UNIVERSITY OF CALIFORNIA. 
 
 AGRICULTURAL EXPERIMENT STATION. 
 BERKELEY, CAL. 
 
 E. W. HILGARD, Director. BULLETIN No. 114. 
 
 The Causes of "Frogging" and "Bloating" of Prunes. 
 
 DECEMBER, 1896. 
 
Fig. 1. 
 
 Three large prunes showing proper checking of the skin after dipping. 
 Three small prunes subjected to the same treatment, but unchecked. 
 
 Fig. 2. 
 
 Section through the slit of a properly checked prune (enlarged about 20u diameters). 
 A. A.— Ends of the ruptured epidermis. B — Unbroken cells of the Hesh. 
 
THE CAUSES OF "FROGGING" AND "BLOATING" OF FRUNES. 
 
 By P. T. Bioletti. 
 
 During the season of 1895 numerous demands were made on the 
 Agricultural Experiment Station for information concerning the so- 
 called "frogging" and "bloating" of prunes. There was evidently 
 wide diversity of opinion as to the cause, nature, and best methods of 
 avoiding these troubles. Under the direction, therefore, of Prof. E. W. 
 Hilgard, I visited several drying establishments in the Santa Clara 
 Valley, in order to examine into the trouble on the spot. 
 
 As the claim was made that "frogging" and "bloating" were directly 
 attributable to the practice of lye-dipping, or at least that the pricking 
 process prevented these evils, both processes were investigated. As 
 representative of the lye-dipping process, the West Side Drying Estab- 
 lishment was visited at the instance of Mr. S. P. Sanders. From direct 
 personal observation and from the reports of those engaged in drying 
 the prunes, the following facts were obtained: 
 
 The prunes, before drying, pass through a nearly boiling solution of 
 lye, and then through water. A sample of the lye solution taken from 
 the trough as the prunes were passing through, showed 1.6% of alkali. 
 Most of the prunes as they come from the lye are covered with minute 
 cracks or "checks." Some are acted upon too energetically, and come 
 out quite denuded of their skin. For others the treatment is appar- 
 ently too mild and fails to break the skin at all. 
 
 It is prunes of this last class, coming from the process looking smooth 
 and unchanged, that result in frogs. The properly "checked" prunes 
 begin to dry immediately, the skin wrinkling in minute folds. The 
 flesh remains of a light, bright amber color, and the outside of a clear 
 purplish-brown, differing from the shiny black of the pricked fruit. 
 The over-treated prunes simply dry up too quickly and become hard 
 and too dry. The prunes with smooth, "unchecked " skin do not show 
 signs of drying immediately, but remain smooth and round for several 
 days, till they become a dirty brown and the flesh becomes discolored. 
 
 When the good prunes are dry the frogs are picked out by hand, 
 put on separate trays, and dried for several days longer. When dry the 
 frogs are smaller than most of the good prunes, of a duller color, and 
 have a less finely wrinkled skin. The appearance of the skin is similar 
 to that of the pricked prunes with regard to smoothness, but they have 
 not the bright black color of the latter. When the frogs are dry they 
 are thrown in with the rest of the prunes; but being mostly small, they 
 are separated by the grader and fall into the lowest grade. The usual 
 proportion of frogs is said not to exceed, on the average, one per cent. 
 One year, certain orchards will produce the largest proportion of frogs, 
 another year others will do so. In general, it is the orchards producing 
 the largest crops that produce the largest percentage of frogs. In the 
 
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 best drying weather there are few or no frogs produced, while in damp 
 or cloudy weather the proportion is relatively high. If a tray is shaded 
 for any length of time it will contain a larger number of frogs. Fewer 
 frogs come from those orchards where only the prunes which fail naturally 
 from the trees are picked up than from those where the practice is to shake 
 the trees. The first picking and the last contain the greatest quantity 
 of frogs. 
 
 A neighboring drying-yard was examined where the fruit was graded 
 before dipping. The result was that among the first and second grades 
 there were scarcely any frogs; but in the third or smallest grade there 
 were a good many. 
 
 Orchards which produced practically no frogs this year were visited. 
 The trees were found bearing a rather small crop of very fine, large, and 
 well-ripened prunes. The trees were pruned in such a way that no part 
 of the tree was very close-topped, and no part unduly exposed to the 
 sun; the trees being kept fairly open, so as to allow of bearing wood 
 even in the center. They were pruned back enough to avoid all danger 
 from breaking branches, but not enough to make the trees too dense. 
 
 Other orchards which were producing many frogs this year were next 
 examined. The trees were bearing very large crops, and many of them 
 had large branches broken off by the weight of the fruit. The fruit on 
 these broken branches was small and hard, and was not so sweet or so 
 well flavored as that on the rest of the tree. 
 
 A drying establishment where a pricking machine was used exclu- 
 sively was also visited. The prunes while passing over the needles 
 were sprayed with warm water, but no lye was used. In passing over 
 the needles the prunes were pricked on about half their surface; that 
 is, on a central zone, the ends being almost intact. The holes were 
 from one sixteenth to one quarter of an inch apart, and each prune had, 
 besides these holes, from two to about ten little slits made by the 
 needles, to which the proprietor looked principally for the useful effects 
 of the process. A certain amount of juice was lost through these slits, 
 but perhaps not more than is lost from the over-treated fruit in the lye 
 process. The prunes were black and shiny on the outside, of very good 
 appearance, but inside, the flesh was darker-colored than that of the 
 lye-dipped prunes. There were practically no frogs that could be picked 
 out. 
 
 OBSERVATIONS IN THE LABORATORY. 
 
 On September 12, 1895, samples of fresh 'prunes were received for 
 experiment from Mr. S. P. Sanders, as follows: 
 
 1. Prunes that were gathered from branches broken down by the 
 weight of fruit. The fruit was still attached to the branch, though the 
 prunes from the unbroken branches of the same tree had nearly all 
 fallen and been gathered. 
 
 2. Prunes from the unbroken branches of the same heavily laden 
 trees as No. 1. The fruit was small, but in good condition. 
 
 3. Prunes from trees bearing a moderate crop of fine, well-ripened 
 fruit. All were soft and a few showed signs of commencing to dry. 
 
 The prunes (No. 1) from broken branches were treated with a boiling 
 1.6% lye solution. After 20 seconds immersion only one prune was 
 checked. After 40 seconds some skins were split in consequence of 
 partial cooking, but there were no genuine checks. After 60 seconds the 
 
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 prunes had suffered from cooking, but were not checked. A 3% boiling 
 lye solution was then tried. Different samples of the prunes were 
 dipped for 5, 10, 20, and for 60 seconds, with practically the same results 
 as with the weaker solution. 
 
 Of the prunes (No. 2) from the unbroken branches of heavily laden 
 trees, about half felt soft and half hard. The hard and the soft fruit 
 were treated separately, but with practically the same results; about the 
 same proportion of each failing to check properly. Immersion for 20 
 seconds in a boiling 6% lye solution checked about half of the prunes 
 well. Most of the rest were checked more or less insufficiently, while a 
 few were not checked at all. These last presented the same characteris- 
 tics as the fruit from the broken branches, and like them the skin could 
 not be broken short of cooking. 
 
 The first-class prunes (No. 3) were tried first with a 1.6% solution. In 
 ( this strength of solution at the boiling temperature from 20 to 30 seconds 
 was the best time of treatment. More than 40 seconds' immersion com- 
 pletely removed the skin in places. With this strength of lye (1.6%), 
 boiling temperature, and 20 seconds' immersion, all the prunes of this 
 lot checked perfectly. A 3% solution was found too strong, even with 
 only 10 seconds' immersion. A shorter time failed to check the skin, and 
 a longer completely removed it. Experiments at different temperatures 
 showed that if the lye is much below the boiling-point, the skin does not 
 check. If left in the lye longer at these lower temperatures the skin 
 finally cracks like that of the small prunes described above, and instead 
 of innumerable shallow cracks there are a few deep splits. 
 
 All the prunes of this series of experiments were placed on trays and 
 put to dry in the sun in as nearly similar conditions as possible to those 
 which obtain in the drying-yards. The results were exactly the same 
 as those noted at the West Side drying-yard. The prunes which had 
 failed to check turned brown and were long in drying; the others dried 
 in a few days. An attempt was made to determine the presence of a 
 mold or other micro-organism within the tissue of the brown prunes, 
 but without success. A determination of the sugar contents in the two 
 kinds of prunes was made by Mr. Colby, with identical results for each 
 kind. Microtomic cross-sections of the skin were made in order to 
 determine if the relative thickness of the epidermis was in any way 
 connected with the failure to check. A slight difference of thickness 
 was found, but not enough to account for the great difference of behavior 
 in dipping. Ten measurements of the thickness of the skin of prunes 
 (No. 1) from broken branches showed an average of 24.9 fi (microns), 
 while the average of eight measurements of the skin of large, well- 
 ripened prunes (No. 3) was 23.6 p-. Under the microscope the walls of 
 the epidermal cells of the small prunes appeared thicker and retained 
 their shape better under the operations of hardening, imbedding, and 
 cutting, showing that they were of a more resistant nature. 
 
 The following three samples of dried prunes were received later in the 
 season from Mr. S. P. Sanders: 
 
 4. Frogs. These were small, hard, smooth-skinned prunes of the typi- 
 cal dirty-brown color outside, and dark color inside. They averaged 120 
 to the pound, and showed 35.25% of sugar. The skin showed no checks. 
 The higher sugar content as compared to No. 5 is to be accounted for by 
 over-drying. 
 
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 5. Large prunes of good quality. Light purplish color outside, with 
 finely wrinkled skin, and bright amber color inside. These averaged 
 43.2 to the pound, and showed 33.25% of sugar. The skin showed the 
 customary close checking of successfully dipped prunes. 
 
 6. Bloaters. These prunes were very large, a little darker in color 
 than No. 5, and with equally fine-wrinkled skin. The inside was dark- 
 colored in places, but in general resembled that of the prunes of No. 5 
 in texture, though a little harder and more thoroughly dried. They 
 were hollow, and in many cases the stone was split. They averaged 
 37.4 to the pound, and showed 39% of sugar. The skin was finely 
 checked, at least as thoroughly as in No. 5. 
 
 • 
 
 "frogs" and "bloaters." 
 
 At the drying-yards which I visited, the terms "frogs," " frogbellies," 
 and " bloaters " seemed to be used indiscriminately. There are, how- 
 ever two things, very distinct both as to cause and effect, confused 
 under these names. The prunes which I will call bloaters, for the sake 
 of distinction, are generally, perhaps always, large prunes, originally. 
 In drying they swell up to half again their natural size, on account of a 
 gas-producing fermentation around the pit. They become hollow and 
 the pit frequently splits in two. In grading, they all fall in with the 
 largest size, and in the after-dipping, owing to the air space within, they 
 float and can be separated. In quality, as far as eating is concerned, 
 they are almost equal to the first-class prunes, but are not so attractive in 
 appearance, on account of their swollen look. The fermentation that they 
 undergo is slight (in the cases that have come under my observation), 
 as is proved by the high percentage of sugar which they contain. This 
 is higher than in the case of the good prunes; a fact which is to be 
 accounted for, partly by their more complete ripeness when gathered, 
 and partly by the greater amount of drying which they undergo on 
 account of the hollow space around the pit. The bloaters are doubtless 
 large, soft, very ripe prunes, which in falling from the tree become 
 injured in such a way as to allow the yeast germs which exist on the 
 skins of all fruits to find entrance, probably at the hole left by the 
 detachment from the stem. The remedy would be, frequent gathering, 
 in order not to allow the yeast to develop in the shade of the trees 
 before the fruit is dipped and placed upon the drying trays. The 
 prunes which have a tendency to become bloaters, being thoroughly 
 ripened, dry quickly; and unless the fermentation has obtained a good 
 start before they are placed upon the drying-trays it will have no 
 chance to develop enough gas during the process of drying to puff them, 
 unless the drying for some reason is slower than usual. Pricking, if it 
 were deep and thorough enough, might allow the gas to escape as given 
 off and thus prevent bloating. 
 
 The prunes which I will call "frogs," as distinguished from "bloat- 
 ers," are of an altogether different character. They are almost always 
 small prunes and imperfectly developed. This failure to develop prop- 
 erly may arise from various causes. It may sometimes be due to 
 unsuitable or poor soil, but in the cases which have come under my 
 observation it is usually due to overbearing. A tree producing too 
 heavy a crop fails to properly mature its fruit; it cannot supply the 
 requisite nourishment and moisture for so large a quantity. The fruit 
 
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 upon the broken branches is still more insufficiently supplied with the 
 necessary material to complete its development, and it is precisely this 
 fruit which is most prolific in frogs. Fruit which is shaken from the 
 trees is often incompletely developed, and therefore results in many 
 frogs. Trees were observed this year which were prematurely deprived 
 of their leaves by a fungus (Puccinia pruni-spinosse, Pers.). The fruit 
 on these trees was precisely of the character of those which become 
 frogs. In short, anything which prevents the fruit from attaining a 
 normal and complete development and maturity will occasion frog- 
 ging. The way to prevent their production, therefore, is to keep the 
 trees in perfect order, and allow them to produce only a reasonable crop; 
 to gather the fruit only when it has attained the proper stage of matu- 
 rity; and to dry as quickly as possible. Prevention is better than cure; 
 but, if prunes are to be dried which have a tendency to frog, I would 
 strongly advise preliminary grading. This is' more especially important 
 where the dipping process is in use. The smallest grade will then con- 
 tain by far the largest part of the prunes liable to frog, and can be 
 treated in a different manner from the good prunes. The extra expense 
 would be more than counterbalanced by the less amount of sorting-over 
 that would have to be done during the process of drying. As lye- dip- 
 ping has no effect on the frog prunes, they should be pricked, and 
 unless the weather is of the best drying character they should be dried 
 by artificial heat. 
 
 In view of the great influence which a proper system of pruning may 
 have in preventing the production of frogs, a series of photographs of 
 prune trees taken by Mr. Sanders is of great interest. These photo- 
 graphs have been kindly donated by Mr. Sanders to the Station, and the 
 most instructive of them will be reproduced in the forthcoming 
 agricultural report. 
 
 Judicious pruning prevents the production of frogs by regulating 
 the bearing of the trees, not allowing them to overbear one year at the 
 expense of the next, nor allowing some branches to carry more than 
 their share of the load, and by so regulating and adjusting the branches 
 that they will have the maximum bearing strength; that is, that they 
 will bear as heavy a load as possible without breaking. The system 
 advocated by Mr. Sanders and exemplified in his photographs interferes 
 as little as possible with the natural form of the tree. It simply thins 
 out and directs the growth where necessary. It aims to produce a tree 
 with an upright central trunk, from which the branches grow out alters 
 nately instead of opposite to each other in pairs. It avoids the produc- 
 tion of weak crotches, and especially of the forks which are caused by 
 the repeated cutting-back system. These objects are attained by taking 
 the unpruned tree as a model and only cutting where it is necessary to 
 prevent irregularity of growth or of shade, and to distribute the bearing 
 wood as evenly as possible. A tree is thus produced with the maximum 
 breaking strength and one which distributes its energy properly between 
 the production of foliage and fruit and the storage of reserve material. 
 Trees treated in this way do not have to be held together with iron bolts 
 and bands or propped up with wooden crutches. They do not exhaust 
 the reserve materials stored up in their trunks and roots in the produc- 
 tion of an immense crop of inferior fruit, nor expend their energies on 
 an unnecessarily large growth of new wood and foliage. 
 
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 LYE-DIPPING AND PRICKING. 
 
 The difference of opinion regarding the relative value of these two 
 processes is so marked, practical men who have investigated and even 
 tried both methods are so diametrically opposed in their views on the 
 subject, that a priori there would seem to be but one explanation, viz., 
 that it is largely a matter of taste. As a matter of fact, each process 
 has produced dried prunes of excellent quality, which proves that if the 
 conditions are right either process will produce good results. 
 
 The accompanying cut (Fig. 2) may be of some use in explaining the 
 differences of effect between the checks of lye-dipping and the holes and 
 slits made by the needle machines. It represents the microscopic 
 appearance of a cross-section through a check of the proper size. It will 
 be noticed that the skin is simply broken and very little of the flesh of 
 the fruit exposed. A few of the thin-walled cells of the flesh (B) have 
 collapsed, owing to the action of the hot lye, but none of them are 
 broken. The lye check is thus very different from the needle slit, which 
 cuts through several layers of the soft inner cells. This deeper cutting 
 of the flesh would seem at first glance to favor rapid drying. That it 
 does not do so is due, in the first place, to the small number of slits 
 made by even the most efficient machine compared to the number of 
 checks on a successfully dipped prune, and in the second place to the 
 fact that the juice which is allowed to flow out of the cells broken by 
 the needles solidifies and chokes up the slit. When a prune is over- 
 treated by the lye process, the skin (A) rolls back, exposing more of the 
 flesh and in extreme cases completely denuding it. 
 
 The advocates of lye-dipping claim: 
 
 1. That the prunes are more thoroughly cleansed; 
 
 2. That they dry more quickly; 
 
 3. That the skin is rendered less tough; 
 
 4. That the finished product is of better appearance; 
 
 5. That the flavor is better. 
 
 On the other hand, the advocates of pricking claim: 
 
 1. That the hot water they use cleans the fruit perfectly; 
 
 2. That the prunes dry more evenly; 
 
 3. That no frogs are produced; 
 
 4. That the finished product is of better appearance; 
 
 5. That the flavor is better. 
 
 With certain reservations and explanations it may be said that all 
 the claims of each side are true. With some pricking machines the 
 washing is not very thorough, but with the best arrangements the 
 cleaning is quite satisfactory. The lye-dipped prunes dry, according to 
 the weather, in from two to seven days less time than the pricked prunes. 
 But the greater uniformity in the drying of the pricked fruit probably 
 quite offsets this, especially where there are many tough-skinned prunes 
 which the lye does not affect. 
 
 The third claim of the advocates of lye-dipping, that the skin is 
 rendered less tough, hardly cuts any figure, as the lye acts only on 
 those prunes which have naturally a tender skin, and leaves the tough- 
 skinned ones untouched. It is with regard to the fourth claim, that is, 
 with regard to the appearance of the fruit, that there is most to be said 
 on both sides. All that the unbiased observer can remark is that the 
 
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 finely wrinkled skin and light amber color of the lye-dipped prunes 
 are very attractive to those whom they attract, while the same remark 
 applies to the smooth, shiny skin and black color of the pricked prunes. 
 As it is a matter of the consumer's taste, the market should quickly 
 settle this question. It may be added that the smooth skin and black 
 color are characteristic of the best French dessert prunes, dried without 
 either lye-dippihg or pricking; and as habit is so powerful in matters of 
 taste, this fact will certainly have influence. With regard to flavor my 
 own impression is that the difference is so slight as to be scarcely 
 noticeable, and to be quite obscured by the least diversity in the 
 original quality of the fruit. 
 
 The conclusion, therefore, would seem to be that if we are dealing 
 with good raw material, that is, fine, well-ripened prunes and good 
 drying weather, the choice of method must depend on which appearance 
 is most attractive to the average consumer. 
 
 There are, however, other cases to be considered. In the case of 
 unevenly developed, mixed lots of fruit the pricking process affords 
 more security against the frogging of a portion of the fruit, and the 
 product is much more uniform in appearance and in time of drying. 
 Uniformity in time of drying is very important, as the difficult and 
 expensive hand-sorting on the drying-trays is thus reduced to a mini- 
 mum. The mere pricking of a needle-hole evidently facilitates drying 
 but very little; the exuding juice almost immediately closes up these 
 fine holes. It is otherwise with the slits, which, being similar to the 
 effect produced by the lye process, facilitate drying much more. Still 
 the flow of sap consequent on the penetration of the needles into the 
 flesh tends to close up the finer slits also and thus retards drying, while, 
 however, imparting to the surface that shining appearance which the 
 lye-dipped prunes only obtain by an after-treatment. 
 
 It thus appears as though both processes had their place in the prune- 
 drying industry, each serving best under special conditions; and that 
 the smaller sizes especially would, on the whole, be more advan- 
 tageously treated by the pricking than the lye-dipping process.