COMMERCIAL DEHYDRATION 19 



dried; apples, for example, arc not sun-dried commercially. Prunes 



and clingstone peaches may l>e cither sun-dried or dehydrated, hut 

 freestone peaches, apricots, figs, pears, and nectarines are nearly 

 always sun-dried, as are most raisin grapes, although considerable 

 quantities of seedless grapes are now dehydrated. Dehydration of 

 clingstone peaches and golden-bleached raisins is comparatively new. 

 Increasing numbers of prunes and figs- are being dehydrated. 



Unlike most vegetables, fruits to be dried are rarely blanched in 

 steam or hot water. Prunes and sometimes figs and grapes may be 

 dipped in a hot lye solution as a means of checking (cracking) the 

 skins to facilitate drying. The darker colored fruits — prunes and 

 some varieties of grapes and figs — often are simply washed. 



Preparation of the light-colored fruits usually includes sulfuring. 

 Peaches, apricots, pears, apples, grapes for golden-bleached raisins, 

 and some figs are sulfured. In this process the prepared fruits arc 

 exposed to the fumes of burning sulfur and absorb varying amounts 

 of sulfur dioxide, which protects the color and makes other improve- 

 ments in quality. Sulfur makes the cells more permeable, thus 

 hastening drying, and helps to preserve ascorbic acid (vitamin C), 

 though to a large extent it destroys thiamine ( vitamin BJ . 



Fruit is sulfured on trays in an enclosed chamber or small build- 

 ing, large enough, usually, to hold one or two loaded trucks. The 

 sulfur is preferably burned in a shallow pan on the floor, and the 

 trays are stacked one above the other in zigzag formation to facili- 

 tate the circulation of the fumes. Sulfuring should be as light as 

 possible to accomplish the desired results. 



Fruits are not generally dried to as low a moisture content as 

 vegetables. The greater amount of sugar in fruit, though it aids 

 keeping quality, increases the tendency to case-harden if drying is 

 long continued at too fast a rate. Some fruit products, however, 

 such as apple nuggets — dried apples ground and further dried under 

 vacuum — are dried to a moisture content of 3 percent or lower. 

 Fruit flakes and powders also have a very low moisture content. 



RETENTION OF NUTRITIVE VALUES 



Dehydrated vegetables and fruits supply practically as many cal- 

 ories as fresh ones, and they retain most of their mineral values. It 

 is not possible, at the time of writing, to make any accurate general 

 Statement regarding the retention or loss of vitamins in dehydra- 

 tion. Onlv a comparatively few scattered studies have been made 

 so far, and those mainly with products dehydrated under laboratory 

 or pilot-plant conditions rather than in commercial plants. The 

 figures from these few analyses show a wide range of Losses. More 

 extensive studies of vitamin losses in dehydrated products are under 

 way, however, and when sufficient data are collected it will be pos- 

 sible to form a more accurate picture. Research will continue to 

 stress the development of methods that will make it possible to 

 retain the maximum amounts of vitamins. 



Figures on losses in storage are even scantier than those on losses 

 during dehydration itself. The indications are that they can be 

 heavy under adverse conditions. 



It should be remembered, of course, that practically all methods 

 of food processing reduce the amounts of some of the vitamins in 



