UNIVERSITY OF CALIFORNIA COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA THE DEHYDRATION OF PRUNES A. W. CHRISTIE BULLETIN 404 August, 1926 UNIVERSITY OF CALIFORNIA PRINTING OFFICE BERKELEY, CALIFORNIA 1926 CONTENTS Introduction 3 Development of dehydration 3 Statistics 4 Eelation of dehydration to sun-drying 5 Comparative quality , 5 Comparative yield 7 Comparative costs 11 Summary of comparisons 17 Principles of dehydration 17 Heat requirements 18 Fuels :... 19 Heating systems 19 Air flow requirements 20 Methods of securing air flow 22 Humidity considerations 25 Calculation of dehydrater requirements 27 Selection of a dehydrater 29 Dehydrater manufacturers 30 Patent situation 31 Some construction principles 34 Arrangement of equipment 36 Operation of dehydraters 37 Dipping 37 Green grading 38 Traying 39 Temperature 40 Humidity 42 Drying time - 44 Storage 45 Summary of operating methods 47 Selected references 47 THE DEHYDRATION OF PRUNES 1 A. W. CHRISTIE 2 INTRODUCTION California is the leading state in the production of prunes, supplying the greater part of the domestic consumption in the United States as well as a considerable foreign exportation. The total acreage of prunes in 1925, exclusive of the plantings of that year, was 198,572 acres, of which 73.6 per cent was in bearing. The production in 1925 amounted to 145,000 dry tons and, as the more recent plantings come into bearing, the annual production will increase considerably. Unlike most fruits, prunes are rarely sold or canned fresh but are marketed almost entirely in the dried form. Therefore, drying is a most important operation and the profitable production of dried prunes is dependent on successful drying of the crop. i DEVELOPMENT OF DEHYDRATION Sun-drying has been the standard and, until recent years, prac- tically the only method of drying prunes in California. Artificial drying in evaporators was common during the early years of the prune industry but was not so successful at that time as the sun-drying which displaced it. While the adoption of improved cultural practices resulted in steady improvement of the quality, size and yield of prunes, very little attention was given to improved methods and equipment for drying. The few artificially heated dryers which had been used were built by growers with little knowledge of the fundamental principles of dehydration. As a result, these older dryers (see Fig. 1) were comparatively inefficient and expensive to operate and drying in them was slow and uneven. Consequently, they were only used to supple- ment sun-drying when weather conditions prevented natural drying. The popular impression, which amounted to a conviction with most growers, was that artificial drying could not compete with sun-drying, either in quality of product or economy of operation. The fact that all prunes produced in the Pacific Northwest were always artificially i This bulletin supersedes Bulletins 330 and 337 in so far as they concern the construction and operation of dehydraters for prunes. 2 Assistant Professor of Fruit Products, Associate Chemist in the Experiment Station. 4 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION dried, because of the impossibility of sun-drying in that region, was seemingly disregarded as having no application to California conditions. The interest in dehydration and its development during the world war contributed materially to an understanding of the economic application of the principles of heating and ventilating engineering to the evaporation of moisture from prunes. The unusually heavy rains of September, 1918, which spoiled prunes worth millions of dollars, greatly stimulated interest in dehydration. The work of state and federal investigators was instrumental in pointing out the proper Fig. 1. — An old-time inefficient dryer with " windmill' * fan and gravity oil burner. (Note man stirring prunes on sun-drying trays.) construction and operation of dehydraters and in showing their advantages over the natural drying of prunes. Growers began to appreciate these advantages and manufacturers began designing and constructing dehydraters to fill the growing demand for their use with prunes as well as with other products. STATISTICS There has been a steady growth in the dehydration of prunes in California, particularly since 1919, as illustrated by the figures in Table 1, showing the increasing number of dehydraters and the annual tonnages of prunes dried therein. A census of all dehydraters was conducted by the writer during 1921, 1922 and 1923. The figures for 1924 and 1925 are based on information furnished by dehydrater manufacturers, prune packers and others, and are believed to be approximately correct. Bull. 404 THE DEHYDRATION OF PRUNES Table 1.— Growth of Prune Dehydration Number of dehydraters used on prunes Dry tons Proportion Year Dehydrated Total driedf dehydrated 1921 48 126 178 244* 310* 2,946 13,356 16,810 23,780* 30,750* 100,000 132,000 131,000 139,000 145,000 2.9% 10.1% 1922 1923 12.8% 17.1% 21.2% 1924 1925 * Estimated. t Compiled by Dried Fruit Association of California. Many of the most experienced and successful prune growers have installed and are continuing to use dehydraters to the exclusion of sun-drying. Their success with this modern method of drying is influencing other growers to make use of the advantages of dehydration. RELATION OF DEHYDRATION TO SUN-DRYING Success in drying prunes is measured by three main results : 1. Production of the finest quality of dried product permitted by the nature of the fruit harvested. 2. Production of the largest size and greatest weight of dried prunes in relation to the composition of the fresh prunes. 3. Lowest cost of drying consistent with fine quality and high yield. Therefore, it becomes of interest to consider the relative merits of sun-clrying and dehydration with respect to these three vital results. COMPARATIVE QUALITY The satisfactory drying of prunes requires not only the reduction of their moisture content to an amount which prevents spoiling by molding or fermentation but also certain modifications in color and flavor which have become trade standards. The skin should be black, the flesh a light amber color and have a sweet prune flavor free from sourness or caramelization. If every prune season consisted through- out of hot dry weather conducive to rapid drying, there would not be necessarily any significant difference between naturally and artifi- cially dried prunes. However, it is not uncommon for part of the drying season to consist of cold, damp weather accompanied by fog or showers. In some years, the duration of such unfavorable drying b UNIVERSITY OF CALIFORNIA EXPERIMENT STATION weather has been sufficient to cause the total loss of a considerable part of the crop through molding or fermentation. In other seasons of less unfavorable weather, actual spoilage losses have been slight but a material percentage of the prunes have suffered injury to the color and flavor of the flesh because of a partial fermentation during the time drying was temporarily arrested. Such injury does not prevent the sale of the prunes, but definitely lowers their quality and conse- quently their market value. Furthermore, the percentage of prunes which do not dry promptly or properly is proportional to the unfavor- able condition of the weather and, as a result, considerable quantities of variously termed "bloaters," "frogs," "chocolates," "slabs," etc., must often be culled out to maintain the quality of the remainder of the crop. This not only causes a loss in yield but necessitates expensive hand sorting. Proper dehydration not only gives rapid and continuous evapo- ration of the excess water but absolutely prevents deterioration in quality through mold or fermentation. Furthermore, since dehydration is conducted in a closed building by currents of warm air free from wind blown dust, it prevents contamination of the fruit by dirt or insects as in sun-drying. In brief, dehydration returns a clean, thoroughly dried product fully retaining the quality of the original fruit. Dehydration is in line with the modern demand for sanitary production of foods. Not all dehydrated prunes have been of the best quality. In some cases inferiority has been due to the poor quality of the prunes before dehydration. Since dehydration is merely a dependable, controllable method for evaporating the excess water from fruits without injury to their quality, it cannot be expected to improve on the original qualities of the fruit. Rain damaged prunes, salvaged by dehydration, should never be classed or judged as dehydrated prunes. In other cases, inefficient construction or operation of dehydraters has resulted in injury to the quality of the prunes during dehydration or in deterioration after dehydration because of improper or insufficient drying. However, the great bulk of prunes dehydrated in recent years has been accepted and sold as being of the best market quality. An example of the quality of dehydrated prunes is seen in the fact that certain orchardists in the Sacramento Valley, who previously had never obtained the highest grade for their dried product, have, since the installation of dehydraters, consistently obtained the highest grade and price for prunes from the same orchards after dehydration. Cer- tain packers freely state their preference for the dehydrated product. BULL. 404 J THE DEHYDRATION OF PRUNES 7 Other packers, because of unfavorable experiences with occasional lots of improperly dehydrated prunes, have unjustly condemned dehydra- tion. Since the average quality of dehydrated prunes is unquestion- ably above that of sun-dried prunes, it would be more logical to condemn sun-drying. However, since the quality of dehydrated prunes has become more generally appreciated, such unwarranted prejudices are rapidly disappearing and the present consensus of opinion is that the quality of properly dehydrated prunes is at all times equal to and often superior to that of the sun-dried. COMPARATIVE YIELD It has long been known that if, as a result of unfavorable weather conditions, prunes undergo a partial fermentation during sun-drying, the yield of dried product is materially reduced. This is explained by the fact that when the sugar in the prunes is fermented by ever present yeasts, it changes into alcohol and carbon dioxide gas. Since both these compounds are volatile, they evaporate into the surrounding air and thereby cause a loss in weight of solid matter proportional to the extent of the fermentation. When the prunes become sufficiently dried, the action of micro-organisms is arrested and further loss in weight on this account is prevented. The temperatures normally used in dehydraters are above the temperatures at which fermentation organisms act and consequently such' losses are prevented by dehydration. Therefore, it was thought that when fruit underwent rapid and continuous sun-drying without visible signs of fermentation that no losses other than water evaporation occurred and consequently the maximum possible weight of dried product was obtained. Kecent investigations show that even under the most favorable sun-drying conditions prunes suffer a loss in sugar. This is explained by the fact that the living tissues of prunes contain enzymes which cause respiration, or the change of some of the sugar to carbon dioxide gas and water, compounds that evaporate from the prunes. As long as the fruit remains on the tree, respiration losses are replaced by the photosynthetic action of the leaves which manufacture sugar for translocation to the fruit. When the fruit is removed from the tree, it no longer absorbs sugar and, since the tissues remain alive and active for a considerable time thereafter, a small but definite loss of sugar occurs during the long sun-drying and does not entirely cease until the prunes are nearly dry. While the brief heating incident to lye-dipping tends to reduce subsequent respiration losses, the rapid 8 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION drying at the relatively high temperature of a dehydrater soon kills living tissues, which stops respiration and consequently prevents loss in weight therefrom. The relatively short time required for dehy- dration is probably of greater importance in minimizing respiration losses than the elevated temperature. Comparative Tests. — In order to obtain exact data on the relative yields of sun-dried and dehydrated prunes, comparative field tests were made in several prune growing districts, In each test a lot of prunes varying from 200 to 2000 pounds, freshly harvested from a restricted area of orchard, was selected in such a way as to obtain prunes as nearly uniform in size and condition as possible. The prunes were lye dipped in the customary way and, as the dipped prunes were discharged from the dipper or grader, they were received alternately on sun-drying and dehydrater trays, and the gross, tare and net weight of each tray recorded. A repre- sentative sample of the fresh prunes was sealed in a jar with a small amount of preservative and placed in freezing storage for subsequent analysis. Sun-drying was conducted by exposing the prunes to direct sun- shine until about three-fourths dry and then the drying completed in the stacked trays. Warm, dry, clear weather was the rule in every test made except that on Robe de Sergeant prunes at Visalia, in which case rain occurred during the latter part of the drying period and the prunes suffered considerable fermentation before reaching dryness. In fact, with this exception, all lots were dried under optimum sun-drying conditions for the locality and time of year. Dehydration was conducted only in modern air-blast dehydraters operated at a maximum temperature of 160° to 165° F. The time of drying varied from 22 to 37 hours, being affected primarily by the size of the prunes. As soon as drying was complete, the net weight of each lot was determined and a representative sample placed in a sealed jar for subsequent analysis, Table 2 gives the yields of dried prunes per 100 pounds of fresh prunes and the number of dried prunes per pound, first, as actually obtained and, second, when calculated on a uniform moisture basis of 20 per cent. All tests gave a greater weight of dehydrated prunes. However, in every case the increase was partly due to a higher moisture content left in the dehydrated prunes and when this variable is eliminated, the increase in yield is less. In one case (Healdsburg) the yield is actually reversed in favor of sun-drying, but this dis- crepancy is probably due to unevenness in the prunes or an unaccount- able error. Dehydration permits exact and uniform control of the Bull. 404] THE DEHYDRATION OF PRUNES moisture content of the prunes when removed from the dehydrater. As a result, a greater weight of prunes is often obtained by dehy- dration through prevention of over-drying which frequently occurs in sun-drying. Naturally, wherever the dehydrated prunes gave a Table 2.- —Comparative Yields of Sun-Dried and Dehydrated Prunes Date District Variety Test No. How dried Per cent water after drying Sept. 8, 1922 Sept. 8, 1922 Sept. 8, 1922 Cupertino Cupertino Cupertino French French French French Robe Robe Sugar 1 1 2 2 3 3 4 4 5 5 6 6 7 7 Dehydrated Sun Dehydrated Sun 26.3 21.4 22.9 Sept. 8, 1922 Sept. 27, 1923 Cupertino Visalia 20.1 Dehydrated Sun Dehydrated Sun 17.8 Sept. 27, 1923 Visalia 17.3 Sept. 15, 1923 Cupertino 19.9 Sept. 15, 1923 Cupertino Cupertino Sugar French French French French French French 18.3 Sept. 30, 1923 Dehydrated Sun 23.8 Sept. 30, 1923 Cupertino 20.0 Sept. 28, 1925 Sept. 28, 1925 Sept. 3, 1925 Healdsburg Healdsburg Live Oak Dehydrated Sun Dehydrated Sun 28.9 23.7 15.9 Sept. 3, 1925 Live Oak 11.9 Test 1 — Dehydrated 1— Sun 2 — Dehydrated 2— Sun 3 — Dehydrated 3— Sun 4 — Dehydrated 4— Sun 5 — Dehydrated 5— Sun 6 — Dehydrated 6— Sun 7 — Dehydrated 7— Sun Average for dehydra tion Average for sun dry ing Yield as binned Drying ratio 2.04 2.36 1.80 1.89 2.67 39 49 69 94 12 23 31 65 3 2. 2. 1 2. 2. 2. 2. 2.86 2.26 2.52 Pounds dry per 100 pounds green 49.0 42.4 55.5 52.9 37.5 29.5 40.2 37.2 51*4 47.2 44.8 43.3 37.7 35.0 45.2 41.1 Count per pound 40 43 43 48 51 64 32 34 39 42 89 87 60 65 51 5.5 Yield on 20% water basis Drying ratio 2.20 2.40 1.87 1.89 2.60 3.28 2.49 2.63 2.04 2.12 2.51 2.42 2.53 2.60 2.32 2.48 Pounds dry per 100 pounds green 45.1 41.6 53.5 52.9 38.5 30.5 40.2 38.0 49.0 47.2 39.8 41.3 39.6 38.5 43.7 41.4 Count per pound 43 44 45 48 50 62 32 33 41 42 100 91 57 59 53 54 10 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION greater yield, the greater weight of the individual prunes is reflected in the lower count per pound. # The averages of all tests show a distinctly greater weight and size grade of prunes as a result of dehydration even after calculation to a uniform moisture basis. Table 3. — Changes in Weight and Composition of Prunes After Sun-Drying and Dehydration Test No. Condition Total weight, pounds Pits, pounds Flesh, pounds Water, pounds Solids, pounds Sugar, pounds Solids not sugar pounds Fresh Sun-dried Dehydrated.. Fresh Sun-dried Dehydrated.. Fresh Sun-dried Dehydrated.. Fresh Sun-dried Dehydrated.. Fresh Sun-dried Dehydrated.. Fresh Sun-dried Dehydrated.. Average : Fresh Sun-dried Dehydrated 100.0 42.4 49.0 100.0 29.5 37.5 100.0 37.2 40.2 100.0 47.2 51.4 100.0 43.3 44.8 100.0 35.0 37.7 100.0 39.1 43.4 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.5 5.0 5.0 5.1 5.0 5.1 95.0 37.4 44.0 95.0 24.5 32.5 95.0 32.2 35.2 95.0 42.2 46.4 95.1 38.4 39.5 94.5 30.0 32.7 94. 34. 38. 8.0 11.6 69.7 4.2 5.8 68.8 5.9 7.0 61.6 8.5 11.0 65.0 9.1 11.4 65.5 3.6 5.2 66.1 6.6 8.7 29. 32. 25. 20. 26. 26. 26. 28. 33. 33. 35. 30. 29. 28. 29.0 26.4 27.5 28.8 27.6 29.7 ? 18.9 20.4 15.4 11.8 15.8 19.6 18.7 20.8 24.1 23.8 24.9 21.2 20.4 20.3 17.5 16.3 17.5 19.6 18.3 20.0 ? 10.5 12.0 9.9 8.5 10.9 6.6 7.6 7.4 9.3 9.9 10.5 8.9 8.9 7.8 11.5 10.1 10.0 9.0 9.3 9.8 Table 3 shows the actual pounds of pits, moisture, solids, sugar, etc., in the fresh prunes and the relative amounts of the same con- stituents remaining after sun-drying' and dehydration. All figures in Table 3 refer to 100 pounds of fresh prunes. An examination of these data indicates that : 1. Dehydration results in a greater weight of dried prunes than sun-drying. 2. This increase is in part due to a greater amount of water retained but there is also a greater amount of sugar retained by dehydration. 3. The average amount of sugar remaining after dehydration is approximately the same as in the fresh prunes, while the amount remaining after sun-drying is considerably less. Bull. 404] THE DEHYDRATION OF PRUNES 11 No data are available to show how the sugar was lost, whether through respiration or fermentation, or both. Nevertheless, the data obtained indicate that proper dehydration results in a greater total weight and slightly larger size of prunes than sun-drying. This is of considerable value to growers, not only in the greater total weight of dried prunes sold but in the higher price obtained as a result of larger sizes. Records of relative yields kept by a number of growers support this observation and in many cases the increased return has been sufficient to pay the cost of operating the dehydrater. COMPARATIVE COSTS The fine quality and greater yield and size of dehydrated prunes would probably be of little interest to growers if such gains were counterbalanced by a greater cost for dehydration. Fortunately, however, the construction and operation of dehydraters have attained such efficiency that the advantages of dehydration are obtainable at little or no greater total cost than that of sun-drying in favorable weather, while in unfavorable weather dehydration is often less expensive. Table 4. — Comparative Costs of Sun-Drying and Dehydrating Prunes (Per Fresh Ton) Averages for dehydration Dehydrating 1 Sun-drying 2 Labor (41.6c per hour): Dipping and traying — 1.86 hours $ .75 .73 .89 $ .78 Operating dehydrater — 1.65 hours Scraping trays — 2.19 hours 3.11 Total labor — 5.70 hours $2.37 .95 .69 .11 $3.89 Fuel (5.08c per gallon) : Dipping and drying — 18.7 gallons Power (2.14c per kilowatt hour): Dipping and drying— 32.2 K.W.H .16 .05 Lye (8.5c per pound): Dipping — 1.3 pounds .11 Total operating cost $4.12 $4.21 1 Average of 19 air-blast dehydraters. 2 Average of 11 dry-yards (see Bulletin 388). Operating Costs. — In order to present reliable figures on the cost of operating prune dehydraters, exact measurements of the amount of labor, power, fuel and lye were made during the normal operation of nineteen different air-blast dehydraters. The tests included all 12 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION %SATI0NS IN COSTS OF DEHYDRATING PRUNES PER, FI^ESM TON i dollars. 4s. h **-\ i* ]s Co si given in 63 /.6 8 1 — J0 I III 111 .4 ell A90 B=r.54 ■111 iiiii 16 6 AS* I=.d IIP. /.99 A4 7 J.38 ill 111 57 a.av £1 gl T^g I *.*e U a.V8 £.05 .65 ^7^ Z.8f ~1 /A5* nil /Q7 3.06 I Fig. 2. — Variations in costs of dehydrating prunes. Bull. 404] THE dehydration of prunes 13 commercial makes, all important prune districts and cover the past four years. The plants ranged in daily capacity from 4 to 40 fresh tons, averaging 16.5 tons. The costs cover all steps in the process, beginning with dipping and ending with the dried prunes in storage 6ins. The detailed results of these tests are presented graphically in Figure 2 and summarized in Table 4. Corresponding figures obtained from eleven dry -yards (see Bulletin 388) are also included in Table 4. A comparison of these figures, as presented graphically in Figure 3 shows the average costs of operation for dehydraters and dry-yards to be approximately the same. The cost for power and fuel to operate the dehydrater (average of $1.43 per fresh ton) is more than counterbalanced by the saving in labor (average of $1.52 per fresh ton) aiforded by the dehydrater. Average cost of dehtoijating - U12 per ton -B^ffiSg Labor 5.7 hours - *237 AffiffAfiE COST OF £tTN DlffTNg - *4.21 TOR TON Labor 10.3 hours = *3.&9 Fig. 3. — Comparative costs of sun-drying and dehydrating prunes. As previously reported in Bulletin 337, ''Some Factors of Dehydrater Efficiency," published in November, 1921, the cost of dehydrating prunes in natural draft dehydraters is considerably more than in air-blast dehydraters. Although the natural draft type has no charge for power, the expense for labor and fuel is higher than in air-blast plants. This is further borne out by more recent measure- ments made on three commercially built natural draft dehydraters having a daily capacity of 3 to 4 fresh tons (see Table 5). Table 5. — Cost of Operating Natural Draft Prune Dehydraters* (Per Fresh Ton) Labor, dipping, drying, scraping, 10.0 man hours at 39c $3.90 Fuel, 20.4 gals, at 16c 3.26 Gasoline, for dipper and grader, % gal. at 18c 06 Lye, 1 pound at 8c 08 Total operating cost $7.30 * Average of three dehydraters. Labor Efficiency. — So far as relative costs of drying are concerned, the greatest advantages of the dehydrater lie not alone in the fewer employees required, but in the more efficient use of their labor. After the prunes are dipped and trayed, which is the same as for dehydration, 14 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION prunes for sun-drying are moved a considerable distance into the dry-yard and the trays then spread, one at a time, on the ground. Prunes which, because of large size or unfavorable weather, dry unevenly, must be stirred to promote even drying and minimize the formation of "bloaters" and "slabs." When nearing dryness, the trays are customarily stacked on the ground. When judged sufficiently dry, the trays are stacked on cars and moved to the bin house for unloading. It is also frequently necessary to pick out spoiled or under-dried prunes before the rest can be binned. In case of rain, the labor is further increased because of additional stacking, spreading or sorting. In contrast to these laborious operations, prunes for dehydration are quickly placed in the dehydrater, a car at a time, and when dehydrated are removed and the trays of prunes emptied directly into bins. If the prunes have been green graded and uniformly dried, no sorting or re-drying is customarily required. One experienced operator on the day shift, and one on the night shift, can keep all the operations of dipping, dehydrating and binning under his personal supervision, which is difficult in large sun-dry yards. Since the dehydrater dries the prunes at a steady rate, the workers are stimulated to keep pace with this rate. Such stimulus is less evident in sun-drying. Moreover, workers prefer the shaded, restricted area of the dehydrater to the extended area and hot sun of the dry yard and, as a result, accomplish more work and do it more cheerfully. For these reasons, it has been observed that not only are fewer workers needed for dehydration but that each accomplishes more effective work than in sun-drying. Since the growers' greatest problem during the harvest season is adequacy and efficiency of labor, a dehydrater is of distinct value in solving this problem. Fixed Charges. — A consideration of relative costs would be incom- plete without including comparative fixed charges on the equipment required in the two methods of drying. In calculating fixed charges, it is necessary to include interest on the investment, depreciation and upkeep, taxes and insurance on all land, buildings and equipment used for drying or handling the prunes after harvesting. The fixed charge per ton of prunes dried will be found to vary considerably in both sun-drying and dehydration with the nature of the initial invest- ment in the equipment and with the annual tonnage of fruit dried therewith. In sun-drying, the much greater investment in land and trays often balances the cost of a dehydrater. The area of dry yard is normally estimated at one acre for each 20 acres of orchard, while BULL. 404] THE DEHYDRATION OF PRUNES 15 a dehydrater of equal capacity occupies only about 5 per cent of this area, including the space required for dipping and storing the fruit. While some growers have a piece of land of little or no agricultural value, such as a dry creek bottom, which makes a satisfactory dry yard, most dry yard land has a potential value equal to that of the surrounding orchards. While there is usually no depreciation on such land and upkeep costs can be borne by some annual crop, such as hay, grown each spring, it is only fair to include interest and taxes on the dry yard acreage. Since sun-drying trays are rarely used more than twice a season, while dehydrater trays are usually used at least once every two days, the tray surface required for dehydration is only 10 to 15 per cent of that required for sun-drying. In addition to interest on the com- paratively heavy investment in sun-drying trays, it is customary to allow at least 10 per cent annually for depreciation and upkeep. The nature of the buildings and equipment required for dipping and binning is usually independent of the method of drying, but there is considerably more trackage used in the dry yard. Insurance is often carried on buildings and trays. Taking all these factors into con- sideration, it is customary to figure the annual fixed charges on sun-drying equipment at from 15 to 20 per cent on the total investment, which investment averages $20 per fresh ton of prunes dried per annum. According to figures previously presented in Bulletin 388, the average fixed charge for sun-drying prunes is $3.43 per fresh ton. In dehydration, interest on the entire investment is usually placed at 6 or 7 per cent. Depreciation and upkeep on modern types of fireproof dehydraters is rarely figured at more than 5 per cent, while on trays and other accessory equipment which receive hard usage it is necessary to allow 10 per cent or more. A charge of l 1 /^ P er cen t on the entire investment will normally be adequate to cover all depre- ciation and upkeep. Taxes are variable but rarely exceed 3 per cent on an assessed valuation equal to 50 per cent of the actual value. Many owners of fireproof dehydraters carry no insurance, while some carry insurance only on the wooden buildings, trays, etc., used in connection with the dehydrater. In the case of some of the older inefficient dehydraters built several years ago which have since been either dismantled or remodeled, a heavy charge for obsolescence was incurred. However, the present leading types of commercially built dehydraters have become so standardized and have proved so efficient that it is probably not necessary to include a charge for obsolescence. Adding these charges together, it is found that the total fixed charge on a substantially built fireproof dehydrater, together with 16 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION accessory buildings and equipment, will average 16 to 17 per cent per annum on the investment, which investment averages $22 per fresh ton of prunes dried per annum. The detailed figures on fixed charges in 6 modern air-blast dehydraters given in Table 6 show an average fixed charge of $3.65 per ton for dehydrating. Comparing this with the corresponding Table 6. — Fixed Charges on Prune Dehydraters (Per Fresh Ton) Plant Total investment Season tonnage Interest at 7% Deprecia- tion at 71% Taxes Insurance Total S B $13,000 12,850 6,000 7,000 6,770 19,300 575 524 399 521 201 733 $1.58 1.72 1.05 |li.94 2.46 1.84 $1.69 1.84 1.12 1.01 2.64 1.97 $ .40 .37 .16 .20 .27 .39 $ .18 $3.85 3.93 A 2.33 W 2.15 K 5.37 E .04 4.24 Average $10,820 492 $1.60 $1.71 $ .30 $ .04 $3.65 Fig. 4. — The first Puccinelli Dehydrater built (1921). Note young prune trees in former dry yard. average of $3.43 for sun-drying, it is seen that fixed charges for dehydration are only slightly greater than for sun-drying. Adding the comparative operating costs given in Table 4 to the corresponding fixed charges, it is found that the average total cost of dehydrating is $7.76 per fresh ton as compared with $7.64 x for sun-drying, which difference is not significant. Using the even amount of $8 a fresh ton and an average drying ratio of 2.5 to 1, it is evident that the cost of drying prunes is approximately one cent a dry pound. It may be 1 One of the oldest and largest non-profit cooperative dry yards in Santa Clara County makes a charge of $8 a ton for drying and storing prunes. BULL. 404] THE DEHYDRATION OF PRUNES 17 concluded that the average total cost of dehydration is no greater than for sun-drying if based on present prices for complete new equipment in each case. Growers, who already have adequate sun-drying equipment in most cases purchased at considerably less than present prices, may find the investment and therefore the fixed charges on a new dehydrater to be temporarily somewhat greater than on their dry yard. However, many such growers have installed dehydraters, sold most of their trays and made their dry yard land more profitable by planting it to trees (see Fig. 4), feeling that the economic advantages of dehydration are more than sufficient to balance a slightly greater fixed charge. Growers who must provide new or additional drying equipment for young orchards coming into bearing will, with few exceptions, find it advantageous to employ dehydration rather than sun-drying. SUMMARY OF COMPARISONS Summarizing the foregoing comparisons, it can be fairly said that dehydration produces prunes of equal or better quality than the sun- dried, generally results in a greater yield and size of prunes, provides insurance against rain damage losses, and the total cost of operating an efficient dehydrater, including fixed charges, need be no greater than for a dry yard of equal capacity. Consequently, growers who can finance the installation of a dehydrater or who can have their prunes dehydrated at a reasonable custom charge will find it to their financial advantage to adopt this modern method of drying as many growers already have done. PRINCIPLES OF DEHYDRATION Dehydration may be defined as the evaporation of water from substances in a current of air, the temperature, humidity and flow of which are subject to control. The fundamental laws of physical science on which dehydration is based have long been known and used in heating and ventilating engineering. The practical application of these principles to the dehydration of fruits has already been presented in detail in several technical publications (see list of references, page 47. Since very few growers build their own dehydraters, it is not necessary in this bulletin to give all details governing design and construction. However, in order that growers may have the necessary information to intelligently select and operate a dehydrater, the following brief elementary presentation of the principles of dehydration is given. 18 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION HEAT REQUIREMENTS By evaporation of water is meant the change of water from the liquid to the vapor state. To accomplish this change requires the expenditure of a definite amount of heat. In sun-drying, this heat is derived from the sun, while in dehydration it is produced by the combustion of fuel. The unit used to measure heat in dehydration is the British Thermal Unit (hereafter referred to as B.T.U.) which represents the amount of heat absorbed in raising the temperature of one pound of water one degree Fahrenheit. If prunes at a temperature of 60° F. are placed in a dehydrater and dried at an average temperature of 150° F., 90 B.T.U. will be required to raise the temperature of each pound of water in the prunes from 60° to 150° F. The heat required to transform one pound of water from the liquid to the vapor state, at 150° F. is 1010 B.T.U. Consequently, the total amount of heat theoretically required to heat the water in the prunes to the average temperature of the dehydrater and then evaporate that water is 90 plus 1010 or 1100 B.T.U. per pound of water evaporated. The actual amount of heat theoretically required will vary somewhat with the original temperature of the prunes and the average temperature at which the dehydrater is maintained. However, for practical purposes, 1100 B.T.U. per pound of water evaporated may be taken as the basic requirement. Heat must also be provided for other purposes. /The walls and roof of the dehydrater are constantly radiating heat which must be replaced in order to maintain the dehydrater at the desired tempera- ture. The solid matter of the fruit and the trays and cars which carry the fruit enter the dehydrater cold and emerge at the maximum temperature and consequently carry away the heat which they have absorbed. Heat is lost from the smoke stack in order to provide the draft necessary to combustion of the fuel and the radiation of the heat generated. Heat is also lost with the warm exhaust air which removes the water vapor and often through cracks in the building or around doors. None of these losses can be entirely eliminated but all can be minimized by proper construction and operation. The overall fuel efficiency of prune dehydraters has been found to vary consider- ably with the type of construction and with the relative temperatures of the inside and outside air. Numerous tests show that efficiently constructed and operated dehydraters generally give an overall fuel efficiency of 40 per cent or higher, sometimes as high as 50 per cent. Taking 45 per cent efficiency as an average figure, the total heat requirement is 1100-^0.45, or 2444 B.T.U. per pound of water evaporated. BULL. 404] TH E DEHYDRATION OF PRUNES 19 FUELS Of the common sources of heat, oil is the most convenient and economical in California for dehydrating prunes and is therefore almost universally used. Wood and coal necessitate additional expense for handling and unless used as a source of steam heat cannot be easily controlled. Cheap natural gas, while an excellent fuel, is not available in most prune districts. Electricity is a convenient and efficient source of heat but even at a rate as low as one cent per kilowatt hour, the cost of electrical heat for dehydration is prohibitive. HEATING SYSTEMS There are three types of heating systems: direct heat, direct radiation and indirect radiation. Direct heat means the absorption of the heat from the burning fuel by the air used to dry the prunes, without the intervention of furnace walls or flues. The hot gases from the combustion of oil or gas are drawn into and mixed with the main air stream in such proportion as to give the resultant mixture the desired temperature. The advantages of this system are : 1. Reduction in fuel consumption through elimination of stack losses. 2. Lower cost of installation. 3. Reduced depreciation and upkeep charges as compared with radiation systems. Common disadvantages of this system have been : 1. The use of higher priced partially refined oils to insure complete combustion. 2. The potential danger of contaminating the fruit with unburned fuel or soot. While there have been a few cases of fruit being injured by this method, many thousands of tons of prunes have been successfully dehydrated by direct heat. Observations show no consistent differ- ence in quality of fruit and fuel charges between the direct heat and direct radiation systems at present in use, but indications point to lower upkeep costs for the direct heat system. Direct radiation means the radiation of heat through the metal walls of furnaces and flues directly into the air used in drying. This is the system in most common use for fruit dehydraters. If properly constructed, this system prevents possible contamination of fruit b}^ unburned fuel and gives relatively high fuel efficiency. Its sole dis- advantage has been the occasional replacement of burnt out flues, 20 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION especially those nearest the high temperature of the furnace. By using flues of such length and radiating surface that the stack tempera- ture is as low as is consistent with adequate draft, furnace efficiencies of 70 to 80 per cent are possible with this system. Indirect radiation means that the heat from the fuel is transferred to the drying air through the intermediate agency of a steam boiler and steam heating coils. Possible advantages of this system are that any kind of fuel can be used and the temperature of the drying air automatically controlled by a thermostatic steam valve. The dis- advantages of the steam heating system are its relatively greater first cost and the fact that it can not at best give an air heating efficiency of over 50 to 60 per cent. Thermal Efficiency. — The over all thermal efficiency can be easily calculated by use of the following formula: Pounds water evaporated X 1100 B.T.U. w _._ «,*»,««. Gallons oil consumed X 142,000 B.T.U. X 10 ° = % fuel efflcienCy - The pounds of water evaporated during a given time, usually 24 hours, is determined from the difference in weight between the fresh fruit entering and the dried fruit leaving the dehydrater. The number of gallons of oil consumed during the same time can be calculated from the following formulae : Vertical Cylindrical Tanks — 3.1416 X % diam. X % diam. X drop in level, (all in inches -f- 231 = gallons) . Rectangular Tanks — Inside width X inside length X drop in level, (all in inches -=- 231 = gallons) . AIR FLOW REQUIREMENTS Air performs two essential functions in a dehydrater. First, it conducts the heat from the air heating system to the fruit which is to be dried and, second, it absorbs and removes the water vapor which that heat has evaporated from the fruit. It is obvious, therefore, that the capacity of any dehydrater to dry prunes depends not alone on the temperature of the air but more particularly on the volume of heated air which is brought into contact with the prunes. More dehydraters have failed to give the expected capacity or efficiency because of inadequate air flow than from all other causes combined. For accuracy in dehydrater calculations air must be considered as a mixture of dry air and water vapor, both of which contain heat. Bull. 404] THE dehydration of prunes 21 When this air mixture comes in contact with moist prunes, a drop in the temperature of the air takes place, indicating that part of the heat in the air mixture has been used in changing water in the prunes from the liquid to the vapor state. Consequently, the amount of evaporation which takes place depends, first, on the drop in tempera- ture of the air, and, second, on the volume of that air passing over the prunes in a given time. For example, let it be assumed that air enters the drying chamber at a temperature of 165° F., and a relative humidity of 25 per cent. By reference to tables of composition, one cubic foot of this air is found to contain .0578 pounds of dry air and .0036 pounds of water vapor or a total of .0614 pounds of air mixture per cubic foot. The amount of heat which this air can give up in dropping one degree is determined by multiplying the pounds of dry air and of water vapor by their respective specific heats 1 and adding these two results together. This gives .0155 B.T.U. as the amount of heat given up by one cubic foot of this air mixture in dropping 1° F. Now if the total number of B.T.U. required per minute to evaporate the water from a given weight of prunes in a given number of hours, together with the heat lost from the drying chamber by radiation, air discharge, etc., be known or estimated, it is possible to calculate the cubic feet of air required by multiplying .0155 B.T.U. by the number of degrees the air drops in temperature in passing through the dehydrater and dividing this result into the number of B.T.U. required per minute for evaporation. This gives the cubic feet of air per minute which must enter the drying chamber. For example, if the above air mixture in passing through the drying chamber has a temperature drop of 35°, each cubic foot of air will give up 35 times .0155 or .5425 B.T.U. of heat. Since each pound of water evaporated theoretically requires 1100 B.T.U. the volume of air which must pass through the drying chamber to evaporate one pound of water per minute would be 1100 divided by .5425 or 2028 cubic feet per minute. However, a part of the heat given up by this air will not be available for evaporation because it will be lost by radiation or leaks or in heating trays, cars, etc. Consequently, an additional amount of air must be provided to compensate for these heat losses. Assuming the air to have an actual evaporating efficiency of 75 per cent, 2704 cubic feet of air per minute will be required for each pound of water to be evaporated per minute. i Specific Beat is the ratio between the heat required to raise (or, conversely, given off by cooling) one pound of any substance 1° F., and that required to raise one pound of water 1° F., the specific heat of water being considered as 1. The specific heat of dry air and of water vapor are taken as .24 and .45 respectively. 22 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION METHODS OF SECURING AIR FLOW Natural draft is the oldest and simplest method of inducing air flow. It depends on the fact that when air is heated it becomes lighter because of expansion and tends to rise, thereby creating an upward current of air. The advantage of this system is that it does not require the use of power driven fans, the energy required for air movement being furnished by the burning of additional fuel. The disadvan- tages are : 1. Inadequate volume and velocity of air for all but small units. 2. Lack of control of air distribution causing uneven drying. 3. Difficulty of securing quick and exact control of temperature and humidity. The natural draft system reached its most extensive development in the ''Oregon Tunnel" dryers, but in recent years many of these have been remodeled to the recirculating fan system in order to obtain the increased capacity and economy of the latter. Natural draft dryers are now only used by growers with small tonnages. Their total cost of operation, including fixed charges, is usually much greater than that of fan equipped dehydraters. Air blast dehydraters are those in which the air flow is produced by power driven fans. The air flow is mainly in a horizontal direction over the trays. The advantages of this system are that it permits exact control of the temperature, humidity, volume and distribution of the air. Although requiring a considerable investment in one or more fans and motors, this extra cost is well repaid by more rapid and uniform drying and greater economy. The fans used in dehydraters are of two main types: disc and propeller fans which have 8 to 14 blades about a central hub blow air in a direction parallel to the fan shaft; centrifugal or multivane fans which have 48 or more short blades on a wheel which revolves within a housing and blows air at right angles to the fan shaft. The steel plate fan is similar but has fewer and larger blades. Dehydraters of small capacity or those with a series of fans for supplying air to several sections have used disc or propeller fans satisfactorily. In large dehydraters where a large volume of air must be circulated through a long drying chamber, the multivane fan is the most efficient because of its ability to produce an adequate flow of air against relatively high frictional resistance. Position of Fans. — This is determined by convenience in the par- ticular design of dehydrater adopted. The position of the fan is also determined by the preference of the designer for either blowing the Bull. 404] THE dehydration of prunes 23 air through the drying chamber under slight pressure or drawing the air through the drying chamber under slightly reduced pressure. There are two usual positions in which the fan is placed: 1. Drawing direct from the drying chamber and returning the recirculated air by blowing it through the heating chamber. 2. Blowing directly through the drying chamber and returning the recirculated air by drawing it through the heating chamber. The first system tends to draw cold outside air into the drying chamber through leaks and has also generally resulted in less even air distribution over the trays. The second system keeps the drying chamber under slight pressure so that any air leakage will be outward but may draw flue gases from leaks in the heating system. A modifica- tion of the first system by placing the fan in the heating chamber and separating the fan intake from the drying chamber by dampers tends to eliminate the above objections by keeping both heating and drying chambers under slight pressure. An advantage of the second system lies in the fact that when the air is drawn from the heating chamber and blown into the drying chamber the churning action of the fan insures a uniform air temperature, and when the air enters the drying chamber under pressure, it is possible to distribute it uniformly over the trays of prunes by proper placing of the fan discharge and, if necessary, by the judicious use of baffles. The air pressure produced by a fan is divided into velocity pressure and static pressure, the sum of which equals the total pressure. Static pressure may be defined as the pressure required to overcome the frictional resistance to the passage of air between trays or through ducts, and may be measured in inches of water column by an instru- ment known as a pitot tube. The static pressure in dehydraters is usually found to be between 1 and 2 inches. In good average practice the static pressure is about 1.5 inches. Long, narrow and crooked passages or obstructions increase static pressure and consequently decrease the volume of air passing. In order that a fan operating at a given speed may deliver the greatest possible volume of air, it is essential that all air passages used for heating, drying and recirculating be as short and straight as possible. No point in the entire system should have a free cross sectional area less than that between the trays and preferably large enough to avoid an air velocity greater than 1000 lineal feet per minute. The aggregate area of the air passages between the ends of the trays should be about 60 per cent of the total cross sectional area of the drying chamber. Air Distribution. — In order to secure the maximum drying efficiency of the air, the stacks of trays should as nearly as possible 24 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION fill the entire cross section area of the drying chamber, leaving barely sufficient clearance for movement of cars as illustrated in Fig. 5. Flexible baffles, commonly made of discarded canvas belting or hose, are advantageously used to prevent excessive flow of air over the top trays, along the walls and under the cars. In short, every effort should be made to cause all the air to flow between the trays. The free air space between the ends of the trays usually varies from one to two inches in height, preferably nearly two inches. If these air spaces are so narrow as to materially restrict the air flow, drying will be slow and uneven. On the other hand, if unnecessarily wide, more rapid drying will not compensate for the decreased holding capacity of the dehydrater. Fig. 5. — Placing a car of prunes in a tunnel dehydrater. (Note how closely the stacks of trays fit the tunnel.) Air Measurement. — A simple method of measuring the air flow in dehydraters is by the use of an anemometer which shows the distance in feet which air moves. By noting the distance air moves in one minute, the velocity is obtained and by multiplying this by the area of the opening measured, the volume of air in cubic feet per minute is obtained. All modern air-blast dehydraters show an average air velocity between trays of 500 lineal feet per minute or over, gen- erally 600 to 700. Velocities below 500 feet are generally associated with slow and uneven drying while velocities in excess of 1000 feet are not economically practicable. Power for Fans. — Fans, and burners, require about 1% horse power for each fresh ton of prunes dried per 24 hours. Electricity is the most convenient and economical source of power. A metal link chain is an efficient fan drive although more expensive than endless water proof leather belts which have also given excellent results in dehydraters. Eubber fabric belts have been found less efficient and shorter lived in dehydrater work. BULL. 404] THE DEHYDRATION OF PRUNES 25 HUMIDITY CONSIDERATIONS Definition of Humidity. — The water vapor present in air is com- monly expressed as relative humidity, or the percentage of the weight of water vapor in a given space to the weight of water vapor which the same space at the same temperature could hold if it were saturated with moisture. Saturated air has a relative humidity of 100 per cent and absolutely dry air of per cent. Measurement of Humidity. — Relative humidity is determined by the comparative readings of two thermometers, one having a dry bulb and the other having its bulb closely covered by a clean wick kept moist by distilled water. These thermometers are placed together in the direct air flow of the drying chamber, usually at the point of highest temperature. The dry bulb thermometer indicates the tem- perature and the reading of the wet bulb thermometer will be lower because of the cooling effect of the evaporation of moisture from the moist wick surrounding the bulb. The lower the moisture content of the air, the more rapid will be evaporation and consequently the lower the reading of the wet bulb thermometer. Use has been made of this simple principle in preparing charts such as the one in Fig. 6, giving the relative humidity of the air for any combination of wet and dry bulb temperatures. Effect of Temperature on Humidity. — The moisture holding capacity of air approximately doubles for every 27° rise in tempera- ture, or, in other words, the relative humidity of air is halved when its temperature is raised 27°. For example, if a given weight of air outside a dehydrater had a temperature of 57° and a relative humidity of 100 per cent, as might be the case on a rainy or foggy day, and if this air were drawn into the dehydrater and heated to 165° F., it would have a relative humidity of only about 6 per cent, or, in other words, the same weight of air could hold 16 times as much water as it originally held. When air is considered in terms of volume, instead of weight, these figures are modified because of the expansion of the air on heating but they serve as a simple explanation of why dehy- draters continue drying independently of the humidity of the external air. Recirculation. — If a dehydrater were hermetically sealed to prevent interchange of air with the outside and the air within continuously recirculated and reheated it would soon reach saturation and drying would cease. If, on the other hand, air were drawn into a dehydrater, heated and then discharged after passing over the fruit only once, an excessive amount of heat would be wasted with the exhaust air. fiuui 8 VcVipeW E-ATUR& § Fig. 6. — Chart for determining humidity from wet and dry bulb temperatures. (Drawn by G. B. Eidley.) BULL. 404] T he DEHYDRATION OF PRUNES 27 Since the first of these conditions is impossible for drying and the second wasteful of fuel, it is obvious that between the two is a condition which permits comparatively rapid drying and at the same time gives optimum fuel efficiency. The volume of air required for absorption of the moisture evaporated from the fruit is on the average only y 7 to % as much as the volume of air necessary to convey the heat required for evaporation. Recirculation means the re-use of a portion of the warm exhaust air to which is added sufficient fresh air to make the resultant mixture after reheating contain no more water vapor than it did before passing over the fruit previously. If this be done, the humidity of the air entering the drying chamber can be maintained at any desired per cent and drying will progress steadily with the minimum loss of heat in the exhaust air. Because of their length and complexity, calculations on control of recirculation are omitted from this bulletin but are available elsewhere (see reference No. 2, page 47). A little experience will enable any operator to so adjust the exhaust air and fresh air intake dampers of his dehydrater as to maintain the relative humidity of the air within the dehydrater at that per cent which experience shows him to give the most rapid drying consistent with reasonable fuel economy. Practical experience has shown that partial recirculation decreases fuel consumption at least 50 per cent without decrease in the rate of drying and all commercially built dehydraters now use this system. CALCULATION OF DEHYDRATER REQUIREMENTS The following typical example of dehydrater requirements is presented as a guide in determining the adequacy of a dehydrater and its essential parts. Let it be assumed that a certain prune grower must have a dehydrater capable of dehydrating 10 fresh tons in a day of 24 hours in order to accommodate the peak load of a normal harvest and that these prunes will have an average drying ratio of 2.5 to 1. Assuming an average drying time of 24 hours, the dehydrater must have a holding capacity of 20,000 pounds of fresh prunes. If the trays have an average load of 3.5 pounds per square foot, 5714 square feet of tray area must be provided for in the dehydrater. If two stacks of 3' X 8' trays, 25 high, are used to the truck, the capacity should be 5 such trucks, if 3' X 3' trays, 13 trucks. To dehydrate 20,000 pounds of prunes with a drying ratio of 2.5 to 1 in 24 hours, necessitates the evaporation of 12,000 pounds of water or 500 pounds an hour. Assuming an overall fuel efficiency of 45 per cent, 2444 B.T.U. per lb. of water evaporated will be required (see 28 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION page 18) or 1,222,000 B.T.U. per hour. Assuming a heat value of 142,000 B.T.U. for a gallon of oil, 8.6 gallons of oil must be burned. Consequently a burner of not less than 9 gallons capacity an hour should be provided. For capacity of oil burners reference should be had to the catalogs of firms manufacturing oil burners for dehydraters. If the direct radiation system be used, it is essential that the total surface area of furnace and flues be adequate to radiate the required heat. The surface required varies greatly with the nature of the furnace and flues and no exact figure can be given. With the common steel radiating furnace and flues, from 400 to 500 square feet is usual for a plant of this size. An evaporation of 500 pounds of water an hour is equal to 8.33 pounds a minute. By reference to the figures on page 21 it can be seen that if the air has a temperature drop of 35° and an evaporating efficiency of 75 per cent in passing through the drying chamber, each pound of water to be evaporated each minute requires the passage of 2704 cubic feet of air. Consequently, an evaporation of 8.33 pounds of water (one gallon) a minute will require 22,532 cubic feet of air a minute. If this entire volume of air is to be delivered by a single fan, a multivane fan will be best. In some types of dehydraters, a number of smaller fans, usually of the disc type, are used to give the total air flow required. No portion of any air passage should have a cross section area less than that of the fan discharge connected to it. If the total air flow is delivered by a single fan, no part of the entire air system should have an area of less than 22.5 square feet, except at the fan. The total free area between the ends of the trays (see page 24) should be about 30 square feet, which will give an air velocity of 750 lineal feet a minute between trays. Most fan manufacturers furnish tables of performance which show for each size of fan the volume of air delivered at a given speed and a given static pressure and the horsepower required. By reference to such tables and to price lists, selection can be made of the fan which will most economically deliver the volume of air required. The static pressure in dehydraters usually varies from 1 to 2 inches. If the dehydrater is constructed with attention to the principles regarding the free area and construction of air passages, an average static pressure of 1% inches is ordinarily used in determining fan capacities. The foregoing paragraphs have given briefly the most important factors concerned in the construction of any dehydrater, namely, its holding capacity for prunes and the amount of heat and air required to dry the prunes in a given time. The figures used in the example BULL. 404] THE DEHYDRATION OF PRUNES 29 are purposely conservative and many dehydraters show greater effi- ciency. For instance, if the drying ratio is less than 2% : 1 or the fuel efficiency more than 45 per cent, correspondingly less heat and air will be required to dry the same weight of prunes in the same time. SELECTION OF A DEHYDRATER The following three courses are open to growers who wish to install a dehydrater: 1. Purchase of a standard commercially built dehydrater. 2. Construction of a dehydrater from plans furnished by a dehy- drater manufacturer or engineer. 3. Construction of a dehydrater from an original design. Most growers find it simpler, safer and just as inexpensive to buy a standard commercial dehydrater as to design or build their own plant. The few growers who have been successful in building their own dehydraters are usually men with previous experience in con- struction or engineering work. Without some technical training or experience in such matters, it is inadvisable for a grower to attempt the construction of a dehydrater. Some growers have the impression that dehydrater manufacturers charge excessive profits but such is not generally the case. The quantity purchase of materials and equipment at low prices and the employment of experienced mechanics enables large dehydrater manufacturers to sell plants at a profit for a price little, if at all, greater than that for which a grower could build a single plant. There have been several instances of successful dehydraters built by growers from plans obtained from dehydration engineers. If such plans have been demonstrated to give an efficient dehydrater, some saving can often be effected by this scheme. In this connection, it should be mentioned that the University of California does not fur- nish plans for dehydraters. Descriptions of the leading types of commercial dehydraters some of which are illustrated in Figs. 7, 8, 9 and 10 are not included because they are preferably obtained from pamphlets issued by the manufac- turers or better by inspecting the dehydraters. Growers are strongly urged to confine their selection of a dehydrater to types which have already been built and operated so as to demonstrate their capacity and efficiency. Persons wishing to manufacture and sell dehydraters should not expect growers to invest in a machine until the prospective manufacturer has demonstrated his claims by operating such a dehy- drater through a prune season. 30 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION DEHYDRATER MANUFACTURERS The following list includes the names of all persons or firms that sold dehydraters for primes during 1925. Persons formerly in this business but not active therein at present or persons seeking to sell dehydraters but who have not yet succeeded are not included : Chapman Dehydrater Company, Bare Building, Modesto. W. W. Cozzens, 10 Broadway, San Jose. Knipschild Dehydrater Company, St. Helena. Oliver Dehydrater Company, Lincoln Avenue and Moorpark St., San Jose. G. B. Ridley, 255 California Street, San Francisco. Progressive Dehydrater Company, 340 Seventh St., San Francisco. Puccinelli Dehydrater Company, Los Gatos. Fig. 7. — Kemoving a tray of prunes from an Oliver Natural Draft Dehydrater. PATENT SITUATION Any grower designing his own dehydrater should do two things before beginning construction, first, have the plans and specifications checked by a competent authority to ascertain if they will accomplish the desired result and, second, ascertain if the design infringes any dehydrater patent. While there is no basic patent covering the dehydration of prunes, most types of dehydraters in use are protected in whole, or in part, by patents. These patents are of the type com- 3 I u 5 3 p p CD TO fc/O o (-< to 1=1 to Bull. 404] THE DEHYDRATION OF PRUNES 33 3 I a go* CfQ •-* P 5 3 p o cs CD a is- p B p S3* 34 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION monly referred to as "construction patents" and care should be taken to avoid legal entanglements occasioned by unauthorized duplication of patented features. One should not be misled by the common misconception that a grower building a dehydrater exclusively for his own use is exempt from patent infringement claims. So far no damage suits have been brought for infringement of dehydrater patents and consequently the situation remains uncertain pending clarification through judicial decision. Dehydrater Patents.- — Of the many hundreds of patents granted on dehydrating equipment, the following have been selected as of the greatest present importance in avoiding possible infringements when new types of dehydraters are built and used. Number Filed Granted Issued to 1,461,224 Dec. 13, 1919 July 10, 1923 J. W. Pearson (associate of Ridley) G. B. 1,413,125 Jan. 15, 1920 Apr. 18, 1922 Claude Rees (Progressive drater Co.) Dehy- 1,404,369 May 1, 1920 Jan. 24, 1922 F. C. Chapman 1,464,338 June 29, 1921 Aug. 7, 1923 R. L. Puccinelli 1,528,223 Feb. 21, 1922 Mar. 3, 1925 C. C. Moore 1,532,303 Dec. 4, 1922 Apr. 7, 1925 W. W. Cozzens 1,543,947 Aug. 13, 1923 June 30, 1925 C. C. Moore A copy of any patent can be obtained by sending ten cents in coin to the U. S. Commissioner of Patents, Washington, D.C. A patent attorney should be consulted on questions concerning infringement. SOME CONSTRUCTION PRINCIPLES While it is not the purpose of this bulletin to enter into a discussion of the relative merits of the several types of dehydraters, the following suggestions will be found helpful : In order to minimize fixed charges the capacity of the dehydrater should not be larger than is necessary to dry the expected tonnage. First cost is a most important factor in fixed charges and it is often sound economy to sacrifice a certain amount in operating efficiency in order to obtain a greater saving on the investment and fixed charges. Custom Dehydration. — Many dehydrater owners have engaged in custom-drying prunes for neighbors. The increased seasonal tonnage reduces the fixed charges per ton and results in a profit to the operator. The custom charge for dipping and dehydrating prunes has usually varied from $10 to $15 a fresh ton, averaging $12.50. Comparing this BULL. 404] TIIE DEHYDRATION OF PRUNES 35 figure with an average total cost of dehydrating of $8 a ton it can be seen that custom-drying is a profitable venture during the few weeks prunes are available for drying, provided the dehydrater is operated at normal capacity. Community Dehydraters. — Growers with small acreages and limited finances will do well to consider the advantages of a community dehydrater. In general, the larger the dehydrater the lower the investment and the less the total operating costs per ton will be. However, in order for a community dehydrater to be successful, the owners should have the true cooperative spirit and the management, both financial and operating, should be carefully planned to accom- modate the crops of the several owners in an efficient, impartial manner. Fire-proof construction is preferred for dehydraters not only because it precludes the loss of the plant by fire which is especially like]}* to occur when most needed during the prune harvest, but it eliminates or very greatly reduces the high insurance premiums on dehydraters built of combustible materials such as wood. Hollow walls, usually built of tile or concrete blocks, are most common, the dead air space serving to reduce heat radiation losses. Double walls of wood, sheet metal, asbestos or other building boards, have also been successfully used and are less expensive to erect. Solid concrete is no longer used much because it is expensive construction and does not retain heat so well as hollow walls. In any case, the. construction should be compact and tight to prevent leakage of heated air. The doors should be especially substantial and tight fitting. As explained on pages 27-29 the vital parts of the dehydrater such as oil burner, furnace, flues, fan, motor, etc., should be of adequate size, strongly constructed and firmly mounted. Accurate wet and dry bulb thermometers should be provided. Recording thermometers, while comparatively expensive, are valuable in furnishing a permanent record of the temperature of the dehydrater at all times, both wet and dry bulbs if desired. The charts of such thermometers are mounted once a day in a locked case and thereby serve as a check on the operator when the owner is absent. Cars and Trays. — The minimum number of cars and trays required for dipping and drying is 50 per cent in excess of the holding capacity of the dehydrater. Experienced operators state that 100 per cent excess is required to obtain maximum flexibility and efficiency. Solid bottom field trays, with sides or ends reconstructed so as to permit adequate air flow between the trays when stacked, are commonly used. However, if new trays are to be provided special dehydrater trays with slat bottoms will generally give slightly more rapid and 36 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION more even drying than solid bottom trays. Screen trays are more expensive and after being used for some time, the sagging of the screen causes uneven drying. For ease in handling the heavy loads of trays, the cars should be equipped with roller bearings and in large dehydraters a winch and cable is necessary for moving the cars through the dehydrater. Fig. 11. — Suggested arrangement for a prune dehydrater. ARRANGEMENT OF EQUIPMENT Since most of the labor in dehydration is required before and after drying, rather than during drying, it is obvious that careful consideration of these operations will result in maximum efficiency and minimum cost of labor. The ideal arrangement of the entire plant is that in which the prunes move from the receiving platform to the storage bins in a continuous, unimpeded circuit by the shortest practical route, the trucks of emptied trays being conveniently returned to the loading point. The compactness of the plant is also of importance in securing a neat appearance and economy of ground space as well as a saving in the construction of roof area, tracks, etc. It is impossible to present a plan which will exactly fit all cases, but the plan presented in Fig. 11 will fit most installations and is suscep- tible of adaptation to any dehydrater now in use. The main features of this plan are: BULL. 404] THE DEHYDRATION OF PRUNES 37 1. All the operations of receiving, dipping, grading, loading and unloading trays are concentrated in one location so as to always be under the direct observation of the person in charge. 2. The path of the prunes is such that they constantly move forward without retracing of routes, thereby preventing interference of the cars of fresh fruit with those holding dried fruit. 3. The emptied trays are available for reloading at a point close to the dipper discharge, thereby avoiding extra handling of empty cars and trays. OPERATION OF DEHYDRATERS The methods of harvesting, dipping and traying prunes are pri- marily the same whether the prunes are to be sun-dried or dehydrated. These have already been described in Bulletin 388, "The Principles and Practice of Sun-Drying Fruits, ' ' and need not be repeated here. The operation of dehydraters is best learned by experience, supple- mented by visits to efficiently operated plants. No one system will fit all types of dehydraters or all varieties of prunes. However, in order that operators may be guided in the right direction, the following principles of operating prune dehydraters are presented : dipping Lye dipping of prunes is as essential in dehydration as in sun drying and should be followed by rinsing in clear water, preferably with sprays. The washing off of the waxy bloom, as well as dirt, and the checking of the skin permits the prunes to be dehydrated more rapidly and evenly than if undipped and gives clean fruit. If lye dipping is property controlled the use of pricker boards is of doubtful value except on very thick skinned prunes, as "bloaters" rarely occur in dehydration. For all but the smallest plants, rotary drum dippers are preferred because of their continuous operation and economical use of labor. Operators of some natural draft dryers report no difference in the drying time of dipped and undipped prunes. Investigations have shown this to be due to the limited ability of such dryers to evaporate water, which enabled undipped prunes to give up their moisture as rapidly as the dryer could remove it. This is not the case in air-blast dehydraters. Over-dipping should be avoided because it causes the prunes to bleed and drip, giving sticky fruit and trays. Tender skinned varieties, such as the Imperial, are often dipped in plain hot water to prevent excessive cracking of the skins. 38 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION GREEN GRADING It is obvious tha,t the more even is the size of fresh prunes on a given car of trays, the more evenly dried the prunes will be when removed from the dehydrater. It has sometimes been claimed that conditions in a dehydrater, especially humidity, could be so controlled as to cause all prunes to dry evenly to the same moisture content regardless of their size or composition. This is not true because when a prune is placed in a dehydrater it continues to give up moisture to the air until its remaining moisture content comes into equilibrium with the moisture content of that air. Unfortunately, the final mois- ture content of a prune which would reach equilibrium with an air humidity which could be economically maintained in a dehydrater is much below the moisture content of the prune when considered suffi- ciently dry to be removed from the dehydrater. Therefore, if all prunes were allowed to reach this equilibrium, an unnecessary loss in weight of dried fruit would result. Consequently, if large or plump prunes are placed on the same car of trays with small or partially dried prunes, it is impossible for all such prunes to have the same moisture content when removed from the dehydrater. In order to minimize this inequality and to obtain as uniform a moisture content in the dried product as possible, it is advisable to grade the fresh prunes into two or more sizes. In addition to promoting even dryness, green grading has another important advantage in increasing the drying capacity of a dehydrater. For example, let it be assumed that a certain dehydrater has an average drying capacity of 10 cars of ungraded fresh prunes in 24 hours or a total of 240 car hours. Now if these prunes were green graded into half "number ones" and half "number twos," the dehydrater could dry these prunes at the rate of 5 cars of "ones" in 24 hours and 5 cars of "twos" in say 20 hours, or a total of 220 car hours. In other words, the average drying time would be reduced from 24 to 22 hours as a result of the faster drying of the "twos" separated by green grading. Therefore, the daily capacity of the dehydrater would be increased by 8.33 per cent or 1666 pounds of fresh prunes, nearly one car load. This advantage may be partially counterbalanced if it is found impos- sible to safely spread as many pounds of "twos" as of "ones" on a tray. In plants having more than one unit or track, the different sizes of prunes are preferably dried in separate lines but even in single unit plants it is possible to realize the advantages of green grading by proper judgment in entering the cars of "twos," usually behind the "ones," In the latter case, often two cars, one each of BULL. 404] THE DEHYDRATION OF PRUNES 39 "ones" and "twos" can be removed as dry at the same time, making room for two cars of fresh frnit instead of only one. The same principle applies to natural draft dehydraters also. Taking all factors into consideration, observation indicates the desirability of green grading in all but the smallest plants. traying For continuous spreading of dipped and graded prunes and mini- mum handling of trays, the diagonal discharge illustrated in Fig. 11. has given satisfaction. A continuous stream of empty trays on a roller conveyor is passed under the ends of the grader, the diagonal discharge being so regulated that the prunes are evenly distributed over the entire tray with little hand spreading. To facilitate removal of the dried prunes from the trays without injury to either fruit or trays caused by sticking, it is necessary to keep the trays clean. It is often necessary to wash them several times in the season. The filled trays are stacked on trucks standing on a track close at hand. Where two grades are being loaded separately, greater flexi- bility in handling will be obtained by using two parallel loading tracks. One of these may run under the grader or tray conveyor because of the low bed of the empty dehydrater trucks, the trays from the emptied truck having been passed over the conveyor for loading the car ahead. The conveyor for each size of prunes is supplied with trays from the corresponding track. In most plants, dipping is done only on the day shift while dehydrating is carried on continuously. In order that the dehydrater may be supplied with prunes all night it is customary to have sufficient loaded trucks by evening to keep the dehydrater filled until the next morning. For these trucks, track space equivalent to over half the capacity of the plant must be provided. Stackers. — In order that the high stacks of trays will pass through a close fitting drying chamber without being broken or knocked off or becoming jammed so as to impede the movement of cars, it is important that the stacks be vertical and centered on the car. Guide posts erected along side the loading tracks help materially in securing a proper stack without particular attention on the part of the men doing the stacking. Automatic stackers are in use at several large dehydraters. These machines consist of a steel frame work supporting two sets of endless chains operating on sprocket wheels and turned by a small motor. The chains are provided with pairs of flexible metal fingers at regular 40 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION intervals which, when a loaded tray is rolled into the stacker from the dipper, pick up the tray, carry it up, over and deposit it down on the car being filled. By means of angle iron guides, a perfect stack is obtained without attention, except to periodically replace loaded with empty cars. The construction is adaptable to any size or number of trays. Observations have shown that the automatic stacker eliminates two men from a five man dipping and loading crew. Since the fixed charges on a stacker are approximately equal to the wages of one man for the normal prune season, its use is in line with economy. Bumpers. — The relative heights of cars and trays are often suffi- ciently irregular so that the free air spaces between trays on adjacent cars do not coincide, in which case the air flow between trays may be impeded. Where such a condition exists, it is necessary to place bumpers on both ends of each car so as to separate the stacks of trays by two to four inches and thereby permit unimpeded air flow. TEMPERATURE \ems of Dehydration. — There are four possible systems by which dehydraters may be operated with respect to temperature. 1. Counter current, in which the fruit enters at one end of the dehydrater at a relatively low temperature and is advanced inter- mittently to the other end for finishing at the maximum temperature. 2. Parallel current, opposite of counter current, the fruit entering at the highest temperature and finishing at the lowest. 3. Combination system, in which the maximum temperature is maintained at the center of the dehydrater and the fruit enters at one end at a relatively low temperature, passes through the highest temperature, while still only partially dried and finishes at the other end at a lower temperature. 4. Constant temperature, in which the fruit is subject to a constant temperature throughout the drying period. The counter current system is the one most commonly used on prunes in California, because nearly all air-blast dehydraters are of the tunnel type, or a long drying chamber through which the cars pass in a continuous stream while the moisture is evaporated by a current of heated air passing in the opposite direction. In the dehydraters designed by Puccinelli, Ridley, Knipschild and others, the air current passes straight through the drying chamber while in the Progressive dehydrater it follows a helical or "corkscrew" motion. BULL. 404] TII E DEHYDRATION OF PRUNES 41 The combination system is employed in the Chapman dehydrater, which is a modification of the tunnel type, with the heated air entering at the center, dividing and passing equally toward both ends. The Cozzens dehydrater is the nearest example to a constant temperature dehydrater. The cars enter through doors along the side of the tunnel and do not move progressively through the tunnel. However, they are removed from one section to another nearer the hot end. This is advantageous when two or more kinds of fruit are being dehydrated simultaneously, for example, peaches and prunes. The parallel current system has not proved satisfactory for prunes and is not used. Critical Temperature. — It is a well known fact that fruit sugars, especially levulose, will gradually caramelize and suffer a loss in weight if subjected to temperatures above 160° F., the higher the temperature the more rapid being the loss. However, the sugar in prunes will not be affected by temperatures considerably in excess of 160° F. so long as the prunes are continuing to give up moisture freely. The tempera- ture of a prune from which water is being evaporated in a dehydrater approaches that of a wet bulb thermometer and is considerably lower than the dry bulb temperature of the surrounding air. It is for this reason that partially dried prunes can be subjected to temperatures considerably above 160° F., without loss in sugar, provided none of the prunes are already so nearly dry that they will be injured by such higher temperatures. When a prune approaches such a reduced moisture content that drying becomes relatively slow, its temperature will approach that of the dry bulb thermometer and it is at this point that prunes should be removed from the dehydrator in order to prevent further drying and possible injury. For these reasons, it has been found unsafe to finish prunes at a temperature above 165° F., and this temperature has therefore been adopted as a standard. Temperatures above 165° F. have been occasionally used without apparent injury to prunes, although it is possible that there was a small undetermined loss in sugar in such cases. While temperatures below 165° F. may produce equally good dried prunes, their use is generally inadvisable because the slower drying results in decreased capacity and a greater cost of operation. The drying of plump prunes should not be commenced at high temperature because the sudden expansion of the prunes will cause them to split and bleed, resulting in a loss in weight and sticky fruit and trays. Furthermore, the desired darkening of the skin is arrested at high temperatures and prunes dried too rapidly at high temperatures 42 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION have been observed to develop a reddish brown rather than a black skin. For these reasons it is considered good practice to enter the prunes at the cooler end of the dehydrater, usually at 120° F. to 140° F., and move them progressively toward the highest temperature for finishiug. HUMIDITY The rate of evaporation from a free water surface decreases with an increase in the relative humidity of the air. However, the cellular structure and syrupy nature of fruit tissues retard evaporation, so that under no condition does the rate of evaporation equal that from a free water surface. When conditions are such that surface evaporation from the tissues exceeds the rate of moisture diffusion to the surface, the surface becomes dry and hard and tends to retard drying. This condition, known as case hardening, can be overcome by reducing the temperature of the air or by increasing the humidity. The maximum rate of drying is attained by using the highest temperature which will not injure the prunes and sufficient humidity to minimize retarded drying caused by case hardening. The humidity at the air-outlet end of the drier should not greatly exceed 65 per cent. In driers employing recirculation the conditions of temperature and humidity may be largely controlled by varying the recirculation. Some operators, observing the effect of increased humidity in reducing case hardening, increased the humidity of the air in the dehydrater to as high as 35-40 per cent at a temperature of 165° F. When such a high humidity could not be reached by recirculation alone, they increased the humidity further by running water into the heating chamber. While such high humidity very largely elimi- nated case hardening it also reduced the moisture absorbing capacity of the air, especially at the colder end of long dehydraters. By going to such an extreme, the drying time of the prunes was increased, the capacity of the dehydrater reduced and the cost of operation increased. Case hardening is only* a temporary condition during drying, not necessarily associated with injury to quality and since it disappears entirely during the subsequent binning and processing of prunes, it is not a condition to be feared, providing the flesh around the pit is sufficiently dried. In order to obtain exact information on the relation of humidity to the drying time and quality of prunes, a series of carefully con- trolled comparative experiments was conducted. A uniform lot of prunes was divided into three parts and dried separately by the Bull. 404] THE DEHYDRATION OF PRUNES 43 counter current system at percentages of relative humidity of 10, 25 and 40 per cent respectively, referred to the finishing temperature of 165° P. All other variables such as tray load, initial and finishing tem- peratures, air flow and drying ratio were identical for all three lots. As can be seen by reference to Fig. 13, each increase of 15 per cent in the finish humidity caused an increase of two hours in the drying time. The prunes finished at 40 per cent humidity showed no case hardening and considerable stickiness, those at 25 per cent considerable case hardening and slight stickiness and those at 10 per cent severe case hardening and no stickiness. 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