VEGETABLE AND FRUIT DEHYDRATION 75 



mg than will onions or cabbage. Carrots and rutabagas are inter- 

 mediate in sensitiveness. Potatoes are almost as sensitive as cab- 

 bage (31). Variety, maturity, and manner and length of storage be- 

 fore use mav have decided effects upon safe dehydration temperatures. 

 The directions for drying specific commodities given in later sections 

 are based upon average qualities of the vegetable varieties usually de- 

 hydrated, and will require varying amounts of modification for various 

 types of raw material. 



Scorching is an effect not only of temperature but also of time. The 

 characteristic marks of damage will appear at temperatures as low as 

 120° F. if the time of exposure is long enough. Very roughly, the 

 time required to produce noticeable scorching will be cut in half for 

 each 15° of rise in temperature. 



The moisture content of the material also appears to have an 

 influence on sensitiveness to temperature, according to Mangels and 

 Gore (31). The conclusion reached as a result of relatively few 

 experiments is that signs of scorching occur in a shorter time if the 

 moisture content is 10 to 15 percent than if it is lower. A few observa- 

 tions on the drying of cabbage at high wet-bulb temperatures have 

 indicated heat damage within a few hours at 110° F. wet-bulb, even 

 though the material was still very moist. 10 Scorching apparently 

 occurs as readily in a vacuum as in air. 



Damage to Nutritive Quality 



The drying processes currently used for fruits and vegetables have 

 little adverse effect on the nutritive value of the products. Carbohy- 

 drates, fats, proteins, and most of the vitamins come through the dry- 

 ing with substantially no loss. The one major exception is ascorbic 

 acid. This vitamin is lost in all stages of preparation and drying, 

 and continues to disappear during storage of the dried product. Loss 

 during the drying step ranges from less than 10 percent to more than 

 75 percent; it occurs both while the material is very moist and when 

 it is nearly dry, at approximately equal rates. The effect of product 

 temperature on the rate of loss has not been determined. Since the 

 disappearance of ascorbic acid is probably caused by oxidation, drying 

 in a vacuum or an inert gas instead of air would presumably decrease 

 the rate of loss. 



Damage to Color and Flavor 



Scorching is recognizable through the development of a yellow or 

 brown color, the appearance of a burnt flavor, or both. Careful meas- 

 urements of color show that some yellowing occurs, and frequently 

 some bleaching of the natural color, even if the drying is completed 

 without such gross damage as would be classified as scorching. These 

 changes are apparently effects of both time and temperature, but the 

 exact connection has not been established. 



Effects of drying on the flavor of the product, aside from scorching, 

 are generally in the direction of loss of characteristic aroma and taste, 

 probably through the evaporation and loss of volatile flavoring sub- 

 stances. The presence of these substances is sometimes conspicuously 



10 The important temperature is that of the piece itself. While the piece is moist this 

 may be close to the wet-bulb temperature of the air ; later it will be close to the dry-bulb 

 temperature. Thin edges and corners will dry rapidly, while the rest of the piece is still 

 much cooler, and may be the only portions that show heat damage. 



