36 MISC. PUBLICATION 5 4 0, U. S. DEPT. OF AGRICULTURE 



readily passing through the ricer free of lumpy material. For the 

 cooking of quartered material in flowing steam the retention time may 

 range from 20 to 40 minutes. 



BLANCHING 



The terms "blanching" and "scalding," as used in the canning, freez- 

 ing, and dehydration industries, refer to the practice of heating the 

 prepared raw food product, in live steam or boiling water, for a short 

 period prior to the principal preservation treatment. In dehydration, 

 blanching is practiced for two primary reasons: (1) To prevent or 

 check the development of undesirable colors, flavors, odors, and the 

 loss of vitamins during dehydration and storage, and (2) to obtain 

 a finished product that will rehydrate and cook rapidly and yield a 

 cooked product of desirable texture. In addition, blanching destroys 

 many of the microorganisms in the raw products. Fruits are not 

 blanched, although blanching has been suggested for certain cut fruits. 

 Recommended blanching treatment varies with the vegetable ; specific 

 recommendations are presented in a later division of this publication. 



The importance of blanching as a means of preserving quality is 

 exemplified by potatoes; if not blanched, they will come from the 

 dehydrator in a discolored condition and after a few days of storage 

 will develop rancid odor and flavor. Similarly unblanched dehy- 

 drated carrots lose their characteristic color during storage and soon 

 develop a stale odor and flavor. With green snap beans blanching 

 influences the time required for reconstitution and cooking and also 

 the quality of the cooked product. In one experiment conducted at 

 the Western Regional Research Laboratory it was observed that un- 

 blanched dehydrated snap beans absorbed about 1.1 grams of water 

 per gram of dry product on soaking in water for 4 hours, as compared 

 with 1.9 grams for material that had been blanched in steam for 2 

 minutes, and 2.3 grams for material blanched in steam 10 minutes. On 

 rehydration, the difference between certain blanched and unblanched 

 materials is apparent. The unblanched material tends to remain 

 shriveled and tough in texture, whereas blanched material becomes 

 plump and tender. 



Adequacy of Blanching as Measured by Tests for Enzyme Activity 



It is generally believed that at least a part of the deterioration in 

 quality that occurs in unblanched dehydrated vegetables is the result 

 of enzyme action. Enzymes are substances present in all living cells, 

 and their function is to accelerate chemical reactions in the cell. In 

 the absence of these accelerators, life would not be possible, since the 

 chemical reactions necessary to life would proceed too slowly or pos- 

 sibly not take place at all. It may be assumed, then, that if the enzymes 

 are inactivated or destroyed, some of the chemical reactions leading 

 to the development of undesirable qualities in dehydrated vegetables 

 will be retarded or prevented. Since enzymes can be destroyed by 

 heat, it has been assumed that the tendency of blanched dehydrated 

 vegetables toward longer storage life than unblanched is due, at least 

 in part, to the inactivation or destruction of the enzymes as the result 

 of blanching. 



On the basis of this assumption, tests for the presence or absence of 

 certain enzymes have been adopted as criteria of the adequacy of 



