384 MICROBIOLOGY OF SPECIAL INDUSTRIES. 



green coloring matter so as to have a clear liquor in the can, and second, 

 to drive water into the vegetables so that all will be tender. There is a 

 consequent tendency toward a paleness in products which undergo the 

 blanching process. Sometimes an artificial color is produced by the 

 addition of small amounts of copper sulphate, but this practice has never 

 become general in the United States. 



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

 tables, the high temperature essential to sterilization causes a slight 

 amount of mechanical disintegration, which is not objectionable, however, 

 unless excessive, 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 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. Change of color often 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 oxidation of the chlorophyl 

 as a result of which the vegetables assume an unsightly yellowish or 

 grayish green appearance. Goods may sometimes be discolored by the 

 formation in the container of metallic sulphides which arise through the 

 liberation of simple sulphur compounds, by decomposition of protein 

 portions of the product, the sulphur afterward combining with the tin. 

 Abnormal color is sometimes attributed to the presence of iron in the 

 water used in the process. ^ 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 

 ordinary preparation of the food for immediate consumption. 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, inas- 

 much as this change only occurs in the presence of acids, and in foods 

 with a high acid content the proportion of starch is relatively low. The 

 other amyloses probably undergo little if any change. 



Palatability and Digestibility. It is often contended that canned 

 foods are less palatable than fresh foods of the same kind. This deterio- 



