August, 1921 



BETTER FRUIT 



Page 7 



Methods of Prune Drying In Oregon 



By Ray Powers, Commercial Dehydration Laboratory, Bureau of Chemistry, 



THE prune crop constitutes nearly sixty 

 per cent of the total fruits grown with- 

 in the lower Columbia and Willamette 

 V'alleys of Oregon. Italian and Petite 

 prunes are grown in these regions, but the 

 former comprise almost the entire acre- 

 age. However, Petite prunes are preferred 

 by some evaporators and an attempt is being 

 made to grow them on a large scale. An- 

 ah'ses of the Italian and Petite prunes fol- 

 low: 



•IT.\LIAN PRUNES 

 Moisture Invert Cane Acidity Nitrogen 



Sugar Sugar as SO-3 

 Pet. Pet. Pet. Pet. Pet. 



25.".^ 45.31 8.38 1.4 0.47 



26.8(1 46.03 3.80 1.0 0.375 



26.80 41.93 2.51 1.4 0.46 



24.10 50.00 5.30 1.0 0.419 



27.60 34.47 3.20 1.2 0.3SS 



24.95 48.94 2.08 1.1 0.34 



24.25 42.30 5.96 1.0 0.355 



32.20 40.84 2.51 1.2 0.39 



19.65 37.20 8.29 1.4 0.42 



21.50 37.20 4.60 1.4 0.37 



.\v. 25.36 42.42 4.66 1.21 0.393 



•PETITE PRUNES 



24.20 52.95 4.59 0.80 0.24 



19.55 52.22 4.94 0.80 0.30 



19.95 58.78 3.00 0.80 0.244 



24.55 53.78 5.28 1.20 0.316 



Av. 22.06 54.42 4.45 0.90 0.275 



'Analyses by The Oregon Agricultural College. 



Most of the prunes from these districts 

 are dried, but a small percentage reach the 

 market in a fresh or green state. There 

 would be a decided advantage in stabilizing 

 the prune industry of Oregon if a larger 

 portion could be marketed in the fresh con- 

 dition. This problem, however, is ren- 

 dered difficult because of the frequent 

 rains and humid weather during the har- 

 vesting season. Prunes frequently split or 

 crack during the rainy weather, and in 

 this condition they are exceptionally sus- 

 ceptible to attacks by a fungus disease known 

 as brown rot (Sr/erotinia fructigena). 



Both "splits" and brown rot which fre- 

 quently occur together, were found in the 

 1920 season. The loss from these causes 

 is "estimated as 25 to 35 per cent of the 

 crop, while a loss of 50 per cent is esti- 

 mated as the total loss from all causes, if 

 those prunes not harvested be included. 



The season of 1920 differed from ordi- 

 nary seasons in the fact that it was very 

 humid and rainy, with the result that 

 splitting of the fruit occurred frequently, 

 followed by brown rot. Under these con- 

 ditions drying was rendered difficult and 

 costly, and the quality of the finished pro- 

 duct averaged below the normal. To min- 

 imize these difficulties the prunes were 

 usually graded during the harvesting and on 

 arrival at the evaporator. They were then 

 given the preparatory treatment and placed 

 in the evaporator as rapidly as possible. 



Prunes not only comprise the largest 

 acreage within the lower Columbia and 

 Willamette Valleys, but the season for har- 

 vesting, drying and marketing of the crop 

 is exceedingly short, usually from 18 to 

 25 days. These conditions require and de- 

 mand a drier of large capacity in order to 



U. S. Department of Agriculture 



take care of the prune crop. An evaporator, 

 after giving its season's use on prunes, fre- 

 quently remains idle until the following 

 Si.ason. It is natural under these conditions 

 that the attention of prune growers should 

 be turned toward the construction of an 

 evaporator combining large capacity and 

 output with inexpensiveness of construction. 

 The common style of natural draft drier 

 is the revised Allen type now commonly 

 known as the Oregon Tunnel Evaporator. 

 This evaporator can be tjuilt with large 

 capacity for a very reasonable cost and is, 

 accordingly, in most popular demand among 

 prune growers. The cost of a three tunnel 



an opening within the floor of the drier 

 at the end opposite the stack, and at the 

 lowest point of the tunnel. The heated 

 air passing through this opening is de- 

 flected over the fruit by staggering the 

 trays containing the prunes in such a way 

 that the top trays extend farthest out over 

 the incoming air. The heated air after pas- 

 sing over and between the trays escapes 

 through an opening at the top of the drier 

 where it passes up the ventilator. 



The rate of air circulation through the 

 Oregon tunnel type driers was tested and 

 found to range from 50 to 700 linear feet 

 per minute at the point of intake within 



LOOKING THROUGH THE TUNNELS OF AN EVAPORATOR 



evaporator of the Oregon type, with a ca- 

 pacity of 3.5 tons of fresh prunes, is about 

 $3,000 to $4,000 including furnace and 

 tr.ays. 



THE Oregon drier may be briefly de- 

 scribed as consisting of three or more 

 tunnels usually 20 to 22 feet long, each 

 tunnel being 30 to 36 Inches wide (interior 

 dimensions), inclined at a pitch varying 

 from one to two inches to the foot in such 

 a way that the highest part of the drier is 

 at the exhaust or loading end. The tun- 

 nels are located over a cement or brick 

 heating chamber within which a stove, 

 usually of the hop type, is placed. 



The products of combustion from the 

 stove pass through a series of pipes, arranged 

 back and forth across the heating chamber 

 for radiating heat to the surrounding air 

 before passing out at the stack. On each of 

 the four sides of the heating chamber and 

 near the bottom, holes are provided in 

 the cement or brick walls to allow the en- 

 trance of air for heating and circulation. 

 The sizes of the openings vary with the 

 capacity of the drying chamber, but aver- 

 age 8 by II inches for the three tunnel 

 type of drier. 



The heated air from the furnace cham- 

 ber is admitted to the drier above, through 



the drier. The rate of air movement de- 

 pends upon the outside conditions of tem- 

 perature, humidity and air movement, the 

 temperature to which the air is raised within 

 the furnace chamber, the size and dimen- 

 sions of the heating chamber and stove, and 

 the openings for the admission of cold air 

 into the heating chamber. 



From observations on natural draft driers, 

 during the season of 1920, there was found 

 a common practice of partially closing the 

 intake around the furnace chamber and 

 also the ventilator, in order, it was stated, 

 to "hold the heat." These methods are ob- 

 viously faulty when it is recalled that heat 

 without air circulation is practically useless 

 in drying. 



In all driers visited and where no objec- 

 tions were made to the contrary, the ven- 

 tilators and openings around the furnace 

 chambers were opened. The immediate 

 results were to increase air movement and 

 to require a slightly larger fire to heat the 

 greater volume of air. The ultimate result 

 was in most cases that of decreasing the 

 drying time several hours. In one evapor- 

 ator the air circulation was increased from 

 50 to 3 50 linear feet per minute by this 

 method. Another drier reported a decrease 

 in the drying time from 30-4-0 hours to 

 27-34- hours. The latter drying time could 



