8 MASS. EXPERIMENT STATION BULLETIN 341 



Effect of Temperature Alone on Internal Pressure 



In order to study separately the physical factors involved during the pro- 

 cessing of glass jars, it was necessary to utilize some means whereby only one 

 factor at a time caused any change. Temperature and pressure are the two 

 main physical factors. To study retort temperatures without the influence of 

 external pressure, an oil bath was used because oil would reach a temperature 

 equal to that attained in a retort having 10 pounds steam pressure, namely 

 240° F. Venting points in experimental jars immersed in the oil bath were 

 readily recognized by one to two millimeter fluctuations in the mercury column. 



Figure 2 shows graphically the development of pressures and resultant vent- 

 ing in partially and fully sealed jars filled with water and heated in an oil bath. 

 The data show that the partially and fully sealed jars vented at pressures of 

 0.1 and 1.8 pounds per square inch, respectively. 



The elapsed time before venting occurred was not so important as the 

 pressures at which venting occurred because in heating the oil, care had to be 

 taken to prevent violent agitation. The electric heating unit used to regulate 

 the heat was not subject to fine adjustments. 



2.0 



M 1.5 



R 0.5 



240°F. 



30 40 



Time in Minutes 



Figure 2. Pressure Developed in Partially and Fully Sealed Glass Jars Processed In an 

 Oil Bath through the Temperature Range of 75° to 240° F. 

 (Headspace 75 cc.) 



Effect of Pressure Alone on Internal Pressure 



In order to study retort pressure effects without the influence of heat, an 

 air pressure pump was attached to the petcock of a small pressure cooker. The 

 experimental jar with a tube leading from the cover of the cooker to the U-tube 



