HEAT IN THE PRESERVATION OF FOOD PRODUCTS 527 



object of blanching is first, for the purpose of removing the more or less 

 gummy substance upon the surface of such vegetables as peas and 

 beans; second, to make the product more or less flexible so that it may 

 be packed without breaking, as asparagus; third, to permit the packing 

 of a greater quantity in a can, as spinach; fourth, to force water into 

 the product and cause it to be tender, as in beans; fifth, to secure a 

 more uniform color, as in fruits; and sixth, for its cleansing effect. It 

 is not a bleaching process as many infer from the term. The time used 

 in blanching varies from one to fifty minutes, usually being between 

 two and five minutes. The operation is of no value in reducing the 

 time necessary to properly process canned foods. 



Mechanical Disintegration. In the case of very soft fruits or vege- 

 tables, the high temperature of processing causes a slight amount of 

 mechanical disintegration, which is not objectionable unless exces- 

 sive, as there is little deterioration in appearance and none at all in 

 food value. In the case of meats, practically the only physical change 

 is the shrinkage during the parboiling previous to placing in the cans. 



CHEMICAL CHANGES. Appearance. The chemical changes in foods 

 preserved by heat may be considered under two heads: first, those in 

 which the appearance is modified; and second, those in which the food 

 itself is altered. Some change of color sometimes occurs and results 

 from various causes. In colored vegetables, such as peas, string beans, 

 and asparagus, a part at least of the loss of color is due to the oxidation 

 of chlorophyll. With a few foods, iron sulphides are occasionally 

 formed by a combination of sulphur with the iron of the container. 

 This seldom occurs, however, and is not of great importance. Some 

 fruits packed in glass gradually lose their color by oxidation on exposure 

 to the light. 



Chemical Change. So far as chemical alteration of the food itself 

 is concerned, there is little change and none other than would occur in 

 the preparation of the food for the table. The albumins are coagulated. 

 The fats probably remain unchanged. Of the carbohydrates, the chief 

 action is on the sugars. The cane sugar is wholly or partly inverted 

 by the combined action of the heat and the fruit or vegetable acids. 

 The starch undergoes little if any cleavage, inasmuch as this change 

 occurs only in the presence of acids and in foods with a relatively 

 high acid content, the proportion of starch is low. The other amyloses 

 undergo little if any change. 



