50 BULLETIN 1022, U. S. DEPARTMENT OF AGRICULTURE. 



in others, so that small pressures are to be expected. The amount of 

 pressure actually developed will depend, of course, upon the perfec- 

 tion in construction of the container, the thickness and quality of the 

 rubber rings, and the pressure exerted by the device holding the cap 

 in place. These pressures are practically negligible. 



Investigations upon the resistance of rubber rings and other sealing 

 equipment designed for use with glass are receiving attention at the 

 hands of the manufacturers of glass containers, and Bitting (6) has 

 recently published the partial results of work upon this subject. 



The fact that with some substances an undesirable metallic taste is 

 imparted to the food by the tin makes the use of glass containers de- 

 sirable ; and knowledge of the fact that, with fruits especially, deli- 

 cate flavors may be retained in the product by completely sealing the 

 jar before processing and then cooking at temperatures below 100° 

 C. will doubtless lend impetus to investigations of this sort and result 

 in the manufacture of glass containers better suited than those now 

 employed for use where pressures are developed. 



On account of their rigidity the pressures and vacuums obtained in 

 glass jars hermetically sealed before processing should give figures 

 more closely approaching the theoretical values than those obtained 

 with tin. Owing to the fact that, in present practice, most, if not all, 

 of the air escapes during the processing, the vacuum obtained is close 

 to the theoretical value when the jars are sealed immediately at the 

 end of the processing period. Unless complete sealing is effected at 

 once, however, air is drawn into the jar and the vacuum obtained is 

 less, depending upon the rate of cooling and the length of time 

 elapsing before the jar is sealed. 



SUMMARY. 



(1) In tin cans containing various quantities of water, changes 

 in pressure vary somewhat from the calculated values, owing to the 

 distortion of the can under the changed conditions. 



(2) With water the rate of change of pressure and the rate of 

 change of temperature at the center of the can agree closely and are 

 very rapid where the external medium is water and very slow where 

 the external medium is air. 



(3) With food materials in which a free liquid fills the inter- 

 spaces the rate of change of pressure and of temperature is very 

 rapid; but while the maximum temperature is reached promptly, 

 the maximum pressure, on the other hand, is never reached during 

 the ordinary processing periods, the pressure continuing to rise 

 slowly as long as the high retort temperatures are maintained. 



(4) In cans filled with material of heavy consistency, the rate 

 of change of temperature at the center of the can is very slow. In 

 contrast with this, the rate of change of pressure is very rapid at 



