9 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. (a 
: | 
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. applicatits | 
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 a processed under ik | 
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 ie 
cooling curve. Furthermore, during processing permanent physical — 
changes which alter ereatly 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 ad 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 variety 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 
