VACUUM-PRESSURE IN GLASS CANNING JARS 11 



Although from Figures 3 and 4 one might conclude that a fully sealed jar 

 vents only once or twice during the whole processing period, this is not the case, 

 for in water-bath processing one can see the continually escaping gas bubbles 

 once the jar vents. This venting continues steadily until the processing period 

 is completed. Headspace is also of little consequence in fully sealed jars. The 

 amount of pressure developed depends more on the condition of the rubber 

 ring, the tension of the bail, and the fit of the cover. An increase in the filling 

 temperature and in the initial temperature of the water bath serves only to 

 cause a more rapid rise of internal pressure, as shown in Figure 4. 



Internal Pressures Developed in Pressure-Cooker Processing 



While the fully sealed method was found desirable to use in water-bath 

 processing, the advantages of the method were still more noticeable in pressure 

 processing. It is in the latter method of heat preservation that the largest 

 losses of liquid are encountered. Representative data are presented in Figure 5. 

 If a jar is partially sealed, placed in a pressure cooker, and processed at a steam 

 pressure of 10 pounds per square inch, the pressure within the jar will also be 

 10 pounds. "When the pressure is released from the pressure cooker, the excess 

 jar pressure will also be released at the same rate. Thus, liquids and vapors 

 are lost to such an extent that there is a noticeable loss in volume of contents of 

 the cooled jar. Although a fully sealed jar, heat processed under similar con- 

 ditions, will also develop an internal pressure of approximately 10 pounds per 

 square inch, it will release this pressure so slowly, upon release of the cooker 

 pressure, that there will be practically no loss of liquid from the jar contents. 

 It was to prevent the loss of jar contents and to improve the appearance of the 

 finished product that the fully sealed method was originally tried for pressure 

 processing. Yet, with the improved appearance there is no sacrifice of keeping 

 quality. The internal changes occurring during the heat processing are prac- 

 tically the same in both methods. This is clearly shown in Figure 5. When 

 the jars were filled cold and heated in a pressure cooker at 240° F., curves 

 similar to Figure 5 were obtained. 



Ordinarily, pressures in partially sealed jars rise and fall with the cooker 

 pressure, but occasionally they may lag behind the cooker pressure because of 

 a faulty cover seal which causes the jar to act like a fully sealed jar. Also, from 

 the data one can see that after the petcock of the cooker was closed, each jar 

 apparently vented only once or twice. This was due to fluctuations of cooker 

 pressure beyond the control of the operator. A decrease or an increase of as 

 little as 0.25 pound per square inch in the cooker made a noticeable corre- 

 sponding change in the jar pressure. Theoretically, if a cooker pressure does 

 not fluctuate but rises constantly to the desired point and remains there, the 

 fully or partially sealed jar will not vent until the cooker pressure is released. 

 However, this is improbable in actual practice, for even automatically con- 

 trolled pressure cookers shut off at regular intervals during the processing 

 period long enough for venting to take place. 



Fully sealed jars are slightly slower in attaining retort pressure than partially 

 sealed jars, but also release their pressures more slowly in cooling, thus equal- 

 izing the time held at a definite pressure. 



