166 MISC. PUBLICATION 5 4 0, U. S. DEFT. OF AGRICULTURE 



As shown in figure 72 there was a definite relationship between the 

 temperature of drying, the finished moisture, the time in the drier, 

 and the rate and effectiveness of rehydration. It was obvious that 

 samples dried at a dry-bulb of 200° to 180° F. and a wet-bulb of 95° 

 to 140° did not rehydrate so rapidly or so completely as did samples 

 dried at lower temperatures. It will be noted that none of the samples 

 dried at a minimum dry-bulb of 180° and over to less than 3 percent 

 moisture rehydrated successfully. 



Reducing the size of the piece by grinding increased the rate of 

 rehydration in 9 out of 12 samples. After 10 minutes of boiling with- 

 out soaking these ground samples were as well rehydrated as were the 

 whole slices after 17% hours of soaking and 20 minutes of boiling. 

 Even in these ground materials the ability of the sample to rehydrate 

 was decreased as the temperature in the drier was increased and as 

 the finished moisture was decreased. Interpretation of these results 

 would have been difficult or impossible if single tests had been made, 

 and the final evidence was obtained only when the ground samples 

 were shown to react in the same manner as the whole slices, thus in- 

 dicating that rehydration failure was a characteristic of the entire 

 tissue in the sample and was not merely case hardening or other surface 

 effect. 



Since the purposes of rehydration tests in the plant are to check 

 the technique of drying and to insure high quality in the finished 

 products at all times, a clear understanding of the distinction between 

 rehydration and cooking should be made. In the strict sense, rehy- 

 dration refers to the replacement of water in the dry sample, and 

 when the material is held long enough at low temperature, a coeffi- 

 cient of rehydration of 100 will be attained and the shape of the 

 sample will be completely restored. The only change in the sample 

 results from imbibition of water. Cooking, on the contrary, is heat 

 treatment of food for the purpose of bringing about physical and 

 chemical changes, and the only criterion for degree of cooking in 

 vegetables is a change from crisp, hard tissue to a soft, tender texture. 

 The natural taste and flavor of the vegetable should be retained. 

 It has been pointed out that the rate of rehydration is greatly 

 accelerated at high temperatures ; thus rehydration and cooking may 

 proceed at the same time but at different rates, and the change in 

 the chemical composition of the solids at 212° F. may result in 

 complete disintegration of the tissue before rehydration is complete. 

 Moreover, from the standpoint of quality testing there is a limit 

 to the time that vegetables can be held if the finished product is to 

 be edible. Off-flavors develop at less than boiling temperatures, and 

 inedible products result from long boiling. The method for recon- 

 stitution, then, must take into consideration the relative time at 

 which the sample will be held at room temperature and boiled. 

 Establishing these times for each vegetable will make it possible to 

 arrive at conditions that are compatible with optimum quality. 



Nature and Condition of Material 



It is difficult to explain the fact that products of one plant may 

 rehydrate differently from those of another, even though the equip- 

 ment and process are similar in the two plants. It is possible that 



