TEMPERATURE CHANGES IN CANNING FRUITS AND VEGETABLES. 5 



the can attains or approaches closely that of the retort in less than 

 15 minutes. The prime advantage of the apparatus as described by 

 these authors is that it allows the determination of the temperatures 

 at various parts of the can at any time, thus giving a true idea of the 

 rate of heat flow within the material itself. It is obvious, however, 

 that the readings obtained under the conditions described are no 

 more accurate than may be obtained by the direct reading of the 

 mercury thermometer. 



Castle (G) in 1919 called attention to the fact that the depth of 

 the water bath about the jars directly affects the rate of change of 

 temperature at the center of the containers, the shallower the bath 

 the slower the rate of change. She also pointed out that in the inter- 

 mittent process the first cooking may so compact the material that 

 the heat penetrates more slowly in the second and third heatings. 

 This was found to be true for leafy vegetables. No differences were 

 obtained in the rate of change of temperature in blanched and un- 

 blanched string beans, and she erroneously concluded that blanching 

 does not permit closer packing of this product. 



Thompson (13) in 1919 published a preliminary report upon a 

 large amount of valuable work dealing with temperature-time rela- 

 tions in various fruits and vegetables during processing, in which he 

 made use of thermocouples. From these tests he developed mathe- 

 matical formulas the object of which was to make possible the cal- 

 culation of the temperature at the center of the can at any time 

 during the processing, starting at any initial temperature. 



Such formulas would be of great value if they could be made 

 applicable to the handling of all food substances canned. In using 

 such formulas, however, it is necessary to assume that all heat trans- 

 ferred is by conduction or else that any convection is very local. This 

 would make the method inapplicable, apparently, for determining 

 temperature changes in cans of substances such as string beans and 

 peas, in which there is free convection, and would limit its useful- 

 ness to the canning of substances of heavy consistency, such as corn 

 and squash. Inasmuch as the use of these formulas depends upon a 

 constant factor, k (which in itself varies with different methods of 

 processing, different containers, different kinds of food materials, 

 differences in packs, and in some cases differences in varieties, stages 

 of maturity, and other factors), it would seem that the establishment 

 of the necessary constants would be very difficult and would in itself 

 necessitate determinations which would give directly the original 

 time-temperature facts desired. Furthermore, in certain substances 

 the heat is carried inward during the first part of the processing 

 period by convection, and in the latter part almost entirely by conduc- 

 tion. In other cases there is a change in the material in the can 

 during processing resulting in the reverse of this, the heat passing 



