TEMPERATURE CHANGES, ETC., DURING CANNING OPERATIONS. 7 



glass arm A with the test can F, located in the retort ft, as 

 described for temperature tests in United States Department of 

 Agriculture Bulletin No. 956, entitled "A Study of the Factors Af- 

 fecting Temperature Changes in the Container during the Canning 

 of Fruit and Vegetables." Ketort temperature is controlled by the 

 thermometer T. During the test the pressure which causes the level 

 of the mercury to fall in arm A is compensated for by raising arm 

 B, and the mercury in arm A is therefore maintained at the constant 

 level x, corresponding to the zero graduation on scale D. The vol- 

 ume of gas in the retort side of the apparatus is kept constant and 

 the pressure in centimeters or inches, as desired, read directly from 

 the -f- portion of the scale. Similarly, the vacuum is read by lowering 

 arm B, to compensate for the rise of mercury in A, and the vacuum 

 is read directly from the — portion of the scale. The can being at- 

 tached to the cover of the small retort, the flexible tube E allows the 

 prompt removal of the test can from the retort in a few seconds and 

 the cooling of the can either in air or in water, as desired. 



This apparatus is very sensitive to slight changes in pressure, 

 demonstrates very quickly any leaks present, and has proved 

 very satisfactory. By means of this apparatus the writers have been 

 able to conduct tests upon all classes of substances in various quanti- 

 ties with different sizes of cans at any initial temperature and retort 

 temperature desired. 



DETERMINATION OF TEMPERATURE. 



Data upon temperature changes were obtained in the same manner 

 as described in United States Department of Agriculture Bulletin 

 No. 956, and all temperatures, both of retort and can, were made with 

 special carefully standardized mercury thermometers. 



In the paper previously reviewed, Bigelow and his collaborators 

 object to the use of a mercury thermometer for this purpose, on the 

 ground that heat is conducted down the stem to the mercury bulb. 

 According to Smithsonian conductivity tables, the conductivity of 

 glass and water is practically the same. It is evident, therefore, that 

 the conductivity of glass is about the same or slightly less than the 

 minimum for any food substance. Since, also, the distance from 

 the ends of the can to the center is greater than the radius, the heat 

 should reach the mercury bulb through the food from the side before 

 it could be conducted down the stem of the thermometer. In the 

 thermocouple used by Bigelow and his collaborators, which is in- 

 serted into the can in the same way as the thermometer, the thermal 

 junction is soldered to the end of a bare copper tube, which in turn 

 is soldered to the end of a brass tube, the external portion of which 

 during processing is subjected to the full temperature of the retort, 

 which allows of rapid conduction of heat into the can. With the 



