VACUUM-PRESSURE IN GLASS CANNING JARS 5 



Furthermore, the possibility of later spoilage is increased, because as the liquid 

 is forced out of the jar, it may carry with it particles of solid matter which get 

 lodged between the rubber sealing ring and the lip of the jar, thus providing 

 an entrance for spoilage organisms. 



The pressure in the retort was reduced to zero rapidly and slowly to determine 

 how this factor affected vent loss. The pressure was reduced to zero within 

 one minute when released rapidly and within eight to ten minutes when re- 

 leased slowly. It is evident from a study of Figure 1 that a slow release of 

 pressure from the cooker is the preferred method in order to avoid vent losses. 



Relation of Headspace to Vent Loss 



The results of the experiment show that the greater the original headspace in 

 the jar, the less the vent loss during heat processing. In partially sealed jars 

 with a headspace of 56 cc. the average vent loss was almost five times as great 

 as in similar jars with a headspace of 97 cc, when the retort pressure was 

 released rapidly; and almost three times as great when the retort pressure was 

 released slowly. This same trend was noticeable in the fully sealed jars but to 

 a lesser degree. The loss in the jars with the smaller headspace averaged two 

 grams more than that in jars with the larger headspace. Fully sealed jars may, 

 therefore, be filled to a higher level in packing than the partially sealed jars. 

 However, it is not advisable to completely fill jars. A headspace of one- 

 quarter inch is satisfactory for canning most food products. 



Relation of Length of Processing to Vent Loss 



The only significant difference between jar pressure and retort pressure occurs 

 when the retort pressure is being released. The pressure within the jar lags 

 behind the retort pressure until the difference becomes sufficient to raise the 

 cover of the jar and release the pressure within, thus allowing the two pressures 

 to come to equilibrium. This discharge of gases under pressure sometimes 

 carries with it liquid from the jar, causing a vent loss, particularly when the 

 jars are only partially sealed. A s the greatest release of jar pressure occurs when 

 the retort pressure is released, the duration of the processing after the maximum 

 pressure in the jar is attained does not affect the vent loss. For example, a jar 

 held at 240° F. for 30 minutes would have as much vent loss as one maintained 

 for two hours at the same temperature. Observations on many jars show that 

 the greatest losses of liquid occur in partially sealed jars at the time of pressure 

 release. 



Time of Venting of Fully and Partially Sealed Jars 



A comparison of the time elapsing before venting takes place in the partially 

 sealed and the fully sealed jars is indicated in Table 1. Of the 800 jars used, 

 400 were filled with water at 70° F. and 400 with water at 170° F., and all 

 were immersed in a 180° F. water bath. In both cases, half the jars were 

 partially sealed and half were fully sealed. Venting was readily discernible 

 by means of the bubbles rising from the jars. As all the jars vented in the first 



