VACUUM-PRESSURE IN GLASS CANNING JARS 15 



Water Displacement Method. — The second method eliminates the error of 

 adhesion between rubber ring and cover. 



The jar with its contents is weighed after processing, then immersed in water 

 in an inverted position and the seal broken, permitting the headspace to fill 

 with water in proportion to the vacuum in the headspace. With the jar still 

 inverted, the water levels inside and outside the jar are made the same, the cap 

 is replaced, the clamp tightened down, and the jar is then removed from the 

 water, wiped off, and reweighed. The difference in weight between the second 

 and first weighings gives the amount of water which was sucked in. The lid 

 is then removed and the jar filled completely with water, including the space 

 under the glass cover, after which it is weighed again. This weight minus the 

 first weight gives the volume of headspace. The vacuum can be determined 

 from the weight of the water sucked in and the volume of the headspace, as 

 shown in the following example: 



a. Weight of jar and contents after processing 980 grams 



b. Weight after opening under water 1,030 grams 



c. Gain in weight (b-a) 50 grams (cc.) 



d. Weight of jar completely full 1,050 grams 



e. Original headspace volume (d-a) 70 grams (cc.) 



b-a 



f. Vacuum - — x 30 21.4 inches 



d-a 



Effect of Type of Seal on Vacuum Formation 



Table 2 shows the results of vacuum determinations made on fully sealed 

 and on partially sealed jars filled with water at 70° F. and processed in a boiling 

 water bath for 5, 10, 15, and 20 minutes. There was little difference in the 

 vacuums formed in jars processed fully sealed and partially sealed for the same 

 length of time. Because of inherent differences in methods of canning in glass 

 and tin containers, the former usually have a higher vacuum after thermal 

 treatment. This is because the tin can is completely sealed and the glass jar 

 incompletely sealed during thermal treatment. 



A vacuum of 20 inches or more is so well above the safety line that a difference 

 of one or two inches due to the condition of the seal at the time of processing 

 is not going to affect the keeping quality of the contents of the jar. The average 

 vacuum considered safe for preserved foods in tin cans is 12 inches according 

 to Clark, Clough, and Shostrom (1). 



The data in Table 2 were secured also to check the efficiency of the two 

 methods of determining vacuum in all-glass jars. The two methods were 

 found to be in satisfactory agreement in their results. 



In order to discover the effect of the type of seal during pressure processing 

 on subsequent vacuum formation, a series of pint jars was processed 5, 10, 15, 

 and 20 minutes in a pressure cooker at 10 pounds steam pressure. Duplicate 

 jars were partially and fully sealed. On one set of jars the pressure was released 

 slowly from the retort; on the other set the pressure was released rapidly. The 

 results showed that the vacuums formed in jars processed in pressure cookers 

 were practically the same regardless of type of seal when processed and regard- 



