VACUUM-PRESSURE IN GLASS CANNING JARS 9 



manometer was placed inside of the retort and the cover clamped down tightly. 

 The jar contained water at 70° F. and had a one-half-inch headspace. The 

 pump was started and air flowed into the retort. Seven pounds was the maxi- 

 mum pressure obtainable in the cooker as indicated on its gauge. Jar pressure 

 readings were made as usual on the mercury column of the manometer. Pres- 

 sures obtained in three partially sealed jars were 0.68, 6.77, and 6.50 pounds 

 per square inch, respectively and for three corresponding fully sealed jars: 

 0.1, 0, and pounds per square inch. 



There was a slight water loss in the partially sealed jars but none in the fully 

 sealed jars. The low rise in pressure in the first partially sealed jar is thought 

 to be due to the rapid increase of retort pressure which made its way into the 

 partially sealed jar only to the extent of 0.68 pounds per square inch before the 

 excessive retort pressure completed the seal. On the duplicate trials with the 

 partially sealed jars, pressure was drawn into the retort more slowly, with the 

 result that there was a corresponding rise in jar pressure and retort pressure. 

 On duplicate trials with fully sealed jars there was no rise in jar pressure — a 

 further indication that the system did not leak. 



Internal Pressures Developed in Water-Bath Processing 



Partially sealed jars of water processed in a water bath did not build up any 

 appreciable pressure. The mean for eight jars was approximately 0.1 pound 

 per square inch. The fully sealed jars, however, showed a maximum pressure 

 of 6.19 pounds per square inch after heating slowly for 55 minutes in a water 

 bath through the temperature range of 70° to 212° F. In the eight jars, the 

 pressure range at time of venting was 2.42 to 6.19, with a mean of 4.82 pounds 

 per square inch. In the partially sealed jars the venting occurred in from 6 to 

 10 minutes, with an average of 7 minutes; for the fully sealed jars the range 

 was 21 to 55 minutes, with an average of 37 minutes. These data may appear 

 misleading in indicating that the venting of fully sealed jars does not occur 

 until after a long heating period. This was not the case as observed in actual 

 practice where the jars were placed in fairly hot water. Actually, venting 

 occurred within a few minutes, as shown in Table 1. Repeated observations 

 were made on jars of raspberries, blueberries, tomatoes, and other small fruits 

 that were fully sealed before processing in a water bath. They invariably 

 vented in 1 to 3 minutes. Jars showing no vacuum, thereby indicating no vent- 

 ing, have not been found, whether water-bath or pressure processed. This 

 shows that venting always occurs even when the jars are fully sealed. Figures 

 3 and 4 show typical curves for jars processed in a water bath. 



Although the experimental jar apparatus was used to secure the results shown 

 in Figure 4, the curves demonstrate more clearly the slight margin of difference 

 in results obtained with the two methods of sealing. When fully sealed jars 

 were placed in a water bath at 190° F. and heated to 212° F., venting occurred 

 at from 2.7 to 3.3 pounds per square inch in from 5 to 9 minutes. As the venting 

 is the main cause of the vacuum, if both fully and partially sealed pint jars 

 with the same headspace vent at about the same time and to the same degree, 

 they will both have the same amount of vacuum when the jars are cooled after 

 processing. 



