VEGETABLE AND FRUIT DEHYDRATION 29 



when the product is being introduced at normal rates. The coil should 

 be located well above the sludge basin described above, so that normal 

 thermal circulation will not continually agitate the sludge. 



After the product has been chemically treated in the lye vat and 

 drained, it goes directly into a washer for removal of skin and rinsing 

 residual caustic solution from the peeled surface. The simple corru- 

 gated-surface rotary-drum washer equipped with moderately high- 

 pressure water sprays has been used satisfactorily. It is suggested 

 that the washer be proportioned to permit a retention time of not less 

 than 1% minutes. 



A simple test for detecting the presence of caustic on the surface 

 of a washed potato consists of placing a drop of alcohol solution of 

 phenolphthalein on the potato. The appearance of a pink stain will 

 indicate that not all of the caustic has been removed, in which case 

 additional washing is necessary. 



The time of immersion in the caustic bath will depend upon several 

 factors ; namely, the strength of the caustic bath and its temperature, 

 and the type, variety, and age of the product. A typical curve showing 

 the approximate retention time for potatoes in different lye concentra- 

 tions is shown on figure 13 (48). 2 Probably the best results can be 

 obtained with moderately dilute solutions, inasmuch as the weaker 

 solutions can be much more readily removed from the product in the 

 washer than concentrated solutions. It is desirable that control runs 

 be performed on each type of product to determine the correct reten- 

 tion time prior to full-scale operation. 



Several means have been devised for maintaining constant bath 

 concentrations, but none has been firmly established. A simple means 

 is to keep close watch on the peeling results and add lye as required. 

 After a brief period of operation, the operator will become skilled in 

 determining the caustic requirements of the system per sack of product 

 peeled. 



Lye (commercial caustic soda) is obtainable in either flake or cake 

 form. The former is more expensive but is more readily handled. 

 Cake lye is more difficult to handle because it must be either broken 

 up or melted through the use of a steam jet thrust into the container. 

 It has the advantage, however, of being cheaper than the flake lye. 

 In some localities commercial 50-percent caustic soda solution may be 

 obtained in tank-car lots. It may be stored in steel tanks, which must, 

 however, be provided with heating coils, since the freezing point of 

 the heavy solution is about 53° F. The solution can be measured to 

 the peeler by means of a weir or small measuring tank. 



Retort peeling. — Retort or steam peeling has recently attracted at- 

 tention, along with brine and flame peeling, as a heat process for 

 peeling potatoes. Essentially, retort peeling consists of subjecting 

 a product to steam under pressure for a short period, thus loosening 

 the skins so that they can be removed by washing and scrubbing. 



At least one commercial plant reports satisfactory results. Pota- 

 toes are subjected to steam under 50 to 60 pounds per square inch of 

 gage pressure for a period of 30 to 45 seconds. This pressure corre- 

 sponds to a temperature of about 300° F. At this temperature the 

 flesh immediately adjacent to the skin is cooked to the softening point 

 and the skins are removed in a washer. 



In any type of equipment now commercially available, the proce- 



8 Italics numbers in parentheses refer to Literature Cited, p. 216. 



