VACUUM -PRESSURE RELATIONSHIPS IN 

 GLASS CANNING JARS 



By C. R. Fellers, Research Professor, A. S. Levine, Research Assistant 

 and W. A. Maclinn, Instructor, in Horticultural Manufactures 



INTRODUCTION 



The all-glass container has several advantages over the commercial tin can 

 which account for its extensive popularity among home canners and small- 

 scale, quality producers. The glass container has a re-use value and attractive 

 appearance, and does not require expensive sealing equipment. 



The little previous original research on the subject of the glass container has 

 received its stimulus from glass manufacturers and unorganized home canners. 

 Ford (2, 3, 4) has studied extensively the processing of glass jars, but these 

 studies were concerned with the metal closure types. Similar studies conducted 

 largely on tin cans have been reported by Magoon and Culpepper (8, 9). The 

 factors involved during processing (sterilizing) both in a water bath and under 

 steam pressure seem to have been largely neglected. Maclinn (6) in 1933 

 started an investigation of this phase of canning and made a preliminary study 

 of the Massachusetts method of processing the all-glass fruit jar. By this 

 method the wire bail or clamp is placed in the tightened position prior to pro- 

 cessing. This is commonly referred to as the "fully sealed" method. 



The primary object of this paper is to present an account of the action of 

 some physical factors involved, and incidentally to establish beyond all doubt 

 the safety and efficiency of the "fully sealed" or Massachusetts method in the 

 preservation of canned foods. 



DESCRIPTION OF ALL-GLASS JARS AND HOW THEY ARE PROCESSED 



The all-glass container consists of a glass jar and cover, a flexible rubber ring, 

 and a wire bail which clamps the cover into the sealing position. The jar type 

 commonly known as "E-Z Seal" was used in this investigation. When the 

 wire bail is in place but not clamped tightly, the jar is said to be "partially 

 sealed," as contrasted with the term "fully sealed," meaning that the bail is in 

 the tightened position. (See Plate I.) 



The common method of processing foods in all-glass containers is to submit 

 the jar in a partially sealed condition to a boiling water bath or a steam pressure 

 retort for a specified length of time. This heating destroys microorganisms 

 and insures keeping quality of the product and safety to the consumer. In 

 addition to destroying and inactivating microorganisms, the heat expands the 

 enclosed gases, including water vapor, which create a pressure, lift the lid of 

 the jar, and permit the gases to escape through the partially sealed closure. 

 This release of expanded gases is called "venting." The wire bail is tightened 

 and upon subsequent cooling of the jar a partial vacuum is formed. 



