raisins are dark and very small, with a tart taste and a characteristic but mild flavor, and are highly esteemed for cooking and baking. Muscat. — Natural sun-dried raisins are produced from Muscat of Alexan- dria grapes, principally in California and the province of Malaga, Spain. The California raisins are called loose Muscats if stemmed and not seeded ; seeded Muscat if the seeds have been removed. Unstemmed raisins called layers or clusters were also formerly produced in California. The raisins from the Spanish province of Malaga are carefully dried on the clusters and packed without stemming, and are known in world markets as Malagas. The raisins are very large, grayish black or grayish brown, with the bloom mostly intact, very meaty, but with strong Muscat-raisin flavor and rather tough skins. Valencias, Lexias. — In Spain and in Australia, sizable quantities of Mus- cats are dipped before drying. The caustic-soda dip, as described for the golden-bleached raisins, or a hot dip containing sodium carbonate or potas- sium carbonate along with the lye, is most commonly used. Sometimes olive oil is added. The Spanish raisins are dried in direct sunlight, but may be covered with muslin sheeting at night or during rain. When dry they are stemmed and sold as Valencias. The Australian product is rack-dried, as described for the sultana, and is called lexia, a term sometimes also applied to the Valencia raisins of Spain. FACTORS OF QUALITY IN RAISINS Any condition that affects food value or attractiveness is a factor of quality. Seediness or seedlessness of the unprocessed raisins and varietal flavors are determined by the grape variety used. Certain color, flavor, and texture dif- ferences are caused by the method of drying. These factors of quality and their relation to raisin types have been briefly discussed under "Kinds of Raisins." Within any given kind, additional factors of quality are recognized. Of these the most important concern (1) hue, uniformity, and brilliance of the color; (2) size of the raisin berries; (3) condition of the berry surfaces; (4) texture of the skin and pulp ; (5) moisture content ; (6) chemical composition ; (7) presence of sand, pieces of stems, sticks, nails, or other foreign matter ; (8) presence of decay (rot), mold, and yeast; and (9) insect infestation, or con- tamination by insect remains and excreta. These factors of quality may be influenced by the maturity and the physical condition of the fresh fruit; cli- matic or weather conditions during drying; carefulness and skill in harvest- ing, pre-drying treatment (if any), and drying; sanitation in handling and oaring for the fruit and the equipment ; and storage conditions. Bul. 683] Yield, Composition, and Quality of Raisins 7 Color. — The characteristic color of the various raisin varieties has already been given. Uniformity and brilliance appear to be almost as important as shade or hue. Off-colored berries are always objectionable. Uniformity of color is obtained when maturity of the grapes and pretreatment are uniform and the fruit is properly dried under favorable conditions. With the natural sun-drying method, uniform color within a lot is easily obtained. For the best color, only mature and sound fruit should be used. The raisins should be turned when about one third dry, stacked (or rolled if dried on paper trays) ° when about two thirds dry, and not wet by rain during drying or thereafter. Brilliant color is obtained by preserving the natural bloom on the grapes through careful harvesting and placing on the trays, and by pro- tecting the raisins from rains during or after the drying period. Eaisins from which the bloom has been removed by excessive or careless handling of the grapes, or which have been wet by rains while drying, nearly always appear dull. For uniform color in golden-bleached raisins, the fruit must be not only mature, but also approximately uniform in color. Green-colored fruit produces greenish-yellow golden-bleached raisins, whereas the more desirable golden yellow is obtained from yellow or cream-colored fruit. The grapes should be sorted by hand before or after dipping. This procedure, which is neither costly nor laborious, should be adopted by all producers of such raisins. Practically all dipping installations are provided with a shaker ahead of the dipping vat to remove sand, leaves, loose berries, and rubbish. One or two sorters standing beside the shaker, or beside the conveyer belt leading to the shaker, can remove the green, sunburned, bird-pecked, rotten, overripe, and otherwise undesirable clusters. The off-grade fruit is then dried to produce low-grade raisins or sent to wineries for use as distilling material. Other factors contributing to bril- liancy and uniformity of color are (1) proper dipping to cause many small cracks but few large ones in the skin, (2) rinsing to cool the fruit rapidly and remove the slight alkaline residue of the dipping solution, (3) adequate sulfur- ing to bleach the grapes and to check enzyme activity, 7 and (4) adequate dry- ing (to 15 per cent moisture or less) at moderate temperature (between 140° and 165° F) with low humidity (25 per cent or less of saturation) . 8 The Australian-type sultana raisins (processed by dipping in a solution of potassium carbonate containing emulsified olive oil) are very subject to defects in uniformity and brilliance. At best their natural color is a somewhat dull yellow or light amber — not unattractive, yet not the striking brilliant yellow of the golden-bleached product. Sorting of the fresh fruit, preferably by the picker, with reference to maturity and color is essential. Green-colored grapes produce raisins of an undesirable greenish yellow or greenish brown, requir- ing bleaching by exposure to sunlight or heat when nearly dry. 9 Dark-colored or partially raisined berries remain dark. The immature fruit requires a lower 8 Mrak, E. M., and J. D. Long. Methods and equipment for the sun-drying of fruits. California Agr. Exp. Sta. Cir. 350:1-69. 1941. 7 Hussein, A. A., E. M. Mrak, and W. V. Cruess. The effect of pretreatment and subsequent drying on the activity of grape oxidase. Hilgardia 14(6) : 349-57. 1942. 8 Nichols, P. F., and A. W. Christie. Dehydration of grapes. California Agr. Exp. Sta. Bul. 500:1-31. 1930. (Out of print.) 9 Lyon, A. V. Sultanas "finishing off." Dealing with green tinge. Australian Dried Fruit News 10(2) :10. March 19, 1935. 8 University of California — Experiment Station concentration of potassium carbonate in the dipping solution than the fully mature ; otherwise it suffers carbonate burn and becomes dark brown around the place of attachment of the berry to the cap stem. Broken and bruised ber- ries are also burned by the dip and dry to a dark brown. The best color is obtained from yellow- or cream-colored grapes, mature but not overripe, free from dark-colored or partially dried or broken berries, when dipped in a solu- tion of correct concentration and dried in clear hot weather without being even slightly wetted by dews or rains. If moisture falls during the drying period, the raisins must be covered in order to secure a uniform light color ; otherwise they may become dark brown or nearly black. Light and uniform colors in the soda-dipped Thompson Seedless and the dipped Muscat raisins (Valencia or lexia) are obtained by correct dipping and rapid drying in clear warm weather. Slow drying because of underdipping or unfavorable weather causes darkening. Size of Berry. — The size of the raisins is largely determined by the size and maturity of the fresh grapes. Muscat of Alexandria berries are usually large, Thompson Seedless medium, and Black Corinth very small. Within the same variety the size differs between districts, between vineyards, from vine to vine, and even on the same cluster. Ripe grapes, because of their lower drying ratio, produce larger raisins than green grapes that had the same berry size before drying. After stemming and cleaning, commercial raisins are graded into sev- eral sizes. For Thompson Seedless the berry sizes of the three grades recog- nized are denned in federal specifications as follows : Grade A. — Fancy raisins are raisins which will pass over a grading screen of round perforations 2 %4 inch in diameter. Grade B. — Choice raisins are raisins which will pass through a screen of round perfora- tions 2 %4 inch in diameter, but not through a screen of round perforations !% 4 inch in diameter. Grade C. — Standard (Midget) are raisins which will pass through a screen of round perforations !%4 inch in diameter. These grades — Grade A, Grade B, and Grade C ; or Fancy, Choice, and Stan- dard — have additional quality requirements, and the terms refer to both qual- ity and size. Muscat raisin sizes are given other names : 4 Crown raisins are raisins which will pass over a screen of round perforations 4 % 4 inch in diameter. 3 Crown raisins are raisins which will pass through a screen of round perforations 4 % 4 inch in diameter, but not through a screen of round perforations 3% 4 inch in diameter. 2 Crown raisins are raisins which will pass through a screen of round perforations 3 % 4 inch in diameter, but not through a screen of round perforations 2 % 4 inch in diameter. One Crown raisins are raisins which will pass through a screen of round perforations ~% 4 inch in diameter, but not through a screen of round perforations !% 4 inch in diameter. In California, these crown grades designate only size. Quality grades are further designated as Grade A, Fancy; Grade B, Choice; and Grade C, Stan- dard. These are defined in detail in the Federal Standard Stock Catalog. 10 In Australia crown grades are applied to lexias 11 (dipped Muscat) , sultanas, 10 Federal Standard Stock Catalog, Section IV (Parts). Federal specifications for raisins. Z-R-71a. Nov. 1, L939. 7 p. (For sale by the Superintendent of Documents, Washington, D. C. Price 5 cents.) 11 Anonymous. Lexias. Australian Dried Fruit News 10(2) :8. March 19, 1935. Btjl. 683] Yield, Composition, and Quality of Raisins 9 currants, and also certain dried tree fruits ; 12 and they carry quality specifica- tions, particularly for color, in addition to size requirements. The influence of grape maturity upon raisin size is discussed under "The Relation between Grape Maturity and the Quality of the Raisins." Condition of the Berry Surfaces. — Clean and dry raisins are always most desirable. In this regard, products of natural sun-drying offer few difficulties unless they have been wet by rain or unless many of the fresh berries are broken or mashed. In the latter case even unbroken berries become wet with the juice of the broken ones and may become sticky. Dust and sand collecting on the sticky surfaces may be so tightly cemented to the skin or so deeply imbedded in the pulp of the broken berries that thorough cleaning is difficult. Stickiness in golden-bleached and sulfur-bleached raisins results from over- dipping the fruit or from roughness in filling the drying trays. Overdipping causes large cracks in the skins, through which the juice will exude. Severe overdipping may even cause some of the berries to peel, in which case the dried raisins will be very sticky, liable to cake in storage, difficult to handle, and objectionable to the consumer. A sulfur-bleached product dried in the open may, if sticky, collect dust and sand that may be difficult to remove. Since the golden-bleached raisins are rapidly dried in a relatively dust-free dehydrater, there is no excuse, except carelessness, for producing dirty raisins of this type. Excessive oiliness may cause Australian-type sultana or other oil-dipped raisins to collect dust and sand. The dirt is more easily removed from them than from sticky raisins, although special washing is required. The oil may sometimes become rancid, imparting a disagreeable odor and flavor. Sugaring refers to the crystallization of fruit sugars on the surface and in the pulp of the raisins. It occurs most often in raisins of relatively high mois- ture content, particularly when stored under conditions of high humidity. Australian-type sultana raisins and sulfur-bleached raisins appear more sub- ject to this defect than natural or golden-bleached, although none are entirely free. Texture of Skin and Pulp. — Good raisins should always be plump, pliable, and meaty. They can be produced only from mature fruit. Raisins from imma- ture fruit are deeply wrinkled, inclined to be hard, and often tough. A good index to the texture is the weight-per-unit-volume measurement, devised by Chace and Church 13 and still used in a modified form by a large cooperative organization to grade growers' lots upon delivery. The usefulness of this is largely limited, however, to natural sun-dried raisins of the seedless varieties. Tenderness of skin, a desirable characteristic in raisins for cooking or bak- ing, is largely a function of the grape variety and the drying method. Muscat of Alexandria has a somewhat tough skin, Thompson Seedless medium, Black Corinth rather tender, and Australian-type sultana raisins the most tender. Golden-bleached, sulfur-bleached, soda-dipped Thompson Seedless, and soda- dipped Muscats (Valencias and lexias) are moderately tender. The skin of the natural sun-dried product is toughest. Conditions under which the grapes are grown also influence tenderness or toughness, but only to a limited degree. 12 Anonymous. Board of Management Minutes. Australian Dried Fruit News 11(1) :3, 14. Jan. 22, 1936. 13 Chace, E. M., and C. G. Church. Tests of methods for the commercial standardization of raisins. U. S. Dept. Agr. Tech. Bui. 1:1-23. 1927. 10 University of California — Experiment Station Moisture Content. — When delivered to the stemming and cleaning plant, most raisins contain 10 to 15 per cent moisture. If they are cleaned with water, they will necessarily absorb some of it. If the total rises above 18 per cent, the raisins may deteriorate in storage. Slightly overdried raisins may be "condi- tioned" and brought to the desired moisture content without injury. A badly overdried product — 5 per cent moisture or less — may have a caramelized or scorched taste that cannot be removed. Although it is naturally to the grower's advantage to deliver the raisins at the highest permissible moisture content, it is much safer to dry them to 15 per cent moisture or less if possible. Nutritive Value. — The food value of raisins lies chiefly in their sugars, fruit acids, and mineral salts. In Thompson Seedless raisins the sugar content varies between rather narrow limits according to grape maturity and the drying method. The total acid content varies between rather wide limits and inversely with the maturity of the grapes. According to the experimental data available, the content of some mineral salts varies slightly with grape maturity, whereas other salts remain fairly constant. Composition as influenced by maturity is more fully discussed on pages 21 to 31. Except for some deficiency in vitamin content, the raisins are nearly equal in food value to the fresh grapes. Unsulfured, dehydrated, lye-dipped (soda-dipped) Thompson Seedless raisins, according to Morgan and others, 14 retain practically all the consider- able vitamin- A and -B activity of the fresh grapes. The lye-dipped (soda- dipped) sun-dried raisins retain most of the B and a little A. Sulfured raisins retain the A but almost no B. The natural sun-dried raisins retain most of the B of the fresh grapes, but none of the A. Apparently the Thompson Seedless grapes and raisins contain but little vitamin C or G. Sand and Other Foreign Matter. — About 85 per cent of the California raisin crop is dried on wood or paper trays between the rows of vines. Inevitably, considerable dust and sand will accumulate. If the berries are not mashed or broken, the particles of dust and sand adhere loosely to the raisin surfaces and can mostly be removed by passing over a shaker screen 15 when the product is boxed. Nearly all the remaining dirt is removed in stemming and cleaning. When the raisins are sticky or if they have been wet by rain, which may cement some types of dust to the skins, cleaning becomes more difficult. Occasionally sticks, stones, nails, and other foreign matter will get into the sweat boxes. Small pieces will most likely be removed by the stemmer without doing any particular harm ; but large sticks, stones, or pieces of metal may seriously damage the stemming machinery unless removed before stemming. Decay, Mold, and Yeast. — Rain during drying and the use of rotten or moldy grapes are mainly responsible for the presence of decayed and moldy berries. Very slow drying because of unfavorable weather sometimes permits the growth of yeasts on the surface and in the pulp of cracked berries. The yeast growth, if affecting only a small percentage of the berries and only those mound the cap stem, may not be serious. If a considerable portion of the ber- ries ( 10 per cent or more) have one fourth or more of their surface affected " Morgan, Agnes Pay, Louise Kimmel, Anna Fiold, and Paul F. Nichols. The vitamin eon- tenl of Sultanina (Thompson Seedless) grapes and raisins. Journal of Nutrition 9(3) :369- 82. 1935. Ir> Mrak, E. M., and J. I). Long. Methods and equipment for the sun-drying of fruits. California Agr. Exp. 8ta. Cir. 350:33-34. 1941. Bul. 683] Yield, Composition, and Quality of Raisins 11 with yeast, mold, or decay, the lot may be unmarketable. The grower can hold these defects to the minimum by discarding any clusters seriously affected with bunch rot or bird damage, or by trimming off the bad parts, and protect- ing the raisins from rain. Because rain falls so seldom in California during the drying period, many growers fail to provide the necessary facilities for cover- ing the raisins or quickly stacking the trays. As a result, in seasons when considerable rain does occur, much of the crop may be damaged. Insect Infestation. — Insect damage is caused primarily by three species. The raisin moth (Ephestia figulilella [Greg.] ) is the major pest in the fall when the raisins are dried. The Indian-meal moth (Plodia interpunctella [Hbn.]) and the saw-toothed grain beetle (Oryzaephilas surinamensis [Linn.] ) do their damage chiefly in older raisins. 16 Raisin-moth infestation occurs mainly in the field, particularly after stack- ing ; while trays are spread out, the sun's heat largely prevents infestation. If when paper trays are used they are rolled in biscuit rolls, the infestation will be less than with wooden trays or with paper trays rolled in cigarette form. Cleaning at the time of boxing, by passing the raisins over a shaker screen, as for the removal of sand, will eliminate many eggs and young larvae. Infestations of Indian-meal moth and of saw-toothed grain beetle occur mainly in storage and are particularly troublesome if raisins are held over winter into the following summer. Cold storage, or storage in insect-proof warehouse rooms after fumigation, gives effective control." Sanitary measures, including a thorough cleanup of all materials that might furnish food or breeding places, will aid in control. Every effort must be made to prevent infestation or to eliminate the insects in the egg or early larval stages. Killing large larvae or adults means leaving in the raisins the remains of insect bodies and pellets of excreta, a serious defect that may be noticed by a qualified inspector. INFLUENCE OF GRAPE MATURITY AND DRYING METHOD ON THE YIELD AND COMPOSITION OF RAISINS The main constituents of ripe grapes are water and sugar. As maturity advances, sugar increases and the water decreases. Raisins have lost most of the water, but retain nearly all the sugar. Obviously, therefore, the greater the percentage of sugar in the fresh grapes, the greater the yield of raisins. Just how much larger a quantity of raisins will result from a unit advance in the maturity of the grapes is less obvious : different varieties, different drying procedures, and different conditions during drying may also produce differ- ences in the yield. 18 These factors, furthermore, affect chemical composition, 16 Simmons, Perez, Heber C. Donahoe, Dwight F. Barnes, and Charles K. Fisher. Tnfesta tion in raisins and its control. U. S. Dept. Agr., Bureau of Entomology and Plant Quaran- tine mimeograph E-414. 4 p. 1 fig. Aug., 1937. 17 Linsley, F. G., and A. F. Michelbaeher. Insects affecting stored food products. California Agr. Exp. Sta. Bul. 676.: 1-44. March, 1943. 18 Bioletti, Frederic T. Eelation of the maturity of the grapes to the quantity and quality of the raisins. International Congress of Viticulture, Panama-Pacific International Exposi- tion, San Francisco, California, Official Kept. p. 307-14. 1915. Bioletti, Frederic T. Dried wine grapes : methods and returns. California State Board of Viticultural Commissioners Bul. 15:6-9. 1919. Lyon, A. V. Mildura Besearch Committee. Experiments with the drying of vine fruits. Sci. and Indus. (Australia) 2:625-29. 1920. 12 University of C .lifornia — Experiment Station physical characteristics, and quality. In an investigation recently completed at the California Agricultural Experiment Station, the relation between advancing maturity of Thompson Seedless and Muscat of Alexandria grapes and the yield and composition of raisins has been determined. A report 19 has been published in technical form, and to it the reader is referred for details of the experiments and of the analytical methods. In the experiments the various methods used for pretreating the grapes and for drying them also influenced the yield and composition of the raisins. These influences, which were briefly discussed in the original report, are analyzed in more detail in the text and particularly the tables of the present bulletin. SOURCES AND PREPARATION OF THE FRUIT FOR THE DRYING TESTS Grapes were harvested at approximately weekly intervals from vines re- served for the experiments. One large cluster or two medium-sized clusters were harvested from each vine at each picking, so that, in all but one case, fruit for the drying tests was obtained from the same vines throughout the season. All clusters of one picking from one group of vines were combined to form a composite lot of 40 pounds or more. Excepting the natural sun-dried lots of 1926 and 1927, in which whole clus- ters were used, all samples were prepared as follows : The individual berries were clipped from the clusters by cutting the cap-stems with scissors. All dried, injured, or discolored berries were sorted out and discarded. The lot was then thoroughly mixed by repeated gentle pouring from one container to another. After the mixing, a sample was removed for the observations and measurements on the fresh fruit; and from the remainder, portions were weighed out for the individual drying tests. The maturity of the fresh grapes was determined by Balling (or Brix) hydrometer 20 readings and by acid titrations on the juice. The grapes of the sample were thoroughly macerated, and the juice extracted by straining or squeezing through cheesecloth. The results of the hydrometer measurements are expressed as degrees Balling and those of the acid titrations as per cent tartaric acid by weight. The size of the fresh berries was determined by counting out 400 or more from a portion of the composite lot and weighing them. For each lot, the results were calculated to weight per 100 berries. METHODS OF DRYING During four seasons— 1926, 1927, 1935, and 1936 — grapes from common sources were dried according to twelve different methods, seven of which are standard commercial processes. The other five are noncommercial. The meth- ods used in pretreating the grapes and in drying them are briefly stated in the paragraphs immediately following. 10 Jacob, II. E. The relation <»f maturity of the grapes to the yield, composition, and quality of raisins. Kilgardia 14(6) :.''>-! 45. February, L942. L '" Balling and Brix hydrometers are equipped with ;i scale graduated t<» read directly the percentage of sugar by weight, and calibrated in a pure cane-sugar solution. When used <>n fruit juices, and on liquids other than pure sugar solutions, the reading is commonly ex- pressed ,-is "degree Balling" or "per cent soluble solids as determined by the Balling or Brix scale hydrometer." The Balling and Brix scales are now identical. Bul. 683] Yield, Composition, and Quality of Raisins 13 Natural Sun-Drying. — The grapes, thinly spread on trays, were exposed in direct sunlight until they reached the desired degree of dryness. In 1926 and 1927, standard wood raisin trays (2x3 feet) were used, and the raisins were dried in the open. In 1935 and 1936 the drying was done on paper-covered laboratory-dehydrater trays inside a large cage covered with %e-i ncn - mesn hardware cloth. The cage eliminated most of the losses that occurred when the raisins were dried in the open. A few samples were spoiled by mold and yeast growth during periods of rain. The raisins were brownish or grayish black, very dry on the surface, with little or no tendency to stick together or to collect dust, somewhat tough-skinned, and characteristic in flavor. Except in having less bloom, the product was identical in character with the natural sun-dried raisins of commerce, the principal type produced in California. Dehydration without Other Treatment. — Lots of grapes similar to those used in sun-drying were dehydrated as for the soda dip with dehydration described in the next paragraph. The raisins were uniform grayish brown, rather tough-skinned, and slightly caramelized in taste. The method is seldom used commercially. Soda Dip with Dehydration. — The grapes were dipped in a solution of 0.2 to 0.3 per cent sodium hydroxide (caustic soda), at a temperature just below boiling (200° to 210° F) for 2 or 3 seconds — until many faint checks showed in the skins after the grapes had been cooled by rinsing in cold water. Then the sample, spread on a paper-covered laboratory-dehydrater tray, was placed in a laboratory dehydrater of the recirculating tunnel type, electrically heated and thermostatically controlled to a minimum of 130° F and a maximum of 140°. The raisins were uniform medium brown, slightly sticky, and medium tender — much like the commercial product occasionally made in this manner. Soda Dip with Sun-Drying. — The grapes were dipped as for the soda dip with dehydration and were then sun-dried. The raisins were medium to dark brown, tender, meaty, and slightly sticky — generally similar to the commercial product occasionally made by this process. Golden Bleach. — Thompson Seedless grapes were dipped in a hot, 0.2 to 0.3 per cent sodium hydroxide (caustic soda) solution and rinsed as in the soda- dip-with-dehydration method. Then the grapes were exposed to sulfur dioxide gas (diluted with air to about 1 per cent by weight) for 2 to 4 hours or until the grapes and stems had bleached to a yellowish white. After sulfuring, the grapes were dehydrated as described for the soda dip with dehydration. Com- mercially the sulfur dioxide used for bleaching is produced by burning sulfur; also the grapes are often dehydrated at about 160° F. Otherwise the procedure in these experiments was essentially like the commercial. The product was brilliant, glossy greenish-yellow to golden-yellow and moderately tender, tast- ing rather strongly of sulfur dioxide — very similar to commercially produced golden-bleached raisins. Sulfur Bleach. — The grapes, dipped and sulfured as in the golden bleach, were exposed to direct sunlight until half to two-thirds dried. They were then finished in the shade. Exposure to direct sunlight was longer than in commer- cial practice. The raisins were pinkish yellow, waxy, and tender, tasting strongly of sulfur dioxide. Except in color, they resembled the commercial sulfur-bleached product. (Sulfur-bleached raisins should be yellowish white.) 14 University of California — Experiment Station Australian Mixed Dip. — The dip was composed of 0.3 per cent sodium hydroxide (caustic soda), 0.5 per cent potassium carbonate (technical grade), and 0.4 per cent (by volume) virgin olive oil. In preparing the dip, the potas- sium carbonate was dissolved in sufficient water to form a 5 per cent solution. The olive oil was emulsified in this strong solution, which was then diluted with nine times its volume of water to form a 0.5 per cent solution of potassium carbonate carrying the emulsified oil. The caustic soda was added last. The dip was used at 176° to 180° F, and the grapes were immersed until faint checks showed in the skins after cooling without rinsing — a matter of 2 or 3 seconds. Drying was done in the shade on a specially constructed rack (fig. 1) some- what similar to those used in Australia. The day after the grapes were placed Fig. 1. — Drying rack used for Australian-type sultana raisins. The grapes are placed on shelves made of wire netting. on the rack, and weekly thereafter until dry, they were lightly sprayed with a 5 per cent solution of potassium carbonate in which had been emulsified 0.4 per cent olive oil. The dip and the drying procedure were essentially the same as are used in Australia. 21 The method was not used with Muscat. When the raisins were dry enough to be shaken from the wire rack, they were removed from it and placed in direct sunlight for 1 or 2 days to change the greenish color to yellow or light brown. The finished raisins were soft and tender, char- acteristically nutty in flavor ; and they varied from light greenish brown to dark brown, the darker colors developing in samples subjected to foggy or rainy weather during the drying period. Other lots receiving the same predrying treatment were dehydrated as described for the soda dip with dehydration. Since Australian-type sultana raisins are never dehydrated in commercial practice, results of the treatment are not given separately in the tables, but are included in the averages given for all methods. 21 Lyon, A. V. The mixed dip for sultanas. Australian Dried Fruit News 9(2) :1, 11. March 7, 1934. Lyon, A. V. The scientific side of fruit growing. An account of the work al the Meibein Station. Australian Dried Fruit News 8(11) : 4-5. July 20, 1933. Bul. 683] Yield, Composition, and Quality of Raisins 15 Australian Cold Dip. — The dip was a 5 per cent solution of potassium car- bonate (technical grade) in which was emulsified 0.4 per cent (by volume) olive oil. It was used at 95° to 100° F. The grapes were immersed until about three fourths of the bloom had been removed — usually 1 to 4 minutes. After dipping, they were dried in the shade on the wire rack and were sprayed at intervals during drying as after the Australian mixed dip. The composition of the dip and the drying procedure were essentially the same as those recom- mended by de Castella for Australian use. 22 When practically dry the raisins were removed from the rack and quickly washed in a 0.5 per cent solution of potassium carbonate containing 0.4 per cent emulsified olive oil, the purpose being to remove the heavy deposit of carbonate left by the dipping and spray- ing. The washed raisins were spread on trays and exposed to direct sunlight for 1 or 2 days to allow them to dry and to change the green to yellow or light, brown. The product varied from light greenish brown to medium brown and in other respects resembled that of the Australian-mixed-dip method, but had even better texture. The sultana raisins of Australia and South Africa are made according to either the mixed-dip or the cold-dip method from the variety called Sultana in those countries but identical with the Thompson Seedless of California. Other lots of grapes were given the Australian-cold-dip predrying treat- ment but were dehydrated instead of rack-dried. Since the method is not commercially used, the results are not tabulated separately, but are included in the averages for all methods. California Soda-Oil Dip. — This dip was a 4 per cent solution of "Wyan- dotte" powder (soda ash, commercial anhydrous sodium carbonate) on which was floated a thin film of olive oil. Its temperature during use was held between 95° and 100° F. The grapes were immersed until about three fourths of the bloom had been removed — usually 30 to 60 seconds. They were dried on trays in direct sunlight. The raisins were medium brown, soft, moderately tender, and characteristic in flavor. Often they were lightly crusted with flakes of sodium carbonate deposited by the dipping solution; but this residue was almost completely removed in the stemming and cleaning. The process was once used extensively in the Sacramento Valley, but has now been largely abandoned. Other lots received similar predrying treatment but were then dehydrated, a method never used commercially. Since neither of these two methods is now commercially important, the results are not given separately in most of the tables, but are included in the averages for all methods. TIME REQUIRED FOR DRYING The variety of grapes, the pretreatment, the manner of drying, and the weather conditions all affected the rate of drying. The influence of these fac- tors is shown in table 1, which gives the minimum, maximum, and average time required to dry samples of Thompson Seedless and Muscat of Alexandria grapes by each of the methods used. The data are complete for the Thompson Seedless, and were obtained from observations on the drying of 348 individual 22 Castella, F. de. Sultana drying by the cold-dip process. Victoria (Australia) Dept. Agr. Jour. 23(12) : 716-31. 1925. 16 University of California — Experiment Station lots. With the Muscat only six drying methods were used, involving a total of 76 individual lots, and there was serious difficulty in finishing some of the sun- dried and rack-dried lots because of the late ripening of Muscat at Davis and because of unfavorable drying conditions encountered late in the season. All sun-dried and rack-dried lots of Muscat started after September 11 in 1935 and after September 27 in 1936 either were lost entirely through becoming moldy, or had to be finished in the dehydrater. The data on the drying time for TABLE 1 Drying Time for Grapes Treated and Dried by Various Methods Method of treatment and drying Natural sun-drying Soda dip with sun-drying Sulfur bleach with sun-drying. . . . Australian mixed dip with rack- drying Australian cold dip with rack- drying California soda oil dip with sun- drying Untreated with dehydration Soda dip with dehydration Golden bleach with dehydration Australian mixed dip with de- hydration Australian cold dip with dehydra tion California soda oil dip with de- hydration Thompson Seedless Minimum 12 days 6 days 7 days 10 days 14 days 13 days 34 hours 10 hours 9 hours 10 hours 20 hours 23 hours Maximum 52 days* 28 days 28 days 47 days 54 days* 41 days 47 hours 24 hours 24 hours 25 hours 27 hours 29 hours Average 25 days 12 days 12 days 19 days 25 days 20 days 41 hours 18 hours 17 hours 19 hours 23 hours 26 hours Muscat of Alexandria Minimum 20 days 10 days 18 days 49 hours 23 hours 20 hours Maximum -t 70 hours 42 hours 34 hou Average 28 days* 15 dayst 25 days 60 hours 33 hours 25 hours * A few lots of these treatments failed to dry properly and were discarded when they became moldy, or were finished in the dehydrater. t All lots of these treatments started after September 11 in 1935 and September 27 in 1936 were lost because of failure to dry. t Average of the lots that were finished. § These methods were not used with the Muscat. the dehydrated lots of Muscat are sound, but those on the sun-dried and rack- dried lots indicate only the length of the drying period under favorable weather conditions. Muscat of Alexandria dries more slowly than Thompson Seedless when both varieties are treated alike ; apparently the larger size of the Muscat berries is the principal reason for the difference. The rate of water loss from equivalent surface areas of untreated grapes of the two varieties was about the same. The average weight of the Muscat berries used in the experiments was 4.229 grams ; thai of the Thompson Seedless 1.404 grams. If these figures represent also the relative volumes of the berries, then the approximate surface areas* 1 of single berries of the two varieties would be respectively 12.6 and 6.1 sq. cm. Since the Muscat berries are approximately three times the size of the Thompson Seedless, a given weighl of Muscat would contain only one third as many berries as the same quantity of Thompson Seedless. The total surface area of 28 These calculations have been made on the basis of spherical shape, since neither variety has spherical berries, the figures given are not quite actual, bu1 are Indicative of the relative areas of the berry surfaces. Bul. 683] Yield, Composition, and Quality of Raisins 17 Muscat grapes was therefore roughly two thirds of the surface area of the same weight of Thompson Seedless (12.6 as compared with 3 x 6.1, or 18.3). As table 1 shows, an average of 60 hours was required to dehydrate the un- treated Muscat grapes, whereas the average for similar lots of Thompson Seed- less was 41 hours. All the predrying dipping treatments used caused the grapes to dry faster than untreated grapes dried in a similar manner. The hot dips (soda dip and Australian mixed dip) were most effective in hastening the drying. The soda dip that is used in preparing the soda-dipped, the sulfur-bleached, and the golden-bleached raisins reduced the drying time to one half or less of that required to dry untreated grapes by dehydration or by sun-drying. The sulfur dioxide used with the golden-bleach and sulfur-bleach methods had but little additional effect on the time. It was, however, a preservative during the drying period. Slight growths of molds and yeasts, during bad weather, damaged several of the soda-dipped sun-dried lots. No such damage occurred in any of the sulfur-bleached lots. The Australian mixed dip was nearly as effective as the hot lye in hastening drying, and more effective than the Aus- tralian cold dip. The California soda-oil dip had the least effect on the drying time. The degree of "checking" — cracking of the skins — obtained with the hot dips influenced the rate of drying. The minimum drying times shown in table 1 for the grapes treated with one or the other of the hot dips were obtained with overtreated lots. Different lots of grapes sometimes respond to the dipping treatments in markedly different degree. Tender-skinned fruit from shaded portions of the vine checked more quickly in the dip than the more resistant fruit that had developed in exposed positions. Also fruit from heavily loaded vines required, as a rule, a shorter dipping time than did fruit from vines carrying only light to moderate crops. The composition and temperature of the dips were maintained nearly constant for all lots treated by the same method. Effort was made to adjust the time of immersion according to the requirements of the fruit. That these efforts were only partially successful is evident from the rather wide variations that occurred in the drying time of the dehydrated lots. Undertreated fruit dried slowly; and undertreatment with the Australian mixed dip resulted in nonuniform, sometimes dark colors in the rack-dried raisins. Results with the cold dips were less variable. Rack-drying was slower than direct exposure to the sun. Since the rack was shaded by a roof (fig. 1), the temperature of the grapes during the day was lower. Undoubtedly the lower temperature caused by the shading was re- sponsible for the slower drying on the rack. Although the grapes on the wire netting of the rack were better exposed to air circulation than those on the trays in direct sunlight, and were protected by the roof of the rack from occasional dew, these advantages did not compensate for the effect of the shade. All samples were protected by covering during periods of rain. Grapes treated with the Australian cold dip dried on the racks in about the length of time required for natural sun-drying, and those treated with the Australian mixed dip dried on the racks in about four fifths of that time. As with commercial work, the number of hours required for drying in the dehydrater was about the same as the number of days needed for sun- or rack- 18 University of California — Experiment Station dried lots that had received the same pretreatment. The temperature in dehy- drating the experimental lots was kept lower than that used commercially for the golden bleach and soda dip with dehydration, in order to avoid the cara- melization that may occur in the undipped lots at high drying temperatures. Air temperatures at Davis, where this work was done, are also lower during the drying period than in the San Joaquin Valley. With sun- or rack-drying, weather and length of day influenced the rate of drying. The differences recorded in the length of the drying period of the natural sun-dried lots are almost entirely caused by differences in the weather conditions and in length of day. Drying in the sun and on the rack was most rapid in hot, windy, low-humidity weather and practically ceased during rain or fog. With the facilities available for the experiments, it was not possible to determine precisely how changes in temperature, humidity, rate of air move- ment, and the like affected the length of the drying period when other factors were controlled. The data at hand indicate, however, that with a daily mean temperature of 80° F about 12 days was required for natural sun-drying of Thompson Seedless ; with 70° about 20 days; and with 60° about 40 days. Some of the differences in minimum and maximum time required for dehy- drating equivalent lots of fruit (table 1) were caused by differences in the rate of air flow over the grapes, occasioned by the relative position of the trays in the dehydrater. Although the trays were shifted several times while the grapes were drying, the changes did not entirely compensate the effect of position. Still other recorded differences were caused by varying humidity of the incoming air. Recirculation of air was the only means available for regulat- ing the humidity in the dehydrater ; but control by this means, as attempted in the early runs, proved unsatisfactory. Thereafter recirculation was used mainly to conserve heat in order to enable the heating unit of the dehydrater to maintain the desired temperature; and the humidity was held as low as conditions allowed. EFFECTS ON YIELD OF RAISINS Determination of Yield. — Every drying test was started with a weighed lot of fresh grapes. The dried raisins of each lot were weighed. Moisture was deter- mined by oven-drying (p. 22) ; and then the weight of the raisins obtained from each drying test was adjusted, by calculation, to that of raisins contain- ing 15 per cent moisture. The weight of the fresh grapes divided by the weight of the raisins (at 15 per cent moisture content) gave the drying ratio, or the pounds of fresh grapes required to produce 1 pound of raisins. Drying Ratios with Thompson Seedless. — Table 2 shows the results obtained with Thompson Seedless for each important method of drying. Other methods used in the experiments, but having little or no commercial importance, are omitted. The averages given in the right-hand column, however, include all methods. In constructing table 2 and similar ones for Thompson Seedless (tables 5, 7, 9, 11, 14, and 15) all lots in which the fresh fruit tested between 17.5° and 18.4° Balling are averaged together and presented as a group. Simi- larly, lots testing between 18.5° and 19.4°, between 19.5° and 20.4°, and so fori h, have been averaged. For each group, the left column of the table shows the average Balling degree, which is taken as the index of maturity. Bul. 683] Yield, Composition, and Quality of Raisins 19 TABLE 2 The Drying Ratio* of Thompson Seedless Grapes in Relation to Grape Maturity and to the Drying Method Average maturity of grapes weight of grapes Drying ratio, , with various methods of pretreatment and weight of raisins drying Natural sun-drying Soda dip with sun-drying Sulfur bleach; Australian mixed dip with rack-drying; and Australian cold dip with rack-drying Soda dip with dehydration Golden bleach Average of all methods ° Balling 17.8 19.6 21.0 22.0 23.1 24.2 24.7 26.9 27.9 29.7 ratio 4.85 4.51 4.26 3.97 3.82 3.63 3.50 3.26 3.10 2.87f ratio 4.98 4.44 4.07 3.87 3.69 3.52 3.45 3.11 3.05 2.79 ratio 4.84 4.43 4.14 3.83 3.67 3.52 3.44 3. lit 3.01 2.82 ratio 4.73 4.33 4 02 3.79 3.62 3.48 3.43 3.12 3.06 2.80 ratio 4.82 4.30 4.02 3.80 3.63 3.44 3.39 3.10 3.01 2.78 ratio 4.86 4.42 4.11 weight of fresh fruit * Drying ratio = weight of raisins at 15 per cent moisture t Some lots of these treatments were finished in the dehydrater at 110° F temperature. TABLE 3 The Calculated Drying Ratio and Yield of Natural Sun-Dried, Sulfur-Bleached, and Golden-Bleached Raisins from Thompson Seedless Grapes at Various Stages of Maturity* Drying n weight of grapes Yieldf raisins from 1 tc Average maturity of grapes weight of raisins Natural sun-drying Sulfur bleach Golden bleach Natural sun-drying Sulfur bleach Golden bleach ° Balling 18 ratio 4.88 4.62 4.39 4.18 3.99 3.82 3.66 3 51 3.38 3.25 3.14 3.03 ratio 4.72 4.47 4.25 4.05 3.86 3.70 3.54 3.40 3.27 3.15 3.04 2.93 ratio 4.65 4.40 4.18 3.98 3.80 3.64 3.48 3.34 3.22 3.10 2.99 2.88 pounds 410 433 456 478 501 524 547 570 592 615 638 661 pounds 424 447 471 494 518 541 565 588 612 635 659 682 pounds 431 19 20 21 22 455 478 502 526 23 550 24 25 574 598 26 622 27 646 28 670 29. . 694 * Maturity measured by hydrometer on expressed juice and given in degrees Balling (or Brix). t Calculations of drying ratio and yield are made on the basis of Balling degree X drying ratio for natural 87.8, for sulfur-bleached = 85.0, and for golden-bleached = 83.6. 20 University of California — Experiment Station The drying ratios obtained with the sulfur bleach, the Australian mixed dip with rack-drying, and the Australian cold dip with rack-drying were essen- tially identical and hence are averaged together. The figures for those methods are also nearly the same as the averages for all methods. The differences obtained in drying ratio among the several methods involving dehydration are too small to be important. The figures of table 2 may seem to indicate that the most favorable drying ratio was obtained with the golden bleach; but the apparent differences between the golden bleach and the soda dip with dehydra- tion are not statistically significant. All methods involving dehydration gave more favorable drying ratios than did any method involving sun-drying or rack-drying. Natural sun-drying gave the poorest ratios, and the California soda-oil dip was next poorest. When the drying ratios for the individual drying lots were tabulated, a discovery was made : with Thompson Seedless dried by any one method, the Balling degree of the fresh fruit of the sample multiplied by the correspond- ing drying ratio gave a value that remained essentially constant over the entire range of maturity studied. The value of the constant for each method of dry- ing was as follows : Method Constant 24 Natural sun drying 87.8 Dehydration without pretreatment 84.5 Soda dip with dehydration 84.1 Soda dip with sun-drying 85.5 Golden bleach 83.6 Sulfur bleach 85.0 Australian mixed dip with rack-drying 85.0 Australian mixed dip with dehydration 84.3 Australian cold dip with rack-drying 85.0 Australian cold dip with dehydration 84.1 California soda-oil dip with sun-drying 86.4 California soda-oil dip with dehydration 84.5 Any of these figures divided by any degree Balling between 18° and 29° will give, on the basis of these experiments, the drying ratio to be expected in dry- ing Thompson Seedless at that degree of maturity by the respective method. The theoretical drying ratios in table 3 have been calculated on this basis. The yields from 1 ton of fresh grapes, as given in table 3, were calculated by divid- ing 2,000 pounds by the respective drying ratio. Comparisons between tables 2 and 3 show reasonably close agreement between the actual drying ratios obtained in the experiments (table 2) and the calculated values (table 3). As already explained, the values for the Australian mixed dip rack-drying and the Australian cold dip rack-drying methods are practically identical with those for the sulfur-bleach. Also the figures given for golden bleach are nearly the same as those for any other method involving dehydration. Drying Ratios with Muscat of Alexandria. — Table 4 gives the drying ratios obtained in the experiments and the calculated yields per ton (2,000 pounds) of Muscat grapes for the two most common methods of drying. Data obtained for other methods are incomplete ; and since these other methods are not im- portant commercially, they are omitted from the table. The averages given in (Va^Ij) ° -' The Standard errors ( \ — ■-- I of these constants range from ± 0.18 to ± 0.40. Bul. Yield, Composition, and Quality of Raisins 21 the right-hand column, however, include all methods. In constructing table 4 and similar ones for Muscat (tables 6, 8, and 10), lots of similar fresh-fruit maturity are averaged together and presented as a group. For each group, the left column of the table shows the average Balling degree. The soda dip with sun-drying method gave better yields than the natural sun-drying. With the Muscat, as with Thompson Seedless, advancement in grape matur- ity produced an increase in the yield of raisins, but not in proportion to the Balling degree of the fresh fruit. When the drying ratios obtained with any one method were multiplied by the corresponding Balling degree of the grapes, the product did not remain constant, but drifted slightly upward with advanc- TABLE 4 The Drying Ratio and Yield of Natural and Soda-Dipped Sun-Dried Raisins prom Muscat Grapes at Various Stages of Maturity Average maturity of the grapes Drying ratio,* weight of grapes weight of raisins Natural sun-drying Soda dip with sun-dry inj Yield of raisins form 1 ton of grapesf Natural sun-drying Soda dip with sun-drying Increase per degree Balling, t average of the natural sun-drying and soda dip ° Ballinq 17.1 18.6 19.7 21.4 23.3 25.1 27.4 ratio § 4.48 4.21 3.89 3.65 3.38 3.17 ratio 4.73 4.38 4.16 3.85 3.54 3.35 3.07 pounds 446 475 514 548 591 631 pounds 423 457 481 519 565 597 651 pounds 22.7 24.1 22.6 21.1 20.8 20.4 * Drying ratios actually obtained in the tests. t Yields of raisins calculated by dividing 2,000 pounds by the drying ratio. % Increased yield per degree Balling from 1 ton of grapes over the next lowest stage of maturity appearing in the first column of the table. The first figure is for the soda dip only. All other figures in the column are averages of the two methods. § Dashes indicate data not available. ing maturity. This upward drift in the drying ratio x Balling degree is re- flected by the figures in the right-hand column of table 4, which show a slightly lowered increase in yield per degree Balling advance in maturity as the fruit became riper. The difference in behavior of the two varieties in this respect is logically attributed to the presence of seeds in Muscat and their absence in Thompson Seedless. If the Muscat seeds are assumed to be nearly mature in the greenest fruit used and to change but little as the sugar content of the grapes increases, their effect on the relation between the drying ratio and the Balling degree would be as observed in these tests. EFFECTS ON COMPOSITION OF RAISINS The chemical composition of green grapes, from the time the berries set until ripening begins, remains relatively constant and is much the same as that of other green, succulent tissues of the vine. With the start of ripening, how- ever, come marked changes. 25 Of these the most important, from the growers' 25 Bioletti, F. T., W. V. Cruess, and H. Davi. Changes in the chemical composition of grapes during ripening. Univ. California Pubs., Agr. Sci. 3(6) : 103-30. 1918. 22 University of California — Experiment Station point of view, are (1) a rapid increase in sugar, (2) a rapid decrease in acid, (3) development of color, and (4) development of varietal flavor. Such changes occur in all varieties, but the rate of the changes and the end points reached are varietal characteristics modified by environmental conditions, chiefly temperature. Thus varieties differ in time of ripening, length of the ripening period, color and flavor, and the amounts of sugar and acid in the ripe fruit. In addition, these characteristics are much influenced by climate and soil. Within a given variety, the time of ripening is largely determined by sea- sonal heat accumulation after blooming has occurred. 26 Thompson Seedless attain approximately 18° Balling when 2,000 (Fahrenheit) degree-days 27 of heat above 50° F have been reached after blooming and will be about 25° Balling with 3,000 degree-days similarly accumulated. A warm summer, there- fore, causes early ripening ; a cool summer, late ripening. The amount of acid in the ripe grapes relative to the sugar content (Balling to acid ratio) is influ- enced by the temperature during the ripening period 28 (that is, the 3 to 6 weeks immediately before harvesting) rather than by the accumulation of heat throughout the summer. With a cool ripening period, the acid, relative to the sugar content, is high ; with a hot period, it is low. When grapes are dried into raisins, the largest change in composition occurs in the water content. Fresh, ripe grapes usually contain 70 to 80 per cent water. The water content of raisins when delivered to the processing plant is usually between 10 and 15 per cent. About 90 to 95 per cent of the water of the fresh grapes is therefore lost in the drying. Flavors and colors change during drying. Grapes of characteristic flavor, such as Muscat, usually produce rai- sins whose flavor is distinctive, but different from that of the fresh fruit. Sugars, acids, salts, and other solids are concentrated in the raisins to a degree approximately, but not always exactly, represented by the drying ratio. The changes occurring in the grapes during ripening cause differences in the composition and quality of raisins made from grapes of various degrees of maturity. Treatment before drying and manner of drying also cause small differences in raisins made from similar lots of fruit. Laboratory analyses on experimental lots of raisins produced in 1935 and 1936 included determinations of the content of water, sugar, acid, water- insoluble solids, potassium, calcium, magnesium, and phosphorus. The methods 20 used, except in determining moisture, were standard procedures, sometimes with slight modifications. Water. — The water content was determined by vacuum-oven-drying, at 70° C (158° F), of a thin smear of ground raisin pulp on a previously dried and weighed filter paper. In weighing, the paper and the sample being weighed 26 Winkler, A. J., and W. O. Williams. The heat required to bring Tokay grapes to matur- ity. Amer. Soc. Hort. Sci. Proc. 37:650-52. 1939. 27 "Degree-days" is a convenient term for expressing summation of effective heat. For oxamplr, if the mean temperature for a period of 5 days was 80° F, the summation would be (80° -50°) x5 = 150 degree-days; and, if the mean temperature for August was 79°, the summation for the month would be (79-50) x 31 = 899 degree-days. 28 Winkler, A. J. Maturity tests for table grapes. California Agr. Exp. Sta. Bui. 529:1-35. 1932. (Out of print.) 20 These methods are more fully discussed in: Jacob, H. E. The relation of maturity of the grapes to the yield, composition, and quality of raisins. Hilgardia 14(6) : 321-45. 1942. Bul. 683] Yield, Composition, and Quality op Raisins 23 were always enclosed in a tared closed petri dish to avoid change in weight due to change in moisture content. The results of the moisture determinations were used mainly in adjusting the weight of the raisins, by calculation, to a common basis of 15 per cent moisture. The actual water content of the finished product ranged from 8.5 per cent to 19.0 per cent, with about 83 per cent of the Thompson Seedless samples and 71 per cent of the Muscat falling in between 12.0 and 16.0 per cent. Most of the samples below 12 per cent were sun-dried Thompson Seedless; most of those above 16 per cent sun-dried Muscat. TABLE 5 The Sugar Content* of Thompson Seedless Raisins in Relation to Grape Maturity and to the drying method Average ISf Sugar content with various methods of pretreatment and drying maturity of the grapes Natural sun-drying Soda dip with de- hydration Soda dip with sun-drying Golden bleach Sulfur bleach Australian mixed dip Australian cold dip Average of all methods f Balling 17.8 19.6 21.0 22.0 23.1 24.2 24.7 26.9 28.0 29.7 Average. . . per cent 66.6 68.3? 68.9 69.0 69.4 69.3 70.0 69.3 69.2 69.5{ 69.0 per cent 67.9 68.9} 69.4 70.5 70.9 71.1 71.7 71.1 69.8} 71.2J 70.3 per cent 68.4 69.3J 70.6 70.2 70.6 71.0 71.3 70.7 70.0} 70.6} 70.3 per cent 69.3 71.4} 70.4 70.9 71.1 72.0 71.9 71.8 71.8 71.7} 71.2 per cent 68.2 69.7} 70.2 71.0 71.0 71.3 71.4 70.9 69.4} 72.1} 70.5 per cent 67.7 -§ 69.9 70.1 70.8 70.8 70.7 70.4 70.0 70.3} 70.1 per cent 66.1 66.5} 68.8 69.4 69.5 69.7 70.1 69.4 70.7 70.1} 69.0 per cent 67.9 69.1 69.8 70.4 70.6 70.8 71.0 71.0 70.3 71.0 70.2 * Sugar was determined by the Quisumbing and Thomas method, calculated as invert sugars, and is expressed in percentage of the raisins at 15 per cent moisture content. t Twelve drying methods are represented in the averages for the 17.8° and the 21.0° to 26.9° groups, inclusive; eleven methods in the 19.6° group: and eight each in the 28.0° and the 29.7° groups. } Result from only one determination. All other figures are averages of determinations on 2 to 6 lots. § Dash indicates data not available., Sugar. — Because preliminary analyses revealed no sucrose, the customary inversion procedure to include sucrose in the sugar determinations was omit- ted. Reducing sugars, determined on water extracts of the raisins by the Quisumbing and Thomas method, 30 were calculated as invert sugars and ex- pressed in percentage of the raisins at 15 per cent moisture content. Table 5 summarizes the results for the Thompson Seedless. The individual lots of raisins were grouped according to maturity of the fresh grapes as for table 2. For each group, the left-hand column of the table shows the average Balling degree and is taken as the index of maturity. The sugar content of the Thompson Seedless raisins increased slightly with advancing maturity of the grapes up to 23° or 24° Balling; thereafter the data on sugar show no further appreciable change. These trends are best illustrated in the right-hand column of table 5, which gives the averages of all the drying methods used. Grapes averaging 17.8° Balling dried to raisins averaging 67.9 30 Association of Official Agricultural Chemists. Official and tentative methods of analysis. 710 p. Washington, D. C. 1935. 24 University of California — Experiment Station per cent sugar (as invert sugar) ; 24.2° Balling, 70.8 per cent sugar ; still riper grapes, about 71.0 per cent sugar. Since chemical determination of sugar is more or less empirical, individual determinations on the same sample may vary as much as plus or minus 0.5 per cent from the average. In the right-hand column of table 5 a sufficient number of samples have been averaged, except in the two highest Balling groups, to reduce the error of the averages to well below that limit. With the figures given for each of the different methods of drying, however, fewer samples are represented in the averages, and differ- ences of 0.5 per cent or less may not be significant. Apparently the highest sugar content was that of the golden-bleached rai- sins ; the lowest (about 2.2 per cent lower) , that of the natural sun-dried and the Australian cold-dipped rack-dried raisins. The raisins of the other four methods listed in table 5 are intermediate in sugar content, and no differences may be assumed to exist among them. When dried without the use of artificial heat, the grapes treated with a hot dip evidently produce raisins higher in sugar than undipped or cold-dipped grapes similarly dried. A possible ex- planation might be in the more rapid death of the tissues : there is a shorter period for respiration than with cold dip or no dip. The tissues of hot-dipped grapes appear to die within a few minutes or at most a few hours, being rapidly discolored unless treated with sulfur dioxide. With the small berries of the Thompson Seedless, sufficient heat might penetrate to kill most or all of the tissues within a few seconds ; but deep penetration is hardly to be expected with the larger Muscat of Alexandria. Undipped grapes and those treated with the Australian cold dip retain living tissue for some days after being placed on the drying trays or racks. As long as living tissue is present, energy-producing materials, such as sugars and certain acids, are used up in respiration. The undipped dehydrated raisins (not given in table 5) had the same sugar content as the soda-dipped dehydrated lots, a fact that substantiates this theory. The temperature of the dehydrater — 130° to 140° F — rapidly killed all grape tissues ; hence no appreciable differences in respiration may be assumed to exist among dehydrated lots. Whether or not losses caused by respiration were entirely responsible for the lower sugar content of the natural and Australian- cold-dipped raisins has not been determined. Table 6 summarizes results of the sugar determinations on Muscat of Alex- andria raisins. Only the three commercially important methods are listed separately. Three additional methods are represented in the average of all methods in the right-hand column. The lots have been arranged with refer- ence to maturity of the fresh grapes, as for tbale 4. The left column shows the average Balling degree of each group. The sugar content of raisins from vari- ous lots of Muscats of similar maturity, as indicated by the Balling degree of the juice, fluctuated oftener and more markedly than with Thompson Seed- less. These fluctuations were intimately associated, in inverse ratio, with fluc- tuations in the amount of water-insoluble matter (the sugar content decreases fis the insoluble solids increase" 1 ). The variations in the insoluble matter were, in 1u l-ii, caused mainly by differences in seed content. The seeds in Muscat of Alexandria vary in number from none to four and in size from very small to 81 The calculated coefficient of correlation between sugar con (cut and water-insoluble solids content of the Muscat raisins in the experimental lots was -0.891 ± 0.0149, which shows a high degree of negative correlation between these substances. Bul. 683] Yield, Composition, and Quality of Raisins 25 relatively large. The seed content varies with different clusters, different vines, different vineyards, and different seasons. To obtain strictly comparable re- sults on the sugar content of various lots of Muscat raisins, the seeds should be removed from the samples before the analyses. Adequate facilities for this work were not available in the laboratory when the experimental samples were analyzed. The sugar content of the Muscat raisins averages roughly 4 per cent lower than that of Thompson Seedless ; the insoluble-solids content about 5 per cent higher (tables 9 and 10) . The general trend of the sugar content of the raisins TABLE 6 The Sugar Content* of Muscat of Alexandria Raisins in Relation to Grape Maturity and to the Drying Method Average maturity Sugar content with various methods of pretreatment and drying of the grapes Natural sun-drying Soda dip with de- hydration Soda dip with sun-drying Average of all methodsf ° Balling 17.1 per cent -X 63. 0§ 63. 2§ 64. 1§ 66. 2§ 65. 8§ 66.8 64.9 per cent 63. 2§ 63.7 65.5 66.7 68.1 68. 1§ 67.5 66.6 per cent 61. 9§ 62.9 65.1 66.5 66.6 66. 4§ 67.5 65.8 per cent 63.1 18.6 63.4 19.7 21.4 23.3 65.2 66.4 67.2 25.1 66.9 27.4 67.2 Average 18.6 to 27.4 . . . 66.1 * Sugar was determined by the Quisumbing and Thomas method, calculated as invert sugars, and is expressed in percentage of the raisins at 15 per cent moisture content. f Averages of the results on lots dried by six methods in 1935 and 1936. $ Dash indicates data not available. § Results from only 1 determination. All other figures are averages of determinations on 2 or more lots. in relation to grape maturity is essentially the same in the two varieties : it increased slowly with advancing maturity up to 23 or 24 degrees Balling. Beyond that stage, apparently, it did not appreciably increase. Table 6 shows a slightly higher sugar content in the soda-dipped dehydrated than in corre- sponding soda-dipped sun-dried lots. Also, both these methods produced rai- sins of higher sugar content than did natural sun-drying. Acid. — The acid-content determinations of the fresh grapes were made by direct titration of 10-cubic-centimeter portions of the extracted juice with standardized sodium hydroxide solution, phenolphthalein serving as the indi- cator. With the raisins, larger portions of the water extracts of the samples were titrated. The results have been calculated as tartaric acid and expressed as percentage by weight of the grape juice or of the raisins at 15 per cent moisture content. Table 7 gives results for the Thompson Seedless; table 8, for the Muscat of Alexandria. The figures of table 7 show a decrease in the acid content of Thompson Seed- less raisins as grape maturity advanced. The right-hand column, giving the averages of all methods, shows the decrease most clearly. The decrease was rapid with the greenest fruit, slowed up in the mid-portion of the ripening 26 University of California — Experiment Station range, and nearly or quite ceased as the fruit became very ripe. The acid con- tent of the grapes (second column of table 7) shows a similar trend, but with smaller differences from one stage of maturity to another. In the fore part of the ripening range, the acid content of Thompson Seed- less raisins is slightly less than the product of the acid content of the grapes multiplied by the drying ratio (table 7) for the particular group. Judging from the results obtained early in the season with the least mature fruit, some acid is lost in drying. Since the loss appears greatest in the natural sun-dried TABLE 7 The Acid Content* of Thompson Seedless Eaisins in Eelation to Grape Maturity and to the Drying Method Acid content* of the grapes Acid content* of raisins with various methods of pretreatment and drying Acid content Average maturity of the grapes Natural sun- drying Soda dip with dehy- dration Soda dip with sun- drying Golden bleach Sulfur bleach Aus- tralian mixed dip Aus- tralian cold dip Average of all methodst of grapes X drying ratio; average of all methodst ° Balling 17.8 per cent 0.79 per cent 3.09 per cent 3.55 per cent 3.65 per cent 3.97 per cent 4.05 per cent 2.97J per cent 3.09 per cent 3.40 3.84 19.6 0.69 2.53} 2. 82* 2.49J 3. 11J 3.90J -§ 2.65 2.84 3.05 21.0 0.66 2.38 2.45 2.48 2.71 3.01 2.20 2.26 2.46 2.71 22.0 0.59 2.10 2.11 2.00 2.58 2.60 1.99 1.94 2.10 2.26 23.1 0.54 1.88 1.86 1.77 2.41 2.34 1.90 1.83 1.90 1.98 24.2 0.51 0.46 0.42 1.79 1.65 1.57 1.73 1.75 1.60 1.54 1.63 1.50 2.08 2.08 1.80 2.31 2.10 1.80 1.78 1.55 1.45 1.77 1.56 1.44 1.79 1.69 1.51 1.79 24.7 1.58 26.9 1.32 28.0 0.42 1.45 1.50 1.50 J 1.88 2.07J 1.35 1.39 1.59 1.27 29.7 0.44 1.32J 1.50 1.47 1.94J 1.70} 1.47 1.39 1.53 1.24 Average 0.55 1.98 2.09 2.00 2.46 2.59 1.85 1.93 2.08 2.10 * Total acid, as determined by titration with phenolphthalein as the indicator, calculated as tartaric acid and expressed in percentage by weight of the grape juice (second column) or of the raisins at 15 per cent moisture content. t Twelve drying methods are represented in the averages for the 17.8° and the 21.0° to 26.9° groups, inclusive. Eleven methods are represented in the 19.6° group, and eight each in the 28.0° and the 29.7° Balling groups. t Results from only 1 determination. All other figures are averages of determinations on 2 to 6 lots. § Dash indicates data not available. lots and least in the dehydrated, some acid may have been used up in respira- tion before the grape tissues died. At the mid-point in the ripening range the acid content of the raisins is about the same as the apparent acid content of the grapes multiplied by the corresponding drying ratio ; but above the mid-point it is slightly greater ; below slightly less. The discrepancies are greatest in the greenest and the ripest lots. The results for Muscat (table 8) show a similar trend. That the acid content increased when very ripe grapes were dried to raisins is improbable. More likely, the method used with the fresh juice did not truly show the total acid content of the grapes. In grapes much of the total acid content represents potassium bitartrate (cream of tartar), and the remainder is presumed to be free acid. The amount of potassium bitartrate relative to the free acid present increases as the grapes ripen ; in very ripe grapes it may account for one third or more of the total titratable acid. Since the solubility of this compound varies with the composition of the solution and particularly Bul. 683] Yield, Composition, and Quality of Raisins 27 with the active acid present, 32 one cannot say from the information at hand whether in these tests the very ripe grapes contained so much potassium bitar- trate that some of it had crystallized out of the juice. But if this did occur, some of that which had crystallized would have been lost with the discarded pulp and sediment. With the raisins, on the other hand, hot-water leaching was used to obtain the water extracts ; hence all the potassium bitartrate was un- doubtedly extracted and would be represented in the acid content of the raisins. The sulfur ed raisins, both golden-bleached and sulfur-bleached, are clearly higher in acid content than any other comparable lots. The sulfur dioxide present accounts for part of this higher acid content. Whether or not it might be responsible for all the difference between the golden-bleached and the soda- dipped dehydrated and between the sulfur-bleached and the soda-dipped sun- dried raisins was not determined. These differences appear rather large, however, to be thus explained. In acid content the differences between corre- sponding lots of golden-bleached and sulfur-bleached are not significant, and the raisins produced by the other methods listed in table 7 are essentially alike. A difference of slight statistical significance apparently exists between the figures for the soda dip with dehydration and those for the Australian mixed dip. While drying, however, the raisins of the latter method had been sprayed with an emulsion of olive oil in a 5 per cent potassium carbonate solution ; one may therefore reasonably conclude that the slightly lower apparent acid con- tent of the Australian-type sultana results from small quantities of carbonate residue adhering to the outside of the berries despite the superficial washing given in finishing them. The Muscat of Alexandria grapes show a trend in the acid content much like that of Thompson Seedless. The acidity, as measured on the expressed juice of the fresh berries, decreased as maturity advanced up to about 23° Balling. Thereafter the data (table 8) indicate no further decrease. The Mus- cat raisins, on the other hand, show a continued decrease in acid content (table 8) with advancement in maturity of the grapes over the entire range studied. The decrease was most rapid in the fore part of the ripening range; with further advancement in maturity it was less rapid. The ripest Muscat grapes used in the experiments were 27.4° Balling as compared with 29.7° in Thomp- son Seedless. To facilitate comparison of methods, the figures for comparable maturity groups — 18.6° Balling to 27.4° Balling — have been averaged, and these aver- ages appear in the last line of table 8. Of the three drying methods tabulated separately, the soda-dipped dehydrated raisins are highest in acid content, the soda-dipped sun-dried raisins lowest, and the natural sun-dried raisins inter- mediate. Water-Insoluble Solids. — The water-insoluble-solids determination has in- cluded all substances that failed to be dissolved by 6 hours' leaching with warm 32 The "pH" is a measure of the active as distinguished from the total acidity ; it is an inverse function of the concentration of hydrogen ions furnished by an acid on dissociation. The lower the pH, the greater is the effective acidity. For further information see : Genevois, L. La solubilite des tartrates de potassium et de calcium dans les solutions alcooliques acides, les mouts et les vins. Annales de la Brasserie et de la Distillerie 32:310-15, 326-28, 337-43. 1934. 28 University of California — Experiment Station TABLE 8 The Acid Content* of Muscat of Alexandria Raisins in Relation to Grape Maturity and to the Drying Method Average maturity of the grapes Acid content* of the grapes Acid content* of raisins with various methods of pretreatment and drying Acid content of grapes X Natural sun-drying Soda dip with dehydration Soda dip with sun-drying Average of all methods t drying ratio; average of all methods ° Balling 17.1 18 6 per cent 0.66 0.58 0.46 0.44 0.42 0.39 37 0.44 per cent -X 2.17§ 1.89§ 154§ 1.48§ 1.38§ 1.22 1.61 per cent 2.95§ 2.55 2.01 1.60 1.56 144§ 1.19 1.73 per cent 2.17§ 1.98 1.72 1.49 1.32 1.175 1.15 1.47 per cent 2.64 2.34 1.92 1.60 1.50 1.36 1.21 1.66 3.10 2 53 19.7 21.4 23 3 1.89 1.68 1.49 25 . 1 1.29 27.4 1.12 Average 18.6 to 27.4 1.67 Total acid, as determined by titration with phenolphthalein as the indicator, calculated as tartaric acid and percentage by weight of the grape juice (second column) or of the raisins at 15 per cent moisture con- t Averages of the results on lots dried by six methods in 1935 and 1936. % Dash indicates data not available. § Results from only 1 determination. All other figures are averages of determinations on 2 or more lots. TABLE 9 Water-Insoluble-Solids* Content of Thompson Seedless Raisins in Relation to Grape Maturity and to the Drying Method Average maturity of the grapes Balling 17.8 19.6 21.0 22.0 23 1 24.2 24.7 26.9 28.0 29.7 Average Water-insoluble-solids* content with various methods of pretreatment and drying Natural sun- drying per cent 7.97 6.95* 6.69 6.06 5.97 5.82 5.43 5.48 5.03 4 861 6 (« Soda dip with dehy- dration per cent 6.36 -§ 6 03 5.83 5.43 6.13 5.10 4.86 5 10 4.66J 5.50 Soda dip with sun- drying per cent 6.49 6.36} 5.57 5.83 Golden bleach per cent 6.10 Sulfur bleach per cent 0.08 6.20J 5.79 5.58 5.22 5.10 4.90 5 21 4.461 4 521 5.31 Aus- tralian mixed dip per cent 7.38J. 63.1 5.98 5 . 68 5.59 5.35 5.30 5.53 5.58 5.86 Aus- tralian cold dip per cent 7.38 6.50J 6.35 6.12 5.90 5.77 5.29 5.41 5 08 5.05 Average of all methods! per cent 6.71 6.69 6.17 5.76 Insoluble solids -:- drying ratio; average of all methods! quotient 1.38 1.51 1.50 1.50 1.57 1.61 1.52 1.60 1.63 1 71 1.55 * Residue from Soxhlet extraction of the raisin pulp, expressed in percentage of the raisins at 15 per cent moisture content. t Twelve; drying methods arc represented in the averages for the 17.8° and the 21.0° to 26.9° groups, inclusive. Eleven methods are represented in the 19.6 group, and eight in the 2^.0° and 29.7° Hailing groups. I Results from only I determination. All other figures are averages of determinations on 2 to 6 lots. § Dashes indicate data not available. Bul. 683] Yield, Composition, and Quality of Raisins 29 water, as well as the small amounts of finely divided material that washed from the sample during the Soxhlet extraction process or precipitated on standing for a few hours and which were subsequently filtered out. In Thompson Seed- less raisins most of the insoluble material consisted of fiber from the skins and pulp, whereas in Muscat about half of it appears to have come from the seeds. Undetermined quantities of dust collected on the sun-dried and rack-dried products, particularly on overtreated lots of soda-dipped or sulfur-bleached raisins that were slightly sticky and on the raisins processed with the Aus- tralian dips, which tended to be oily. Although most of the dust was removed by the cleaning, fairly large fluctuations in the results of the analyses indicate that the finishing process did not always completely remove the foreign matter. Small amounts of resins, fats, and other substances derived from the raisins were, no doubt, also present. Table 9 gives the percentages of insoluble solids in the Thompson Seedless raisins ; table 10, those for Muscat. In Thompson Seedless the content of insoluble solids in the raisins decreases with advancing maturity of the grapes — a trend most clearly shown in the column of table 9 giving the averaged results of all drying methods. The actual percentages of insoluble material ranged from 7.97 per cent in the natural sun- dried raisins made from grapes averaging 17.8° Balling down to 4.40 per cent in golden-bleached raisins made from grapes of 29.7° Balling. A significant inverse relation exists between the percentages of sugar and of insoluble solids. 33 With the rise in sugar content goes a decrease in percentage of insolu- ble solids ; but the former is somewhat greater. As the grapes become more mature, the sum of the percentages of sugar and insoluble solids shows a slight drift upward. No decrease in the absolute amount of insoluble solids per berry or in the percentage of insoluble solids in the fresh fruit is indicated by the data. If the figures for the insoluble solids, as represented in the average of all methods (table 9), are divided by the corresponding drying ratios (from table 2), an approximation of the percentage of insoluble solids in the fresh fruit is obtained for each group of samples. The figures thus obtained (right-hand column, table 9) remain fairly constant for the groups from 19.6° to 24.7° Balling inclusive. Above 24.7° Balling the figures tend to increase slightly. It seems probable that these slight increases may be caused by loss of water from the fresh fruit before harvesting, since the weight of the fresh berries re- mained about the same despite the increase in sugar content. 34 Among the Thompson Seedless lots, the natural sun-dried raisins had the highest content of insoluble matter; the Australian mixed-dipped and Austra- lian cold-dipped were next, followed by the soda-dipped sun-dried and the soda-dipped dehydrated ; and the sulf ured raisins — those golden-bleached and sulfur-bleached — were lowest. In the Muscat raisins the percentage of water-insoluble materials was roughly double that of comparable lots of Thompson Seedless. Also, fluctua- tions among similar lots were greater ; these have already been discussed in 33 In Thompson Seedless raisins the coefficient of correlation between these two quantities is -0.472 ±0.0161. 84 Jacob, H. E. The relation of maturity of the grapes to the yield, composition, and qual- ity of raisins. Hilgardia 14(6) : 321-45. 1942. 30 University of California — Experiment Station relation to fluctuations in the sugar content, with which they are closely corre- lated. 35 As explained in the discussion of the sugar content, the fluctuations in the insoluble-solids percentage of the Muscat raisins appear to be caused TABLE 10 Water-Insoluble-Solids* Content of Muscat Raisins in Relation to Grape Maturity and to the; Drying Method Average maturity Water-insoluble-solids* content with various methods of pretreatment and drying of the grapes Natural sun-drying Soda dip with dehydration Soda dip with sun-drying Average of all methodst ° Balling 17 1. per cent -X 11. 6§ 13. 7§ 12. 8§ 11.35 10 2§ 10.7 11.8 per cent 11.78 12.5 11.4 10 2 10.2 99§ 10.0 10.7 per cent 14 1§ 14.2 11.1 11.2 10 5 9 9§ 10.3 11.2 per cent 12.5 18.6 12.9 19.7. 11.6 21.4 23.3 25.1. . 11.0 10.3 10 1 27 4. . 10.2 Average 1S.6 to 27.4 11 * Residue from Soxhlet extraction of the raisin pulp, expressed in percentage of the raisins at 15 per cent moisture content. t Averages of the results on lots dried by six methods in 1935 and 1936. t Dash indicates data not available. § Results from only 1 determination. All other figures are averages of determinations on 2 or more lots. TABLE 11 The Potassium, Calcium, Magnesium, and Phosphorus Content of Thompson Seedless Raisins* in Relation to Grape Maturity Average maturity of the grapes Potassium Calcium Magnesium Phosphorus Balling 17.8 per cent 0.877 .941 .865 .926 .881 .954 .890 .889 .951 .883 906 per cent 0.061 .065 056 .051 .059 .054 .064 .056 057 050 057 per cent 0.026 .029 .022 .025 .020 .032 .020 032 033 020 026 per cent 0.115 19.6 .108 21.0. .105 22.0. .111 23.1 24 2 24.7 26.9.. 25 () 29.7 A vrerage .106 107 .105 .105 .098 097 106 * Averages of results of determinations made on raisins dried by the natural sun- dried, untreated dehydrated, soda-dip dehydrated, soda-dip sun-dried, golden-bleach, and Bulfur-bleach methods. All results are calculated on the basis of 15 per cent moisture in the raisins. mainly by variability in the number and size of seeds, although the adherence of dust and other foreign matter may have been a minor factor. With Muscat raisins, differences in the percentage of insoluble solids that may be attributed to the drying method are not clear cut. Among the three The coefficient of correlation between percentage of sugar and percentage of insoluble solids in Muscat raisins is -0.891 ± 0.0149. Bul. 683] Yield, Composition, and Quality of Raisins 31 methods listed separately in table 10, soda-dipped dehydrated raisins were lowest in apparent insoluble-solids content; natural sun-dried highest; and soda-dipped sun-dried, intermediate. The averages for the six maturity groups of which comparable lots of raisins were available appear in the last line of table 10. Potassium, Calcium, Magnesium, and Phosphorus. — In order to follow the trend of mineral content of the grapes and raisins over the range of maturity studied, selected samples were ashed and analyzed for potassium, calcium, magnesium, and phosphorus. Thompson Seedless raisins for the analyses were taken from lots dried by the natural, untreated dehydrated, soda-dip dehy- drated, soda-dip sun-dried, golden-bleach, and sulfur-bleach methods. Sam- TABLE 12 The Potassium, Calcium, Magnesium, and Phosphorus Content* of Muscat Eaisins in Eelation to Grape Maturity Average maturity of the grapes Potassium Calcium Magnesium Phosphorus Balling 17.1 19.7 21.4 23 3. per cent 0.777 725 .658 .670 .690 .679 0.700 per cent 0.075 .068 .065 .072 .071 .075 0.071 per cent 0.031 .025 .023 .019 .019 .024 0.024 per cent 0.099 .094 .092 083 25.1 27.4 .092 .087 Average 0.091 * Averages of results of determinations made on raisins dried by the natural sun- dried, untreated dehydrated, soda-dip dehydrated, and soda-dip sun-dried methods. All results are calculated on the basis of 15 per cent moisture in the raisins. pies from lots treated in carbonate dips were avoided. Since the data obtained failed to show differences in mineral content of comparable lots dried by the different methods, the results from all lots belonging in each maturity group were averaged together. In the Thompson Seedless raisins (table 11) the percentage of potassium, calcium, and magnesium appears to remain nearly constant over the entire range of maturity. Judging from the data, the percentage of phosphorus in the raisins may decrease slightly with advancement in grape maturity. Appar- ently, therefore, the increase in the amounts of potassium, calcium, and mag- nesium in the grapes approximately parallels the increase in sugar as the grapes ripen, whereas the rate of increase of phosphorus may be slightly slower. With the Muscat raisins, the samples for analysis were taken from lots dried by the natural, untreated dehydrated, soda-dip dehydrated, and soda-dip sun- dried methods. As with the Thompson Seedless, no differences were found that could be attributed to the method of drying. The data were therefore averaged in maturity groups (table 12) . The potassium and the phosphorus percentage of the Muscat raisins de- creased slightly as the maturity of the fresh grapes advanced. The percentages of calcium and magnesium seem to remain about the same over the entire range of maturity. 32 University of California — Experiment Station RELATION BETWEEN GRAPE MATURITY AND THE QUALITY OF THE RAISINS Judging Samples from Experimental Lots. — Representative samples taken from the experimental lots were submitted to a judging committee represent- ing six of California's most prominent commercial raisin packers. The samples included ten of natural Thompson Seedless made from grapes ranging from 17.4° to 29.7° Balling and ten golden-bleached from grapes ranging from TABLE 13 Influence of Grape Maturity on the Quality of Natural Sun-Dried and Golden-Bleached Thompson Seedless Eaisins Qualitj - score* Average maturity of the grapes Natural sun- dried raisins Golden-bleached raisins ° Balling 17 4 46 46 52 64 66 67 71 88 96 96 — f 18 2 21.6 50 22 9 50 23 3 50 24 5 58 25 1 68 26 5 78 26 8 27.0 81 28.2 92 28.2J 100 29 . 7 92 * Basis of scoring: best possible grade = 100; second grade = 75; third grade = 50; fourth grade = 25; unmerchantable = 0. t Dashes indicate data not available. t The grapes from which this lot of raisins were made had been selected to include only fruit of uniform yellow color. 21.6° to 29.7° Balling. Most of the judges used four grades in evaluating the raisins, and some used intermediate qualifications. In coordinating the results, the judges' grade terms were given numerical values in order to facilitate averaging of the scores ; the best grade used by each judge was arbitrarily given a rating of 100, the second grade 75, the third 50, and the fourth 25. Table 13 shows the average score of each sample and the Balling degree of the grapes from which each was made. Quality of the natural sun-dried raisins improved gradually from 18.2° to 28.2° Balling. The two poorest samples received identical scores, as did also the two best. All natural sun-dried sam- ples were discounted in grade because a lack of brilliancy had resulted from the absence of bloom on the raisins. The fresh grapes had been prepared for drying as described under "Sources and Preparation of the Fruit." The mix- ing of the grapes, by pouring them from one container to another after clip- pi ag them from the clusters, removed most of the bloom. When dried, the raisins had a dullness not characteristic of a well-handled commercial product. This did not affect the scoring of the golden-bleached raisins, for the natural bloom is always removed by the dipping treatment. Bul. 683] Yield, Composition, and Quality of Raisins 33 The judging of the golden-bleached samples gave different results. The scores appear to reflect mainly the relation between the color and the quality grade. The raisins of all samples submitted were of good size and texture ; free from dirt, mold, yeast, and insect infestation; uniform and brilliant in color. The main variation between samples was in the shade or hue. The grapes used were predominantly green in color until about midseason. They produced greenish- or lemon-yellow raisins, discounted in quality grade because of their color. The gradual increase in size and the improvement in texture, reflected in the scoring of the samples of naturals, were overshadowed by the effect of the color, which was about the same in the first three samples. As the season became well advanced, the fruit became more nearly yellow, and the raisins made from late-harvested grapes scored high. The second sample of 28.2° Balling, marked with a double dagger in table 13, was from a lot of grapes especially selected for uniform yellow color. The raisins of this sample were a brilliant golden yellow and received a perfect score. As these results show, the several factors of quality are evaluated differently for different kinds of raisins ; and a given factor, such as color, may be much more important in one kind of raisin than in another. The specific influence of grape maturity on raisin color was not satisfac- torily determined. With maturity measured on the basis of soluble solids (expressed as degrees Balling) such a determination might, indeed, be impos- sible. The color of the dried raisins, particularly in such light kinds as golden- bleached and Australian-type sultana, is governed by the color of the grapes rather than their sugar content. The influence of grape maturity on the chemical composition of the raisins has been discussed in considerable detail in a previous section. To summarize : Early in the range of maturity studied, the sugar content of the raisins in- creased, whereas the total titratable acidity and the insoluble-solids percent- ages decreased. These changes became slower as maturity advanced. Changes in the sugar and insoluble-solids percentages nearly or quite ceased beyond the mid-point in the range of maturity, although the acidity of the raisins con- tinued to decrease slowly. The percentages of potassium, calcium, and mag- nesium of the Thompson Seedless raisins remained fairly constant, whereas that of phosphorus appears to have decreased slightly with advancing matur- ity. In Muscat, percentages of both potassium and phosphorus decreased, with those of calcium and magnesium remaining about the same. The small but definite increase in sugar and the decrease in percentage of insoluble matter indicate that there is an improvement in the food value of the raisins, and hence in their quality, until the grapes reach 23° or 24° Balling. Beyond that stage of maturity no further considerable improvement is indicated by the chemical composition of the raisins. Relation between Grape Maturity and Size of the Raisins. — The size of the raisins is determined by the size of the fresh berries and the drying ratio. The grapes become larger during the early part of the ripening stage, but the data 3 * indicate little or no increase beyond 21° or 22° Balling. Thereafter, until the grapes shrivel on the vines before harvest, the size of the raisin berries is 36 Jacob, H. E. The relation of maturity of the grapes to the yield, composition, and quality of raisins. Hilgardia 14(6) : 321-45. 1942. 34 University of California — Experiment Station approximately inversely proportional to the drying ratio. Table 14 gives the size of the raisins obtained, in the experimental tests, from Thompson Seedless and Muscat grapes at various stages of maturity. The fluctuations in the table are largely the result of seasonal variation in fresh-berry size. Not all degrees of maturity were obtained each season. The drying method influenced the size of the raisins somewhat, as far as it affected the drying ratio. (See tables 2 and 4 for the drying ratios. ) Table 14 groups the individual experimental lots into classes according to the Balling degree of the fresh juice. All drying methods TABLE 14 The Relation of the Size of the Eaisins to Grape Maturity Thompson Seedless Muscat Average maturity of the grapes Weight per 100 raisin berries* Average maturity of the grapes Weight per 100 raisin berries* ° Balling 17.8 19.6 21 22.0 23.1 24.2 24.7 26.9 28.0 29.7 grams 26.1 31.4 35.6 38.2 39.7 39.0 42.9 47.2 46.2 44 8 ° Balling 17.1 18.6 19.7 21.4 23.3 25.1 27.4 grams 88.7 90.4 108.6 116.5 121.6 120.8 129.1 Includes all methods of drying. have been averaged ; and the results are expressed as the weight of one hun- dred raisin berries. As the data clearly show, the size of the raisins increased, along with the maturity of the fresh grapes. In dessert raisins, large size is always desirable. For cooking and baking, size appears less important than color and texture. Bakers prefer small sizes. In the trade, the large Muscat raisins command a higher price than the small ones. Such a differential does not always obtain, however, with Thompson Seedless; if the season's production runs toward large sizes the small raisins may actually sell for as much as the large ones, or even more. Relation between Grape Maturity and Weight per Unit Volume of the Raisins. — The weight-per-unit-volume measurement is at present the most convenient index to the texture of natural sun-dried raisins of the seedless varieties. The test consists in filling a standardized container with stemmed raisins under empirically standardized conditions, then weighing the raisins in the container. In obtaining the measurements on the experimentally dried lots, a calibrated 500-cc wide-mouth Erlenmeyer flask was used, and the pack- ing of the raisins was made uniform by the use of a mechanical shaker. Then the raisins in the flask were weighed. All determinations were made in dupli- cate ; and the duplicates checked, in all but occasional instances, within 2 per cent of each other. Where greater differences occurred, the determinations were repeated. The method was a modification of that originally used by Chace Btjl. 683] Yield, Composition, and Quality of Raisins 35 and Church. 37 In commercial grading the raisins are run through an auto- matically controlled stemmer and cleaner and into a calibrated container of about 5 gallons' capacity, which receives constant and uniform shaking as it is filled. Both the laboratory and the commercial method are empirical ; one can duplicate the results of either only by duplicating the conditions of the respec- tive test. This test gives a good index to texture. All other conditions being equal, a unit volume of the softer raisins will be the heavier. Moisture content, which might be expected to interfere with texture determinations by this method, TABLE 15 The Weight per Unit Volume of Thompson Seedless Eaisins in Relation to Grape Maturity and to the Drying Method Weight ol 500 cc of raisins'with various methods of pretreatment and drying Average maturity of the grapes Natural sun- drying Soda dip with de- hydration Soda dip with sun- drying Golden bleach Sulfur bleach Australian mixed dip Australian cold dip G Balling 17.8 grams 314 329 333 337 349 355 358 368 371 374 grams 305 306 327 336 332 338 349 356 356 369 grams 291 311 348 347 342 348 357 349 353 360 grams 324 317 355 336 334 339 343 351 357 380 grams 297 311 330 328 332 341 342 342 345 367 grams 304 * 349 353 370 366 379 389 393 394 grams 336 19.6 21 0. . 354 350 22.0 361 23 . 1 376 24.2 369 24 7 369 26 9. . 378 28.0 29.7 393 396 Dashes indicate data not available. has two opposing effects that nullify each other. The more water present, the softer and more pliable the raisins. But the higher the moisture content the less perfect is the stemming. Very dry raisins are almost perfectly stemmed by the machine, whereas the cap stems will remain on some of the berries that contain more moisture. Since the cap stems are less dense than the raisins and also tend to prevent compact settling in the container, their presence reduces the weight of a given volume. Hence, within the rather narrow limits of its applicability to commercial use, the results of the weight-per-unit-volume measurement are largely independent of the moisture content. The principal factor affecting texture that is measured by the test on natural sun-dried Thompson Seedless raisins appears to be maturity of the grapes. Table 15 presents experimental results showing the apparent relation be- tween the maturity of the fresh grapes and the weight per unit volume of the raisins. It includes all the important drying methods. Data for the 19.6° and 29.7° Balling classes consist of the measurement on a single sample for each method of drying. All other figures are averages of determinations on two to seven lots. About the same number of lots are represented in each drying method within any one maturity group. 37 Chaee, E. M., and C. G. Church. Tests of methods for the commercial standardization of raisins. U.'S. Dept. Agr. Tech. Bui. 1:1-23. 1927. 36 University of California — Experiment Station With natural sun-drying, as the figures show, the weight per unit volume of the raisins gradually increases as the grapes become riper, and the change extends over the whole ripening period. From these figures one is justified in concluding that the measurement on natural sun-dried Thompson Seedless raisins is a fair index to grape maturity. The figures for the other methods, being less consistent, would not support such a conclusion for any method other than natural sun-drying. Even the undipped lots that were dehydrated (not shown in table 15) showed no consistent increase in weight per unit volume of the raisins as the grapes become riper. With the golden-bleached lots the weight per unit volume had but little relation to the score given for quality when the raisins were judged by repre- sentatives of commercial packers. The lot receiving a perfect score (table 13), made from grapes of 28.2° Balling, weighed 355 grams for 500 cc. The heaviest raisins — 380 grams for 500 cc — made from raisins of 29.7° Balling, received 92. Another lot weighing 355 grams for 500 cc, the same weight as the lot receiving the perfect score, was made from grapes of only 21.0° Balling and rated only 50. As table 15 shows, the Australian-type sultana raisins, from both the mixed- dip and the cold-dip methods, were the heaviest per unit volume. They were also the softest and most pliable. Their weight-per -unit-volume measurements were, however, only roughly indicative of the maturity of the grapes used and not at all indicative of the relative market quality of the various lots. In these raisins, as with other light-colored types, color is more important than texture as a factor of quality. RELATION OF THE WEIGHT PER UNIT VOLUME TO THE ACID AND INSOLUBLE-SOLIDS CONTENT OF NATURAL THOMPSON SEEDLESS RAISINS The work already reported shows that, within the limits of the experiments, the acidity, weight per unit volume, and percentage insoluble solids of the raisins are closely related to the maturity of the grapes. Also the quality grade of the natural Thompson Seedless raisins, in the absence of such serious defects as sand, decay, and insect infestation, appears closely related to grape matur- ity (table 13). It seemed desirable, therefore, to learn how closely the three determinations would agree when made on commercial raisins of diverse origin. Through the cooperation of the California Federal-State Inspection Serv- ice, 38 samples were obtained from 50 lots of natural Thompson Seedless raisins, purchased by the federal government and commercially graded by the weigh t- per-unit-volume method. The grapes and the resultant raisins were of un- known origin except that they came from various locations in the San Joaquin Valley. With these samples the primary purpose was to check the correlation between the weight-per-unit-volume measurement, made by the commercial method, and the acid and insoluble-solids contents of the raisins ; information thus secured might lead to a more satisfactory definition of raisin grades. Each sample, consisting of about 2 pounds, was passed four times through !H St;itc of California, Department of Agriculture, Bureau of Shipping Point Inspection. W. P. A 1 1. welt, Chief. Bul. 683] Yield, Composition, and Quality of Raisins 37 a laboratory grinder to pulp and thoroughly mix the raisins. For the acid determination a 50-gram portion of pulp was slowly boiled in about 600 cubic centimeters of water until all lumps had completely disintegrated (about 2 hours), additional water being put in from time to time to replace that lost by evaporation. The entire mass was then transferred to a 1-liter volumetric flask, and all residue washed from the boiling beaker into the flask. After cool- ing in a water bath to 20° C (the temperature of calibration of the flasks) , the volume was brought to the 1-liter mark by adding water. The contents of the flask were thoroughly mixed by repeatedly upending the stoppered flask, and then the sediment was allowed to settle out until the upper portion of the liquid was practically free of it. The titrations, with standardized sodium hydroxide solution, were made on 100-cc portions of the clear liquid, which in most cases was light to medium brown. An industrial-model Beckman pH meter and phenolphthalein indicator were used to determine the end points. For the titration, the 100 cc of the raisin extract was placed in a titration beaker containing the electrodes of the pH meter and a motor-driven stirrer. The pH value of the extract was read from the meter before titration. Then sodium hydroxide solution (0.0726 normal) was added from a burette until the pH meter read 7.0, at which point the burette reading was recorded. Phe- nolphthalein indicator (2 drops of 1 per cent in alcohol) was added, and the titration continued to the first definite phenolphthalein color change. At this point the burette reading was again recorded, and a reading taken on the pH meter. The titration was continued to a final end point of pH 8.5. All titrations were made in duplicate, and there was little difficulty in obtaining burette readings from the duplicate titrations agreeing within ± 0.1 cc of each other at each of the three points recorded. Duplicate extracts from a number of the samples checked within similar close limits. The phenolphthalein end point was difficult to see because of the interfering brown color of the extract; yet by careful work one could reach the same end point in repeated titrations on the same sample. The records show a considerable variation in the pH of the phenolphthalein end point on different samples. "When, however, repeated titrations were made on the same sample, the same end point and the same pH of the end point were almost invariably reached. To check the consistency with which the end point could be determined, as many as six separate titrations were made on each of several samples. Seldom did the burette readings on the replicated titrations vary more than ± 0.1 cc from each other for a given sample. Table 16 gives the results, calculated to per cent acid as tartaric, in the raisins. Determination of the percentage of water-insoluble solids of the raisins was less satisfactory than the titration for acid. Three methods were tried. In one a 50-gram sample of raisin pulp was extracted by boiling as described for the acid determinations. The insoluble residue was removed by filter paper, using a Biichner funnel with suction, and was washed with water. Even with a high vacuum, the gelatinous nature of part of the residue made the filtration slow. The method was discarded after a few trials. A second method involved plac- ing 50 grams of pulp in a well-washed muslin bag and leaching in warm water for 48 hours or more. This procedure was also abandoned because complete removal of the sugars was not obtained from some samples after as much as 96 38 University of California — Experiment Station TABLE 16 The Weight per Unit Volume,* pH of Water Extract,! Titratable Acid, and Water- Insoluble-Solids Content of Natural Thompson Seedless Raisins! Weight per unit volume and sample no. 37 lbs.; sample 13 37 lbs.; sample 47 37 lbs. 6 oz.; sample 19. 37 lbs. 14 oz.; sample 10 Av. 37 lbs. to 37 lbs. 14 oz. 38 lbs.; sample 48 38 lbs.; sample 42 38 lbs. 2 oz. ; sample 44 . 38 lbs. 2 oz.; sample 49 38 lbs. 3 oz.; sample 45. . . . 38 lbs. 3 oz.; sample 40 38 lbs. 15 oz.; sample 43 Av. 38 lbs. to 38 lbs. 15 oz. 39 lbs. ; sample 41 39 lbs.; sample 7 39 lbs. 1 oz.; sample 39. 39 lbs. 4 oz.; sample 36 39 lbs. 6 oz. ; sample 1 39 lbs. 8 oz.; sample 3 39 lbs. 10 oz.; sample 37.... 39 lbs. 12 oz.; sample 6. . . . 39 lbs. 14 oz.; sample 38... Av. 39 lbs. to 39 lbs. 14 oz. 40 lbs.; sample 8 40 lbs.; sample 33 40 lbs. 2 oz.; sample 35 40 lbs. 3 oz.; sample 34 40 lbs. 6 oz.; sample 1 1 40 lbs. P> oz.: sample 37 40 Ihs. 7 oz.; sample 4 40 lbs. 7 oz.; sample 50 40 lbs. 8 oz.; sample 23. 40 lbs. 10 oz.; sample 9. 40 lbs. 12 oz.; sample 21 40 lbs. 14 oz.; sample 28. . 40 ll>s. 15 oz.; sample 25. . Av. 40 lbs. to 40 lbs. IS oz. pH value Of 50/1000 water extract pH 3.92 3.78 3.93 3.91 3.82 3.82 3.91 3 93 3.93 3.90 3.94 4.11 Of phenol- phthalein end point pH 8.45 8.38 8.40 8.40 8.41 8.40 8.40 8.33 8.35 8.40 8.41 8.31 8.37 8.40 8.44 8.47 8.38 8.25 8.32 8.35 8.52 8.40 8.39 8.43 8.45 8.40 8.45 8.46 8.40 8.45 8.40 8.47 8.40 8.30 8.41 8.40 8.42 Titratable acid as tartaric Titrated to pH7.0 per cent 2.08 2.05 1.93 1.93 2.00 2.07 1.98 1.92 1.86 2 07 1.73 1.85 1.93 1.81 1.68 1.76 1.60 1.76 1.72 1.76 1.74 1.66 1.72 1.72 1.55 1.80 1.74 1.62 1.72 1.50 1.59 1.62 1.69 1.65 1.62 1.59 1.65 Titrated to phenol- phthalein per cent 2.55 2.35 2.35 2.34 2.40 2.31 2.22 1.98 2.07 1.92 2.02 1.98 2.13 2.04 2.14 2.06 2.09 1.88 2.24 2.07 1.91 2.13 1.88 1.90 2.01 1.96 1.95 1.95 1.92 Titrated to pH8.5 per cent 2.57 2.37 2.38 2.37 2.50 2.38 2.33 2.30 2.55 2.17 2.23 2.35 2.25 2.01 2 08 1.96 2 07 2 04 2.17 2.03 2.17 2.09 2.12 1.90 2.27 2.08 1.92 2 15 1.90 1.94 2.02 1.99 2 01 1.98 1 95 2.02 Water- insoluble- solids content per cent 7.1 5.9 6.3 6.1 6.4 6.1 5.8 6.0 6.1 6.3 5 3 5.5 5.7 4.8 6.2 5 1 5 1 5 4 5 5.0 5 1 5 4 5.4 5.2 5 5 5.3 * Measurements made by the commercial method described in the text. t Extract of 50 grains of raisin pulp in 1,000 cubic centimeters of water. t Samples of commercial raisins of the 1941 crop from the San Joaquin Valley obtained through (lie coopera- tion of the Federal-State Inspection Service. TABLE 16 (Concluded) Weight per unit volume and sample no. pH value Of 50/1000 water extract Of phenol- phthalein end point Titratable acid as tartaric Titrated to pH7.0 Titrated to phenol- phthalein Titrated to pH 8.5 Water- insoluble- solids content 41 lbs. ; sample 18 41 lbs.; sample 46 41 lbs. 1 oz.; sample 5 41 lbs. 3 oz.; sample 29 41 lbs. 6 oz.; sample 32 41 lbs. 8 oz.; sample 24 41 lbs. 10 oz. ; sample 22 . . . 41 lbs. 13 oz. ; sample 30. . . 41 lbs. 14 oz.; sample 26... Av. U lbs. to 41 lbs. 14 oz. 42 lbs.; sample 27 42 lbs.; sample 12 42 lbs. 2 oz. ; sample 14 42 lbs. 5 oz. ; sample 17 42 lbs. 6 oz.; sample 15 42 lbs. 10 oz.; sample 16... Av. 4% lbs. to 4% lbs. 10 oz 43 lbs. 2 oz. ; sample 2 44 lbs.; sample 20 pH 3.96 89 3.97 4. OS 3.98 3.91 pH 8.30 8.32 8.30 8.40 8.40 8.40 8.35 8.40 8.41 8.37 8.48 8.40 8.31 8.44 8.40 8.40 8.30 8.37 per cent 1.52 1.69 1.72 1.48 1.50 1.50 1.62 1.60 1.47 1.57 1.61 1.39 1.49 1.43 1.36 1.43 1.45 1.38 1.44 per cent 1.82 1.97 2.10 1.79 1.75 1.85 1.94 1.97 1.76 1.88 1.91 1.70 1.75 1.69 1.71 1.75 1.75 1.62 1.72 per cent 1.86 2.02 2.17 1.82 1.77 1.88 1.98 2.01 1.79 1.92 1.94 1.71 1.77 1.71 1.72 1.77 1.77 1.68 1.75 per cent 5.8 5.3 5.7 5.1 . 4.9 5.2 5.4 4.9 4.5 5.2 5.2 4.9 5.5 hours' leaching in running water. The third method, and the one finally adopted, involved placing 50 grams of the raisin pulp in a previously washed, dried, and weighed muslin bag and boiling in running water for 48 hours. Tests showed only traces of sugar remaining in the sample after extraction in this manner. After extraction the samples were oven-dried, cooled in a desic- cator, and rapidly weighed. Duplicate determinations thus made checked within 5 per cent of each other and within 10 per cent of the results obtained by either of the other two methods described. The data appear in table 16. The fifty samples are arranged in table 16 according to weight per unit volume. The table has been broken at even pounds in the weight-per-unit- volume measurements to show the average acid content and the insoluble- solids content of the samples in intervals of 1 pound weight per unit volume. The weight-per-unit- volume figures, being from commercial grading, cannot be directly compared with the laboratory measurements on the experimental samples given in table 15. The acid and insoluble-solids contents are, however, comparable with similar data given for the experimental lots in tables 7 and 9. A high degree of negative correlation exists between the weight per unit volume and per cent acid as given in table 16 ; as the former increases, the latter decreases. Minor fluctuations occur, but are in general relatively small. Some may appear rather large ; for example, samples 45 and 40 both have a weight per unit volume of 38 pounds 3 ounces. Yet if sample 45, having 2.07 per cent acid (to pH 7.0) , were placed in the 37-pound weight-per-unit-volume group, it would fit well; and sample 40, having 1.73 per cent acid (to pH 7.0) , 40 University of California — Experiment Station would fit nicely in the 39-pound weight-per-unit- volume group. The best corre- lation appears to exist between the weight per unit volume and the per cent acid determined by titration to pH 7.0. Where the titration is carried to higher pH values, the correlation seems to be slightly lower. Apparently there is little or no correlation between the pH value of the raisin extract and weight per unit volume, but a fair negative correlation between the insoluble solids per- centage and weight per unit volume. The following coefficients of correlation have been calculated from the data given in table 16 : Weight per unit volume and per cent acid to pH 7.0 = -0.874 ± 0.0334 "Weight per unit volume and per cent acid to phenolphthalein = -0.851 ± 0.0390 Weight per unit volume and per cent acid to pH 8.5 = -0.841 ± 0.0414 Weight per unit volume and pH of the water extract = 0.342 ± 0.1100 Weight per unit volume and per cent insoluble solids = -0.564 ± 0.0964 Per cent acid to pH 7.0 and per cent insoluble solids = 0.687 ± 0.0747 DISCUSSION The San Joaquin Valley of California enjoys a unique advantage over most other raisin-producing areas of the world in that untreated grapes may here be sun-dried between the rows in the vineyards. This practice is possible be- cause almost rainless weather prevails during the drying season and because the vine rows are wide-spaced. Natural sun-drying is therefore particularly well adapted to our conditions. In the world markets the natural sun-dried Thompson Seedless and Muscat raisins are less highly esteemed for cooking than certain of the lighter-colored, softer-textured types ; but their lower cost of production, their greater stability in storage, and their better dessert qual- ity go far toward offsetting their somewhat inferior culinary quality when they are sold in competition with the light-colored types. They are clearly the preferred raisins on the American market, and in late prewar years they usually held their own place abroad despite artificial trade barriers designed to destroy their principal advantage — a low production cost. Of the light-colored types the golden-bleached raisins, such as are now being produced in California, have the advantages of lower production cost and more brilliant and more stable color than any other type now sold. Aus- tralian-type sultana raisins (dipped in potassium carbonate and olive oil) equal to those made abroad have been prepared on an experimental basis in California. No reason is apparent why such raisins cannot be produced on commercial scale in the San Joaquin Valley. Their production is not, however, recommended at present because the labor required and the cost of materials are greater than for our present natural and golden-bleached types. It is doubtful that their greater value in the British Empire markets would more llian compensate for their higher cost of production. Increased yield and improved quality of the raisins as the grapes advance in maturity have been well demonstrated in the experimental results reported. With Thompson Seedless and within the limits of good commercial practice, the yield appears to be proportional to the total-soluble-solids content of the grape juice as measured by the Balling or Brix hydrometer. The seeds in Mus- cat, cause the increase in yield of raisins with advancing grape maturity to lag Bul. 683] Yield, Composition, and Quality of Raisins 41 slightly behind that indicated by the Balling (or Brix) degree of the grape juice. In the natural Thompson Seedless raisins, as judged by commercial stan- dards and by chemical composition, quality improves as the grapes mature. Judging from chemical composition, improvement stops soon after the grapes attain 23° or 24° Balling. By commercial standards, however, improvement continues at least to near the upper limit of maturity attained by the grapes in the experiments. With the golden-bleached raisins and with other light-colored types, color is even more important for quality than is the sugar content of the grapes. The desired yellow in the raisins, free from greenish tinge, is easiest obtained if yellow grapes are used. The fruit tends to become yellow or whitish yellow as ripening proceeds, but exposure to light also influences the rate of change. When the grapes become very ripe some berries darken from raisining on the clusters or from injuries such as bird pecks. Such berries remain dark when treated and dried for light-colored raisins and constitute a serious imperfec- tion. Fruit containing an appreciable percentage of raisined or dark-colored berries is therefore unsuited to the preparation of a light-colored product. In the experimental lots, dark-colored berries, mostly from raisining, appeared rather regularly in fruit above 26° Balling. This fact does not imply that there is no raisining on the vines below 26° Balling or that raisining always occurs after that stage is reached ; it may come earlier, or not until later. Whenever the fruit starts to raisin on the vines it must be considered overripe for golden- bleached, sulfur-bleached, Australian-type sultana, or other light-colored raisins. The size of the raisin berries is determined by the size of the grapes and by the drying ratio. Early in the ripening range — up to about 21° Balling — the fresh berries become slightly larger. Thereafter their size changes but little until the very ripe fruit shrivels as a result of evaporation of water. Within the range of good commercial practice in raisin drying, therefore, the fresh berries remain nearly constant in size throughout the period of maturity in which the producer is interested, and the dried berries become larger as maturity ad- vances, in inverse proportion to the drying ratio. The data presented on chemical composition, berry size, and scoring by qualified judges leave no doubt that the quality of the raisins made from either Thompson Seedless or Muscat shows definite improvement as grape maturity advances, at least up to 23° or 24° Balling. In these respects the data fully agree with observations in commercial work. Since maturity of the fresh grapes is a highly important factor, some meas- urement on the raisins that would reflect or indicate the approximate stage of grape maturity is useful in establishing the quality or grade of the raisins. Judging from the experimental results, determination of the acid content of the raisins is probably the best index. With the natural Thompson Seedless raisins, the weight-per-unit-volume measurement appears to be next to the acid content in reliability ; but with dehydrated and other light-colored rai- sins, it appears to have little value. If a quick and accurate method for deter- mining water-insoluble solids were available, that might also be useful. At present, however, no satisfactory procedure seems to be known. To determine 42 University of California — Experiment Station the sugar content of the raisins appears almost useless. The best of our present empirical methods of sugar determination are not precise enough to detect with certainty the small differences likely to occur in raisins made from grapes of considerably different degrees of maturity. PROPER TIME FOR HARVESTING RAISIN GRAPES Grapes are usually harvested for raisins when the Balling reading on the juice is somewhere between 20° and 24°. As table 3 shows, at 20° Balling about 456 pounds of natural raisins will be obtained per ton of grapes, whereas at 24° some 547 pounds may be expected — a difference of 91 pounds, or 20 per cent. In California in 1940 the average yield per acre of raisin grapes 39 was 5.07 tons of fresh fruit. Assuming no change in fresh tonnage as the grapes ripened from 20° to 24° Balling, the yield per acre in dried raisins if harvested at 20° Balling may be calculated as 2,312 pounds (456 x 5.07), whereas for raisins harvested at 24° Balling the average yield would probably be 2,773, or an increase of 461 pounds per acre. At $60.00 a ton — the current price for natural Thompson Seedless raisins in December, 1940 — the increased yield of 461 pounds would have been worth about $13.83. At the 1942 price of $110.00 a ton, the increase would have been worth about $25.35. These calculations have been based on raisins of the same quality. As the grapes ripen, however, the raisin quality usually improves. Hence the actual difference in the value of the crop, if harvested at 24° as compared with 20° Balling, would probably be greater than the indicated figure because of the better price likely to be received. The stage of maturity at which grapes for natural sun-dried raisins should be harvested represents a compromise between two considerations: (1) the larger yields and better quality obtained from well-ripened fruit and (2) the more favorable drying weather early in the season than later. In the middle part of the San Joaquin Valley — the main raisin-producing area of Cali- fornia — excellent drying weather may be expected at least until the middle of September. As the season advances the days become shorter, the temperatures lower, and occasional damaging rains more probable. The mean temperature for August at Fresno is 79.5° F., for September 72.0°, and for October 62.3°. In the thirty years 1 913 to 1942 the earliest rain apt to cause serious damage to drying raisins occurred on September 19 (1939). Eight of the thirty years, however, had rain by September 30 and thirteen by October 10. On the basis of the information now at hand several general rules may be formulated : 1. Grapes of less than 20° Balling should not be dried for raisins except as a salvage operation to avoid loss. 2. Grapes for natural sun-dried raisins should reach 23° or 24° Balling, if such maturity can be attained by September 1 or soon after. Picking should start soon after the grapes reach 23° Balling or September 1, whichever is the earlier. 3. All natural sun-dried raisins should be on the trays by or soon after September 15. 30 Data on total crop and bearing acreage and price of raisins from : McDowell, A. M. Grapes. Summary of 1940 marketing season. Federal-State Market News Service, p. 3, 5. July, 1941. (Mimeo.) Bul. 683] Yield, Composition, and Quality of Raisins 43 4. If, for any reason, the grapes cannot be harvested by September 15, they should usually be dehydrated rather than sun-dried. 5. For the making of golden-bleached raisins, yellow color of the grapes is a better index of proper maturity for harvesting than is the Balling degree of the juice. 6. Grapes containing a considerable number of raisined berries should be considered overripe in the production of golden-bleached raisins. SUMMARY Commercial raisins are mostly made from three varieties of grapes. Thomp- son Seedless, Australian-type sultana, golden-bleached, sulfur-bleached, soda- dipped, and oil-dipped types of raisins are made from Thompson Seedless ; Zante currants or currants from the Black Corinth; and Muscat, Malaga, Valencia, and lexia types from Muscat of Alexandria. A few other varieties have some local importance. In raisins, seediness or seedlessness and distinc- tive flavors are characteristic of the grape variety used. Other factors of quality that are influenced by the maturity and condition of the fresh fruit, the method of drying, and the conditions of drying and storage are hue, uni- formity, and brilliancy of the color ; size of the raisin berries ; condition of the surfaces; texture of the skin and pulp; moisture content, chemical composi- tion ; presence of sand, stems, sticks, nails, or other foreign matter ; presence of decay, mold, or yeast ; and insect infestation. Experiments with twelve methods of drying, eight of which are or have been commercially important, were made to determine the influence of grape ma- turity and drying method on the yield and composition of Thompson Seedless and Muscat raisins. Yield with Thompson Seedless was proportional to the Balling degree of the fresh grapes. Seeds in the Muscat cause the increase in yield to lag slightly behind that indicated by the Balling degree of the grape juice. With both varieties, as grape maturity advanced there was an increase in the size of the raisins, the weight per unit volume, and the sugar content of the raisins; a decrease in the acid and insoluble-solids percentages. The changes in sugar and insoluble solids in the raisins nearly ceased when the grapes reached 23° or 24° Balling, but the percentage of acid continued to decrease slowly. As the grapes ripened, percentages of calcium and magnesium remained constant in the raisins of both varieties; that of phosphorus ap- peared to decrease ; that of potassium was constant in Thompson Seedless, but decreased slightly in Muscat. Quality improved as the grapes ripened; the change in the natural sun-dried product was most marked. The color of the fresh grapes markedly influenced the color of light-colored raisin types such as golden-bleached, sulfur-bleached, and Australian-type sultana. Drying meth- ods involving dehydration or a hot dip produced slightly higher yields of raisins having slightly higher sugar content and lower insoluble-solids content than did other methods. Bleaching the grapes with sulfur dioxide increased the acid content of the raisins. Close correlation is indicated between the quality of natural Thompson Seedless raisins and the acid content and the weight per unit volume ; the two latter measurements also agreed well on samples from 50 commercial lots. With light-colored types, neither of these measurements proved to be a good 44 University of California — Experiment Station index to quality, because of the very high value attached to the hue and bril- liancy of the color. Sugar content cannot be used as an index to raisin quality because the differences involved are so small. ACKNOWLEDGMENTS The author gratefully acknowledges the generous aid of his colleagues and of Mr. Charles D. Fisher, chief chemist of the Dried Fruit Association of Cali- fornia, in the preparation of the manuscript; and the cooperation of the Federal-State Inspection Service in supplying samples from commercially graded lots of government-purchased raisins. l. r H«-2.'44(8842)