TEMPERATURE CHANGES, ETC., DURING CANNING OPERATIONS. 5 



the subsequent development of the organisms causing spoilage must 

 be provided for first, but for the sake of the quality it should not be 

 prolonged beyond the time essential to insure the keeping of the 

 product and the safety of the food for human consumption ; in other 

 words, the temperature factor must be under careful control. 



Another factor greatly affecting the quality of the finished product 

 is the amount of air present in the can. The presence of air may 

 result in enormous strains on cans, causing leakage and loss, in the 

 discoloration of the product, and in increased activity of the food 

 material upon the metal of the container. In other ways also it has 

 a direct bearing upon the quality of the product. It is evident, there- 

 fore, that this factor likewise must be kept under control. This may 

 be accomplished by some method of exhaust. 



Inasmuch as under present conditions the exhaust is effected pri- 

 marily by the use of heat and inasmuch as the initial temperature of 

 the material in the can affects the time-temperature curves, it is ap- 

 parent that in determining the proper temperature and duration of 

 the processing period the exhaust must be carefully considered. In 

 other words, the exhausting procedure must be standardized before 

 the processing temperatures can be prescribed and the processing 

 periods determined. 



As is well known, exhausting may be accomplished in three ways : 

 First, by filling the cans with the material, after it has been heated to 

 the desired temperature, as with corn, sweet potatoes, etc.; second, 

 by subjecting the cans of material to a preliminary heating before 

 sealing; and, third, by mechanically removing the air, or sealing in 

 a vacuum chamber. 



Whichever method is used, it is especially important that so far as 

 possible uniform temperatures shall exist throughout the entire mass 

 in the can and that processing shall begin at the earliest possible 

 moment, preferably at once. As has been shown graphically by 

 Bigelow and his collaborators (3, p. 63), unless this is the case the 

 time-temperature curve at the center of the can may actually fall dur- 

 ing the first part of the processing period and thus make impossible 

 an accurate knowledge of the temperature changes taking place dur- 

 ing the processing period. Under such conditions careful standard- 

 ization is out of the question. 



With these things in mind and with a view to gaining a definite 

 knowledge of the temperature changes taking place under carefully 

 controlled conditions as regards the time-temperature relation in 

 food materials sealed at different temperatures and the relation of 

 these matters to pressures developed and vacuum obtained, the pres- 

 ent work was undertaken. 



