2 BULLETIN 196, U. S. DEPAKTMENT OF AGKICULTUEE. 



The tomato is the most important 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 accomplished in retorts where steam is 

 admitted under pressure, in retorts where water can be superheated, or on the open 

 calcium chlorid or oil bath. 



Among the vegetables requiring a high temperature in processing are com, peas, 

 beans, both green and dry, pumpkin, beets, and sweet potatoes. Com is one of the 

 difficult products to can, requiring a temperature of from 245° to 250° F. for from 75 to 

 80 minutes, depending to a considerable extent upon how dry it is packed. If very 

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

 required to raise the center to the temperature of the bath being from 55 to 65 min- 

 utes. In a can of peas this is accomplished 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 com. The necessity for a high temperature is therefore depend- 

 ent upon the ease with which the heat can penetrate the product, as well as the 

 resistance of the organisms. Some products which were formerly processed by boil- 

 ing for a long time are now given a higher temperature for a few minutes, as the prod- 

 uct 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 heaviest process. Milk also contains very 

 resistant 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. 



DETERMrNATION OF TEMPERATURE AND TIME OF PROCESSING. 



In sterilizing, the heat must be applied equally to all cans, and it is therefore neces- 

 sary to deliver steam at the bottom of the kettle, whether open or in a retort, to insure 

 a circulation of the heat. In retorts, whether steam or hot water is used, there must 

 always be a vent open to give off steam in order to hold the heat uniform at all points. 

 The thermometer is the all-important tester, for if it does not show the proper degree 

 of temperature, spoilage will follow. To test the uniformity of temperature in a 

 retort, self-registering thermometers are sealed in a number of cans when placed in 

 the crates, the cans are marked, and when the cooking is completed the thermometers 

 are examined and compared, so that the heat may be adjusted until all give like read- 

 ings. In a similar manner the time required for the heat to reach the center of the can 

 is obtained, experimental lots being run for varying periods, and the temperature 

 noted. The calcium chlorid or oil bath acts in the same way as the open water bath. 



The writer employs two methods of determining the temperature in the center of 

 a can and the rate of penetration. First, a thermometer is placed in a packing joint 

 which is soldered into the can so that the bulb \nl\ just reach the center. By placing 

 a collar an inch above the gasket the can may be submerged in oil and heat applied 

 until a certain temperature is reached. The length of time necessary for the ther- 

 mometer inside the can to reach the same point as that on the outside, or within from 

 2 to 5 degrees of the outside, as experience demonstrates maybe sufficient, must be 

 allowed in the retort and the heating then continued for such an interval as may be 

 found necessary for sterilization. For example, if the spores of certain organisms are 

 killed at 230° F. in 12 minutes, and it should take 20 minutes to cause the content 

 of the can to become heated, it would require 32 minutes as a minimum for process- 

 ing, and as a margin of safety the recommendation would be for a longer time, 

 probably for 40 minutes. 



The second method of determining temperature in different parts of the retort and 

 in the center of cans is to seal a thermocouple in the can and connect it with a record- 



