DEHYDRATION OF VEGETABLES 3 



in studies on the blanching of vegetables for canning, found losses of 1.5 to 20 

 per cent of the water-soluble constituents in water blanching, and relatively 

 little loss in live steam. More recently Adam, Horner, and Stanworth (1942), 

 in quantitative studies on the effect of blanching on vegetable nutrients, sub- 

 mitted data which again showed the superiority of steam blanching in regard to 

 retention of nutrients. 



Little information is as yet available on the nutrient losses that occur during 

 the actual dehydration process, but analysis of foods prepared and dehydrated 

 under properly controlled conditions would indicate that they compare quite 

 favorably in nutritive value with similar products preserved by canning. Morgan 

 and Lackey (1934), in comparative studies on the retention of nutrients in spinach, 

 found 100 per cent retention of thiamin in untreated, dehydrated spinach, and 

 77 per cent retention in spinach which was steamed for 2 minutes before dehy- 

 dration. Canned spinach retained 26 per cent of the vitamin. Their report also 

 showed IOC per cent retention of minerals in the untreated dehydrated product. 

 These data indicate little or no loss of the water-soluble constituents during the 

 actual dehydrating process. It should be noted, however, that steaming for 2 

 minutes is not sufficient to properly blanch spinach. Experiments in this labora- 

 tory show 78 per cent retention of the minerals in spinach subjected to live steani 

 for 7 minutes previous to dehydration, which compares quite favorably with data 

 on the canned product. 



Although dehydrated foods, like foods preserved by other methods, undergo 

 a proportional loss of some of the vitamins and minerals during the processes of 

 preparation, their value as a source of energy should not be disregarded. In an 

 all out war program, and with the increasing scarcity of foods, the problem of 

 supplying an adequate energy intake is also of paramount importance. 



THE DEHYDRATION PROCESS 



Dehydration as applied to foods is generally defined as the evaporation of 

 moisture from the product by artificial heat under carefully controlled conditions 

 of temperature, humidity, and air flow. This process involves bringing currents 

 of relatively dry air into intimate contact with the material to be dried. Although 

 the principles of dehydration are well known, a brief summary of the functions 

 of the various factors, heat, humidity, and air flow, in the dehydrating process 

 are included for a clear understanding of the problems involved in small-scale 

 work. 



Functions of Air 



Air is the common medium used in the dehydration process, acting as a carrier 

 for both heat and moisture. It conveys the heat from the heat source to the prod- 

 uct, and at the same time picks up and carries off the moisture liberated by the 

 product. A given amount of heated air passing over a moist surface undergoes a 

 drop in temperature proportional to the amount of moisture picked up. Other 

 conditions being equal, therefore, the rate of dehydration is directly proportional 

 to the volume of air passing over the product. For that reason, a dehydrating 

 unit must be provided with a means of circulating the air. 



Functions of Heat 



Energy in the form of heat is required not only for vaporization of the moisture, 

 but also to drive the moisture from the cells to the surface of the product where 

 vaporization can take place. The conditions producing these results are fixed 

 by definite physical laws. 



