VACUUM-PRESSURE IN GLASS CANNING JARS 



13 



120 



Pressure Cooker 



10 20 30 40 



Time In Minute a 



♦Water bath boiled (.212° F.). Steam pressure reached 10 pounds. 



Figure 6. Heat Penetration in Partially and Fulled Sealed Glass Jars Processed in a 

 Water Bath and in a Pressure Cooker. 



VACUUM 



A good partial vacuum in the sealed jar is of primary importance in the 

 canning of food. Vacuum is generally expressed in "inches of mercury" and 

 may be described as the absence of "normal pressure." Normal or atmos- 

 pheric pressure is that pressure due to the weight of the air enveloping the earth, 

 and at sea level is approximately 15 pounds per square inch. 



If a jar is closed at room temperature, the air within exerts the same pressure 

 outwards as the atmosphere without exerts on the container. If a part of the 

 air in the jar is removed and the jar is then closed, the remaining air exerts 

 less pressure outwards than the atmosphere exerts upon the outside of the jar, 

 and it is said that a "partial vacuum exists in the container." Atmospheric 

 pressure at sea level is sufficient to support a column of mercury 29.92 inches 

 high, having a cross-section of one square inch. If the air remaining in the 

 "partial vacuum " supports a column of mercury to a height of 20 inches, then 

 the difference between the heights of the two columns, 10 inches, represents the 

 "vacuum." This vacuum is what is meant whenever the word is used in 

 general literature on canned food. 



A vacuum is produced in the all-glass jar either by heating the food before 

 it is packed or by heating the product in the container. The application of heat 

 causes the internal gases and vapors to expand and escape by means of the jar 

 "venting." Upon cooling there is a contraction of the expanded gases, vapors, 



