2 BULLETIN 1022, U. S. DEPARTMENT OF AGRICULTURE. 



changes in foods during canning, but a detailed consideration of the 

 important influence which initial temperatures have upon the form 

 of the heat-penetration curves and their bearing upon sterilization 

 processes was reserved until initial temperatures could be taken up 

 in their proper relations with exhaust, pressure, and vacuum. 



Since the above-mentioned paper was submitted for publication 

 two further contributions have been made. The first of these, by- 

 Thompson (9), enters into a discussion of the theoretical application 

 of physical laws to canning procedures. In addition to the consider- 

 ation of the general phases of the subject and a study of the diffu- 

 sivity constant " K " dealt with in his original paper, this author 

 gives valuable formulas for calculating the approximate time-tem- 

 perature curves for cans of food when processed at different retort 

 temperatures, other factors being the same, and also for calculating 

 the curves for different sizes of cans when processed under like 

 conditions. 



Thompson's study of the time-temperature curves for cooling, how- 

 ever, suggests that such curves might be calculated directly from the 

 heating curves, inasmuch as reversing the temperature conditions of 

 the water bath from hot to cold should result in a curve of the same 

 form as the heating curve. That this can hardly be the case with 

 foods will readily be seen. The viscosity of the liquor varies with 

 the temperature. Except where heat brings about a change in the 

 colloidal substances, the heating curve is always a record of a change 

 from a more to a less viscous condition, while in cooling, without ex- 

 ception, the change is from a less to a more viscous condition. The 

 curves will not be the same, as in this case the final portion of the 

 heating curve will be steeper than the corresponding part of the 

 cooling curve. Furthermore, during processing permanent physical 

 changes which alter greatly the rate of heat transfer within the 

 container commonly occur in food substances ; hence, even the rough 

 approximation of the real cooling curve by calculating from the 

 heating curve would be difficult, if not impossible. While the dif- 

 ferences between the heating and the cooling curves may be negligi- 

 ble from a practical standpoint in some cases, in others they are of 

 very considerable importance. An actual experimental cooling curve 

 would seem to the writers to be the safest way to determine tem- 

 perature changes during cooling. 



In a more detailed and comprehensive manner Bigelow and his 

 collaborators (3) have studied the time-temperature relations of 

 foods during canning. These workers, by the use of a specially con- 

 structed thermoelectric apparatus, have carried on extensive investi- 

 gations upon a wide varietj 7 of canned-food materials and have made 

 important contributions to the knowledge upon this subject. In addi- 

 tion to the consideration of both theoretical and practical matters 



