The investigation here reported was undertaken to determine certain physical principles and their application to dehydration problems in general. The project was not carried to the point where it was possible to consider the modifications necessary for the different varieties of fruits and vegetables. Factors leading to the deterioration of dehydrated products and the relation which the condition of the fresh material may bear to this deterioration are important phases of the problem not here considered. Spoilage of raw food is due principally to the growth of moulds and bacteria. This growth does not occur when the soluble solids are sufficiently concentrated through the reduction, by drying or by other means, of the water present in foods. Even if they are not killed, the moulds and bacteria remain dormant and harmless in the absence of a suitable medium for their growth. Changes in composition, flavour and appearance, however, may also be brought about by the action of the enzymes present in practically all foodstuffs. As these natural catalytic bodies are not always inactivated by the treatment which stops mould and bacterial action they must be considered in working out methods of dehydration. The outstanding advantage of drying as a method of preserving foods is that the weight and bulk of the products are greatly reduced, thus making possible economy in storage and transportation. The production cost of dehydration compares favourably with that of canning. Dried fruits and vegetables are almost as convenient for use in the home as the fresh products. They need no peeling or other preliminary treatment, and soaking and cooking can often be combined. Only the quantity required need be used when the package is opened; the rest will keep in good condition for a reasonable time. “Dried,” “sun‐dried,” “evaporated” and “dehydrated” are the terms most commonly used to describe dried products. Dried indicates drying by any means; sun‐dried indicates drying without artificial heat; and evaporated implies the use of artificial heat. Evaporated refers more particularly to the use of artificial heat in driers depending for their air circulation on natural draught, while dehydrated implies mechanical circulation of artificial heat. The commercial dehydration of fruits has reached a more advanced stage of development than has the commercial dehydration of vegetables, owing largely to the fact that the public is familiar with sun‐dried and evaporated fruits, whereas it knows comparatively little about dried vegetables. During the World War 8,905,158 lbs. of dehydrated vegetables, divided as follows, were shipped to the United States Army overseas: Potatoes, 6,437,430lbs.; onions, 336,780; carrots, 214,724; turnips, 56,224; and soup mixture, 1,860,000. In the years immediately following 1919 the drying of vegetables declined rapidly, and for the last 10 years or more production has been compartively small. To be successful, a dehydration plant must be built where fresh materials are plentiful and reasonable in price. A diversity of products makes possible an operating season long enough to keep the overhead expenses down to the minimum. The products dried, however, should be limited to those for which a ready market exists. The only satisfactory method of operating is to contract for a sufficient acreage to take care of the needs of a plant at a price which will permit both the grower and the drier to make a profit. Material to be dried must be carefully sorted so as to be free of mould, decay and other defects that would lower the grade of the finished product. The stone fruits (apricots, peaches, cherries and plums) must be sufficiently firm to permit mechanical pitting without tearing. Where they are prepared by manual labour they must not be so soft as to stick to the trays. Apples and pears must not be so soft as to crush in the coring and peeling machines. Berries, cranberries and grapes are usually dried whole. Fruit that needs trimming must be avoided, as it not only adds to the cost of operation, but also lowers the grade of the final product. Vegetables, such as beans (snap), cabbage, carrots, celery, corn, parsnips, potatoes, pumpkin, spinach, squash and turnips, are sliced, shredded, diced or cut in desired pieces before drying. Dehydration does not improve the quality of fresh fruits or vegetables, nor does it provide for the satisfactory use of unsound products. At best the process can only conserve the original constituents of the foods, minus replaceable water. Careful handling reduces labour and waste. Bruised tissue is especially susceptible to discolouration and decay. Individual pieces prepared from good stock are more uniform and attractive than those from heavily trimmed stock. Raw materials should be as carefully washed and cleaned for dehydration as for table use. Much of the washing machinery used in canning is suitable for use in dehydration plants. A rotary cylindrical washer equipped with a water‐spraying system is very satisfactory for washing many types of products. Soft or easily broken fruits and vegetables may be washed by passing the trayed material between several sprays of cold water. The segregation of fresh fruits and vegetables according to size facilitates both the preparatory handling and the drying. One type of grader consists of a perforated metal plate, 3 by 10 feet, or larger. The perforations are in sections of varying size, and the plate is inclined and mechanically agitated in order to insure an even flow of the material in one direction. The product is separated according to size by being passed through the perforations. Perforated plates are also used in stacks. Several plates, each stamped with holes of a uniform size, the holes varying in size with each plate, are set one above the other, with 6 inches or more between plates. They are arranged so that the holes are progressively smaller from top to bottom. Another grader sorts out easily rolling materials according to diameter. As a mechanically driven cable rolls the materials along an opening that increases in width, the product falls through and is collected according to size. A grader based on the same principle passes the product down a chute the floor of which consists of rollers placed at increasingly greater distances apart. As the product rolls along the chute it is separated in progression according to size.
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