14 THE CANNING OF FOODS. 



only when it is desired to preserve a very fine appearance. This 

 may be accomplished above 180 F. by maintaining the temperature 

 for a longer time than when boiling, or by repeating the operation 

 on two or more successive days. The object is to prevent breaking 

 the tissue and loss of juices from the fruits by excessive heat. This 

 method of sterilization has been applied experimentally and in 

 private canning with gratifying results, but it involves so much time 

 and labor that it is not used commercially except in a limited way. 

 Sufficient work has not been done to say definitely what products can 

 best be treated in this way nor what temperatures are best suited for 

 different foods. It has been used chiefly with goods in glass, though 

 equally satisfactory results are obtained with foods in tin. 



Cooking at boiling temperature is practiced with nearly all fruits, 

 as the germs present are easily destroyed. Most of the fruits are 

 processed for from 12 to 25 minutes. The tomato is the most im- 

 portant vegetable processed at boiling temperature, which is usually 

 maintained for 50 minutes. 



Cooking at a temperature above the boiling point* is necessary or 

 advantageous for most vegetables, fish, milk, and meats. It is accom- 

 plished in retorts where steam is admitted under pressure, in retorts 

 where water can be superheated, on the open calcium chlorid or oil 

 bath. 



Among the vegetables requiring a high temperature in processing 

 are corn, peas, beans, both green and dry, pumpkin, beets, and sweet 

 potatoes. Corn is one of the difficult products to can, requiring a 

 temperature of from 245 to 250 F. for from 75 to 80 minutes, de- 

 pending to a considerable extent upon how dry it is packed. If 

 very dry, the heat will penetrate to the center of ihe can very slowly, 

 the actual time required to raise the center to the temperature of the 

 bath being from 55 to 65 minutes. In a can of peas this is accom- 

 plished in 6 or 7 minutes, the difference being due to the fact that 

 heat currents are set up in the liquid portion of the peas while they 

 are absent in the corn. The necessity for a high temperature is 

 therefore dependent upon the ease with which the heat can penetrate 

 the product, as well as the resistance of the organisms. Some prod- 

 ucts which were formerly processed by boiling for a long time are 

 now given a higher temperature for a few minutes, as the product 

 has a much better appearance when it is not overcooked. 



Meat products, as a rule, contain highly resistant organisms, besides 

 which the majority of these foods are of such a consistency that the 

 heat penetrates them very slowly. As a class they require the heav- 

 iest process. Milk also contains very resistive germs, but being 

 liquid it heats rapidly; in order to keep it smooth and prevent the 

 portion in contact with the tin from scorching, the cans are turned 

 or agitated almost continuously during the cooking. 



