THE COLLEGE OF |« 
 UNIVERSITY OF CALIFORN 
 
 B 
 
Fig. 1 . The plant layout shown here includes equipment which 
 is necessary in any prune dehydrating operation. Prune growers 
 who have been doing their own dehydrating probably already 
 have variations of this equipment. The layout is an ideal one, 
 showing how equipment can be arranged to provide an uninter- 
 rupted flow of fruit from receiving through all processes to boxing 
 for storage, with maximum efficiency of time and labor. 
 
 * 
 
DEHYDRATING PRUNES 
 
 E. M. MRAK 1 AND R. L PERRY* 
 
 This circular is issued for the California prune grower and the 
 moderate or small-scale operator as a general guide in dehydrating prunes. 
 
 There has been a steady increase in the mechanical drying of prunes 
 in recent years. Thirty years ago virtually all prunes were sun-dried; now 
 it is estimated that over 80 per cent of the crop is mechanically dried. 
 
 This trend has been accompanied by a more general interest on the 
 part of all prune growers in good dehydration methods, as well as an 
 increase of small-plant operators. 
 
 Important details of small-plant operation are covered in this circular 
 with particular emphasis on the steps in preparing fruit for drying, and the 
 method of controlling relative humidity during the drying process. 
 
 For more complete information on the design, installation, and oper- 
 ation of equipment, as well as the technical details of large-plant operation, 
 other publications may be obtained through your County Farm Advisor. 
 
 Similar circulars, prepared by the College of Agriculture, describe the 
 dehydrating of freestone peaches and methods of operating a sulfur house. 
 
 1 Mr. Mrak is Associate Professor of Food Technology and Mycologist in the Experi- 
 ment Station. 
 
 - Mr. Perry is Professor of Agricultural Engineering and Agricultural Engineer in 
 the Experiment Station. 
 
 r=W 
 
 ELEVATOR FOR FILLING 
 BOXES OR MOVABLE BINS 
 
What Advantages Does 
 Dehydration Offer? 
 
 Before 1918 practically all California 
 prunes were sun-dried. Now, the percent- 
 age of prunes dehydrated has risen gradu- 
 ally to over 80 per cent of the entire crop. 
 
 Dehydration is preferred because it 
 generally produces a higher quality of 
 dried prunes. It is cleaner and it reduces 
 losses during unfavorable drying weather. 
 In the long run, dehydration is cheaper 
 than sun-drying. 
 
 Both the quality and dried weight of 
 dehydrated prunes are often above those 
 of the sun-dried product, because spoilage 
 and resultant losses are reduced. Dehydra- 
 tion also results in the retention of a 
 greater amount of fruit sugar than does 
 than sun-drying. 
 
 It is good business practice, in the fruit 
 industry as in all food enterprises, to keep 
 products clean throughout every stage of 
 processing, and dehydration offers the 
 cleaner method. 
 
 Because prunes ripen later (August 15 
 to October 1) than other fruits which are 
 sun-dried, and the drying season may be 
 interrupted by wet weather, dehydration 
 is the safer means of drying. 
 
 A great amount of labor is required in 
 the sun-drying of prunes, and this cost 
 may be reduced by dehydrating the fruit 
 if the grower has his own equipment or 
 belongs to a cooperative dehydrating 
 organization. On the other hand, if he 
 must pay a fixed amount per ton to a com- 
 mercial dehydrator, his own sun-drying 
 plant may prove the cheaper. 
 
 The trend in fruit processing must be 
 toward mechanization in handling the 
 crops, in order to reduce excessive labor 
 costs. As the cost of labor is reduced, the 
 costs per pound of producing dried 
 prunes will be lessened. 
 
 A suggested arrangement of equip- 
 ment for maximum efficiency of time and 
 labor is shown in Figure 1 on pages 1 and 
 2 of this circular. 
 
 Dehydration cannot be expected to 
 
 [4 
 
 improve the original quality of prunes. 
 Neither zvill it return quality to fruit 
 zvhich has been partly spoiled during 
 bad weather under sun-drying meth- 
 ods. 
 
 Inefficient operation of a dehydrater 
 may be costly and result in an inferior 
 product. Case hardening (too-rapid dry- 
 ing of the outside of the fruit while the 
 flesh is still underdried), bleeding (ex- 
 cessive juicing of the fruit during de- 
 hydration), scorching (which results 
 from too-high finishing temperatures in 
 the dehydrater), or smudging (as result 
 of direct-heating oil burners) —all may be 
 caused by careless operation. 
 
 The drying ratio of prunes fluctuates 
 considerably, but usually ranges from 2 
 to 3% to 1, on an average. A yield of 1 
 dry ton per acre is considered low; 2, 
 medium ; and 5 or more, high. As a rule, 
 coastal counties produce prunes of supe- 
 rior quality which have a more favorable 
 drying ratio than those grown in the in- 
 terior counties. However, the warmer in- 
 land areas commonly produce 20 tons per 
 acre of fresh fruit, while in the coastal 
 counties, yields range only from IV2 to 5 
 tons per acre of fresh fruit. 
 
 Pick, Clean, Dip with 
 Care for Good Product 
 
 Steps in the dehydration of prunes are 
 as follows: 
 
 1. Pick prunes by shaking from tree. 
 
 2. Clean the fruit with air blast and 
 sprays of water. 
 
 3. Check prunes by dipping in cold or 
 hot water, or in lye solution. 
 
 4. Grade for sizes, if possible. 
 
 5. Tray fruit (45 to 55 pounds of 
 French, 60 to 75 pounds of Imperial or 
 Sugar varieties per 3 x 6-foot tray) . 
 
 6. Dehydrate (18 to 28 hours) until 
 fruit has but 18 to 19 per cent moisture 
 content. 
 
 7. Cool fruit; scrape from trays, sort- 
 ing out defective pieces; store in clean, 
 wooden boxes. 
 
 ] 
 
Picking the Prunes 
 
 Prunes are best when harvested around 
 the middle of the season for the district. 
 Allowing the fruit to hang too long on 
 the trees (in areas where it does not fall 
 naturally) results in inferior quality and 
 lower dry yields. On the other hand, 
 knocking all the fruit from the trees early 
 in the season will give similar results. 
 
 The number of pickings varies with the 
 orchard and location. In coastal counties, 
 where the fruit falls more naturally from 
 the trees and only light shakings are re- 
 quired, three or four pickings are cus- 
 tomary. In the interior counties, where 
 heavier shaking is needed to bring down 
 the fruit, growers rarely make more than 
 two pickings. 
 
 The first fruit picked in any orchard 
 is usually of inferior quality and 
 should be kept separate from that 
 picked later. 
 
 Excessively hot weather (with tempera- 
 tures above 105° F) may cause the flesh 
 of prunes still on the trees to brown, and 
 form gas pockets in the flesh. 
 
 If it is not possible to process the iruil 
 as fast as it is brought in from the or- 
 chards, the use of cold storage is desirable 
 when available. Freezing storage, on the 
 other hand, results in loss of fruit flavor. 
 
 In an experiment conducted in 1946 
 in the Sacramento Valley, where the pick- 
 ing is customarily done over a period of 
 3 or 4 weeks, a grower of French prunes 
 picked the larger part of his crop within 
 a 10-day period in mid-season, leaving 
 the remainder of the fruit on the trees. 
 As all of the fruit could not be handled 
 at once in his plant, he hauled the picked 
 prunes to a commercial cold-storage 
 plant 10 miles from his ranch. Here it 
 was held in storage at 40° F for a month. 
 When finally dehydrated, the dried prunes 
 were far superior to those from the same 
 orchard, made from fruit which had been 
 left on the trees for a week or more longer. 
 The fruit left on the trees for the longer 
 time tended to develop black flesh, gas 
 pockets, and inferior flavor, while the 
 cold-storage fruit had light-colored flesh, 
 no gas pockets, and a good flavor. 
 (Continued on page 7) 
 
 II 
 
 1|$F^ 
 
 Fig. 2. Appearance of sound prunes may be compared with the appearance 
 of puffy prunes in the picture above. In A, the puffy prune at left dries in long, 
 coarse wrinkles while the sound prune, shown on the right, has many small 
 wrinkles. In B, the interior of the puffy prune at left has large gas pockets, and 
 the pit is pulled away from the darkened flesh. These characteristics indicate 
 fermentative spoilage during drying, and are not found in the sound prunes 
 pictured on the right. 
 
 [5 I 
 
HOW TO USE £ thermometers IN DEHYDRATING 
 
 For most efficiency in the operation of the dehydrater, three thermometers 
 should be used: a dry-bulb at the hot (finishing) end, and a pair of thermom- 
 eters—one dry-bulb and one wet-bulb at the cool (wet) end. All thermometers 
 must be located in the tunnel and exposed directly to the full air flow, else 
 false readings will be obtained. 
 
 THE DRY BULB AT THE HOT (FINISHING) END is the key to your 
 operation. When one speaks of "operating a tunnel at 140°" or "drying prunes 
 at 165°" he is referring to the temperature at the finishing end. A tunnel for 
 prunes never should he operated at over 165° F. 
 
 THE DRY BULB AT THE COOL END, together with the wet-bulb, helps 
 the operator to measure relative humidity in the tunnel, and thus governs 
 loading of the tunnel. The reading of this thermometer will always be con- 
 siderably lower than that of the dry-bulb at the hot end. Just hoiv much lower 
 depends on the length of the tunnel, the wind velocity, the dryness of the fruit, 
 and how frequently new cars are introduced. For example, when drying 
 French prunes at 165°, the dry-bulb temperature at the cool end of the 36-foot 
 tunnel probably would read about 130°. 
 
 THE WET BULB AT THE COOL END is to be used in conjunction with 
 the dry bulb to indicate relative humidity. The objective is to maintain the 
 wet-bulb at 15° lower than the dry-bulb reading. Whatever the reading is on 
 the dry bulb, the wet bulb should read 15° lower. If the dry bulb reads 130° 
 then the wet bulb should read 115°. This difference of 15° indicates a relative 
 humidity of 60% at the cool end, the ideal humidity in a counter current 
 tunnel for the drying of prunes and many other fruits. A smaller difference 
 between the readings indicates a higher humidity which should be avoided at 
 all times; a larger difference between the readings indicates a lower humidity, 
 which is safe for the fruit but not completely efficient. The humidity of 60% 
 is high enough to allow for recirculation of a substantial amount of air thus 
 saving heat and making a more efficient operation, and at the. same time is 
 low enough to assure a good dried product. 
 
 The rule of maintaining a 15° difference between dry-bulb and wet-bulb 
 readings at the cool end applies to all temperatures up to 135° dry-bulb and 
 120° wet-bulb. Above 135° dry-bulb, the wet bulb should continue to be held 
 at the 120° wet-bulb reading. 
 
 The wick in the wet-bulb thermometer must be kept clean at all times, 
 and distilled water should be used to moisten it. Only by using a clean wick 
 and pure water will correct readings be possible. Many operators believe, 
 mistakenly, that it is adequate to change the wick once or twice a season; 
 actually, the wick should be changed every ten days or two weeks. 
 
 MAINTAINING RELATION OF DRY-BULB TO WET-BULB (both 
 at the cool end) is simply a matter of adjusting air intakes to allow (a) more 
 air to circulate through the tunnel and less air to be taken in from the outside, 
 thus raising the wet-bulb reading and indicating a higher humidity in the 
 tunnel; or to allow (b) less air to recirculate through the tunnel and more 
 air to be taken in from the outside, thus lowering the wet-bulb reading and 
 indicating a lower humidity in the tunnel. 
 
Guide Charts for Controlling Humidity 
 in Dehydrating Prunes 
 
AT HOT (dry) END 
 
 DA 
 
 RECOMMENDED 
 FINISHING 
 TEMPERATURE FOR <* 
 FRENCH PRUNES 
 
 ■J 
 
 M£ 
 
 160 
 155 
 150 
 145 
 140 
 135 
 130 
 
 ER 
 
 t 
 
 - > 
 
 RECOMMENDED 
 FINISHING 
 TEMPERATURE FOR 
 IMPERIALS 
 
 -] 125 - 
 
 DRY-BULB THERMOMETER 
 
 FINISHING TEMPERATURE SHOULD 
 NEVER EXCEED 165° F. 
 
 TEAR OUT AND HANG AT HOT END OF YOUR DEHYDRATER 
 
AT COOL (wet) END 
 
 IF DRY-BULB 
 
 WET-BULB 
 
 THERMOMETER 
 
 THERMOMETER 
 
 READS: 
 
 SHOULD READ: 
 
 145 
 
 140 
 135 
 130 
 125 
 120 
 115 
 110 
 
 ABOVE 
 *» 135° 
 
 RETAIN 
 120°* 
 
 MAINTAIN A 
 FOR A RELATIVE 
 
 TTOflnDTrroF"&Tf%" 
 
 120 
 
 120 
 120 
 
 115 
 
 no 
 
 105 
 100 
 95 
 
 n 
 
 ^^^SSk-> 
 
 Dry-Bulb 
 
 Wet-Bulb 
 
 TEAR OUT AND HANG AT COOl END OF YOUR DEHYDRATER 
 
Digitized by the Internet Archive 
 
 in 2012 with funding from 
 
 University of California, Davis Libraries 
 
 http://archive.org/details/dehydratingprune383mrak 
 
 V 
 
Prunes may lie on the ground in the 
 orchard for several days, dining which 
 time the fully mature fruit will start to 
 dry and thus remain in good eating condi- 
 tion. However, it should not be left too 
 long on the ground, or it will lose sugar 
 and develop off-color in the flesh, off- 
 flavor, and an inferior texture. 
 
 If the fruit is picked and held too long 
 in boxes before being processed, the flesh 
 may discolor, gas cavities form (fig. 2), 
 and the sugar be lost. Molding and fer- 
 mentation may take place without being 
 visible on the fruit. Frequently this mold 
 can be seen only with a microscope. If it 
 is necessary to hold the fruit it should be 
 stored in a cool and shady place. 
 
 Prunes picked in the warmer part of 
 the day will hold heat and spoil more 
 rapidly than those picked under cooler 
 conditions. The warmer fruit should be 
 processed first. 
 
 Quality once impaired remains so in 
 the finished product. 
 
 Clean the prunes to remove sticks, 
 stones, leaves, and dirt (which are bound 
 to adhere to any fruit harvested from the 
 ground), by passing them through an 
 effective trash remover. This consists of 
 a strong draft of air over which the fruit 
 passes, and strong sprays of clean water- 
 showering over the fruit. Water should 
 not be recirculated, for after continued 
 use it becomes so dirty that the dried fruit 
 may actually show muddy splotches. 
 
 Checking Prunes by Dipping 
 
 Many processors have found that 
 checking the skin of prunes with a lye 
 solution (as is usually done when the 
 fruit is sun-dried) causes them to bleed 
 badly during dehydration, and as a result 
 have resorted to cold- or hot-water washes 
 instead. Hot water is somewhat more ef- 
 ficient than cold water, but use of the 
 latter is adequate. Cold water does not 
 remove the waxy bloom from the skin of 
 the prunes, and is more economical to 
 use because it requires no heating equip- 
 ment. 
 
 Firm, plump fruit checks more easily 
 than shriveled fruit. 
 
 If a lye solution is used, the strength 
 varies from 4 to 12 or even 16 pounds of 
 lye to each 100 gallons of water, accord- 
 ing to the condition of the fruit and the 
 temperature of the fruit and of the solu- 
 tion. The solution should be kept as near 
 boiling as possible. This may be hard to 
 do on cool mornings, when cold fruit 
 entering the vat will quickly lower the 
 temperature of the solution. Watch the 
 fruit to be sure the checking is sufficient. 
 Longer dipping may be needed if the 
 solution cools down; or perhaps it may 
 be necessary to add more lye. (When lye 
 is added to the hot solution there is danger 
 that it may bubble up suddenly.) 
 
 Another method is to spray rather than 
 dip the fruit, either with lye solution or 
 water. In lye-spraying the period of treat- 
 ment is about 10 seconds, concentration 
 of the solution being modified (by add- 
 ing more lye or more water) to the de- 
 gree of checking required (fig. 3). After 
 dipping or spraying with lye, the prunes 
 are washed under sprays of cold water 
 to remove any lye residue. 
 
 Grading for Sizes 
 
 After the prunes have been dipped and 
 washed, grading for sizes may be done by 
 passing the fruit over a perforated shaker. 
 Grading fruit allows for more even 
 dehydration, and increases the actual 
 capacity of the plant by taking advantage 
 of the shorter time small fruit requires in 
 the drier. Grading, however, is seldom 
 done in commercial practice. 
 
 Grading can be done if the plant is 
 large enough to permit different sizes to 
 be dehydrated separately. If the plant is 
 small, however, or is being run on a co- 
 operative basis and thus handling differ- 
 ent growers' lots of fruit, it is better to 
 dispense with the size-grading of the 
 prunes. At present, there is no satis- 
 factory system of quality-grading of fresh 
 prunes, therefore pooling of different 
 growers' lots is not practical. 
 
 [7 
 
7 raying for Even Drying 
 
 Dipped fruit is spread on trays by use 
 of a power-operated shaking device, 
 which spreads the fruit one layer deep on 
 the trays. A tray will hold about 45 to 55 
 pounds of French prunes; from 60 to 75 
 pounds of Imperial or Sugar prunes. 
 
 It is very important that the fruit be 
 spread only one layer deep on the trays. 
 
 If too much fruit is on the trays, de- 
 hydration will be retarded and the prunes 
 may become sticky. The drying time will 
 be increased and the fruit will dry un- 
 evenly. Drying will be especially uneven 
 if the trays are loaded irregularly, one 
 layer deep in some places and two or 
 three layers deep in others. This is apt to 
 happen if the feeding of prunes to the 
 trays at the dipper is done too fast. 
 
 Trays may be stacked on cars either by 
 hand or by use of mechanical stackers. 
 The latter method will reduce the man- 
 hours needed. 
 
 In larger plants, the dehydraters oper- 
 ate both day and night, while the dipping 
 line ordinarily operates only during the 
 day. In order to keep enough cars of 
 
 dipped fruit for 24-hour operation, suf- 
 ficient prunes must be dipped during the 
 day, and some cars must stand all night 
 before entering the dehydrater. This is 
 not the best plan but, because nights 
 usually are cool, will not cause significant 
 damage to the fruit. 
 
 Loading the Tunnel 
 
 In loading the dehydrater, there should 
 be regular time spacing between cars. In 
 most countercurrent tunnels, spacing 
 should be from 1 to 2 hours between cars 
 when the tunnel is loaded. An empty 
 tunnel may be partially loaded more 
 quickly, but it is still important to avoid 
 introducing more than two or three cars 
 at once. Loading evenly insures uniform 
 drying. 
 
 Several Types of 
 Dehydraters in Use 
 
 Dehydraters vary considerably. Among 
 the designs are countercurrent tunnels, 
 small-capacity constant-temperature cabi- 
 nets, and center-inlet cross-flow tunnels. 
 Cabinet driers are occasionally used for 
 
 Fig. 3. The dipping process should result in a definite checking of the skin as 
 shown above. Large cracks or peeling should be avoided. 
 
 [8] 
 
batch operations. The University has de- 
 signed a crossflow type of dehydraler that 
 can be constructed fairly cheaply and will 
 handle the crop from almost any size of 
 orchard, even as small as 6 acres (fig. 4) . 
 A considerable number of the University- 
 planned dehydraters have been erected in 
 California. Plans and specifications for 
 construction of the University dehydraler 
 can be obtained for $1.00. (This and 
 other printed information on the oper- 
 ation of dehydraters and on the use of the 
 humidity chart are obtainable through 
 the office of the county farm advisor.) 
 
 In the countercurrent tunnel the fruit 
 moves through the tunnel (ordinarily 
 long enough to accommodate 12 to 18 
 cars each holding about 25 trays of fruit) 
 
 in the opposite direction to the flow of air. 
 The cars enter at the cool end and move 
 toward the hot end at 1- to 2-hour inter- 
 vals. Air is heated in a chamber adjacent 
 to the tunnel, either at one side or above 
 it, and is forced in by electrically driven 
 fans. The air velocity should range from 
 600 to 800 linear feet per minute. The 
 drying time is usually from 18 to 24 
 hours. 
 
 In the cross-flow dehydrater, the fruit 
 moves at right angles to the air flow. 
 Prunes usually require over 20 hours to 
 dry in this type of dehydrater, with the 
 temperature not exceeding 165° F. 
 
 Rated capacities of dehydraters, and 
 the tons of fruit they will handle, are 
 given in the table on page 10. 
 
 Fig. 4. Movement of air through the fruit at various stages of the drying process is shown 
 above. The dehydrater pictured is the cross-flow type, with axial-flow fan, designed by Uni- 
 versity of California agricultural engineers. The picture shows: /, first position of fruit, when 
 fresh; 2, second position of fruit, partially dried; 3, third position of fruit, nearly dried; E, ex- 
 haust air escaping out of adjustable door; F, fresh air entering beside furnace; G, gases heated 
 in furnace; H, hot air delivered by fan to fruit in second and third positions; /, intermediate 
 air passing from fruit in second and third positions to that in the first position; M, mixture of 
 fresh air, gases heated in the furnace, and recirculated air entering the fan; V, primary fresh 
 air for initiating combustion in the furnace; R, recirculated air passing from the fruit in the 
 first position to the furnace chamber; S, secondary fresh air from completing combustion in the 
 furnace. 
 
 [9] 
 
CARS NEEDED, AND CAPACITY IN TONS OF PRUNES, FOR DEHYDRATERS OF 
 
 DIFFERENT SIZES 
 
 
 Total number 
 
 
 
 
 of cars, with 
 
 Capacity in tons 
 
 of fruit per day 
 
 Type and rated capacity of dehydrater 
 
 trays, needed 
 
 
 
 
 
 
 to operate 
 
 Fresh 
 
 Dry 
 
 University dehydrater (a cross-flow type): 
 
 
 
 
 6-car 
 
 12 
 
 4 
 
 m 
 
 9-car 
 
 18 
 
 6 
 
 21/4 
 
 12-car 
 
 24 
 
 8 
 
 3 
 
 18-car 
 
 36 
 
 12 
 
 41/2 
 
 Counterflow type: 
 
 
 
 
 12-car 
 
 24 
 
 8 
 
 3 
 
 24-car 
 
 48 
 
 16 
 
 6 
 
 36-car 
 
 72 
 
 24 
 
 9 
 
 48-car 
 
 96 
 
 32 
 
 12 
 
 60-car 
 
 120 
 
 40 
 
 15 
 
 72-car. . 
 
 144 
 
 48 
 
 18 
 
 
 
 Rated capacity refers to the number of cars in the dehydrater at one time. At least an equal 
 number of cars should be available outside the dehydrater for loading, drying or defraying. 
 
 Control Humidity and 
 Temperature in Drying 
 
 Prunes are comparatively slow-drying. 
 They require from 18 to 28 hours for the 
 dehydration process, according to the 
 size and condition of the fruit, the type 
 of drier, and the operation of the equip- 
 ment. 
 
 Humidity of the air in the dehydrater 
 during drying is important to the quality 
 of the finished product, and also to the 
 economical operation of the plant. 
 
 If humidity is too high, dripping or 
 stickiness occurs and flesh discoloration 
 may be evident in the prunes; the time of 
 drying is extended; and trays, floor and 
 machinery become dirty and sticky. 
 
 If humidity is too low, heat is being 
 wasted and fuel consumption is higher 
 than necessary because the exhaust air is 
 unnecessarily large in volume and low in 
 moisture. 
 
 Frequent observation of the dry and 
 wet bulbs at the cool end of the tunnel 
 will guide the operator in controlling hu- 
 midity properly at the various drying 
 temperatures. 
 
 To further aid the operator, an ex- 
 planation of how to use three ther- 
 
 mometers, and a convenient guide 
 chart are inserted at the center fold 
 of this circular. The chart can be re- 
 moved and fastened to the dehydrater. 
 
 A relative humidity of 60% at the cool 
 end is the maximum which should be 
 reached at any time during the drying 
 process. This relative humidity is indi- 
 cated by a difference of 15° between the 
 dry-bulb thermometer reading and the 
 wet-bulb thermometer reading at the cool, 
 wet end of the tunnel. This relative hu- 
 midity permits satisfactory drying and 
 will allow the operator to recirculate 
 some air within the dehydrater, thus mak- 
 ing for a much more economical opera- 
 tion. 
 
 Controlling Temperature 
 
 Certain varieties of prunes tolerate a 
 relatively higher drying temperature than 
 do others. 
 
 French prunes are usually dried at the 
 top of their safe-drying range of 150° to 
 165° F. Imperial and Sugar and other 
 large varieties, which require lower dry- 
 ing temperatures to avoid juicing, are 
 most satisfactorily dried at around 140° 
 F. However, since the drying time at 140° 
 F is nearly twice that at 165°, it is com- 
 mon practice to move the large varieties 
 
 [10 
 
from the lower- to the higher-temperature 
 tunnels when they are about half dried. 
 
 At no time should the finishing tem- 
 perature exceed 165° F. 
 
 Don't rush the drying of prunes. When 
 they are dried rapidly the surface be- 
 comes dry and hard while the inside flesh 
 is only partly dried. These prunes will 
 seem drier than they actually are, and if 
 put into storage may mold as the inner 
 moisture spreads to the surface. 
 
 Testing Dried Fruit 
 
 In a manual test, when the fruit is suf- 
 ficiently dehydrated the flesh must be firm 
 and the pit should not slip when a prune 
 is squeezed between the thumb and fore- 
 finger. Sometimes the surface of a dried 
 prune feels quite firm when the fruit is 
 first taken from the dehydrater. 
 
 The moisture content of the fruit as it 
 comes from the dehydrater may be de- 
 termined by the use of a moisture tester. 
 Freshly dehydrated prunes should not 
 register more than 19 per cent moisture 
 content in order to store satisfactorily. 
 
 Do not judge dryness until the fruit 
 is cool. 
 
 Controlling Bleeding of Imperials 
 
 Imperial prunes are difficult to de- 
 hydrate because they tend to bleed during 
 heating. In order to overcome this tend- 
 ency, three procedures have been used. 
 
 1. Dip in water or a light lye solution 
 for a short period of time (15 seconds or 
 less). Dry in the sun until they start to 
 
 wrinkle and then finish drying in a de- 
 hydrater at 140° F. The drying time will 
 vary greatly with the condition of the 
 fruit. 
 
 2. Dip in water and dehydrate at 135° 
 to 140° F for a day. Then permit them to 
 stand in the outside air for a day before 
 completing dehydration in a tunnel with 
 a hot-end temperature of 160° F. The 
 pre-dried fruit may be placed directly in 
 the tunnel at 160° F if standing for a day 
 is not feasible. Late in the season, when 
 the fruit has started to dry and wrinkle 
 before it is harvested, the higher tempera- 
 ture may be used without low-temperature 
 pre-drying. 
 
 3. After dipping, dehydrate at low 
 temperature of 135° to 140° until drying 
 is complete. This procedure has the dis- 
 advantage of requiring about 60 hours, 
 and is feasible only when plenty of tunnel 
 capacity is available, such as in the early 
 part of the season. 
 
 Culling for Storage 
 
 The prunes should be allowed to cool 
 after they are taken from the dehydrater. 
 They should then be scraped from the 
 trays into clean, wooden boxes for stor- 
 age. During this process, inspect the 
 prunes and hand-sort out such culls as 
 slabs, cracks, and scabs. 
 
 For storing, boxes are preferred to bins 
 as being more sanitary and less apt to 
 cause damage to the fruit. Dried prunes, 
 as a rule, are not held long in ranch stor- 
 age, and are delivered to the packer in 
 sacks or boxes. 
 
 i ii i 
 
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