EARLY THEORIES ON PRESERVATION. 11 



taken to heat the contents well before sealing; later the cans were 

 heated, then vented, and again heated. Sometimes a second venting 

 was given. The belief was general that every trace of air must be 

 removed. This theory was held until recently, particularly for meat 

 and fish products, corn, and others difficult to process, as there was 

 no recognition of the more resistive varieties of bacteria found on 

 some substances. The theory of exclusion of air has had followers in 

 the practical operations up to the present time. 



The next theory advanced was that it was the vacuum which pro- 

 tected the food substances. This was believed by the workmen 

 almost from the beginning, as they found that heating and then 

 sealing the can resulted in more or less of a vacuum. This theory 

 was closely associated with that of excluding the air, and those 

 believing in it developed mechanical methods for producing the 

 vacuum; these, however, are not of as much practical importance in 

 canning as expected. The cold vacuum is useful in preserving meat 

 and fish products which have been cured by salting, drying, or smok- 

 ing, for fruits which have been partly cured by drying, sugaring, 

 etc., and for jams, jellies, preserves, etc. The vacuum aids in pre- 

 serving where the organisms causing spoilage require oxygen for 

 growth. 



In factory practice more or less of a vacuum is secured by passing 

 the cans through an exhaust box which will heat them to 180 or 

 more, or by adding hot brine or sirup before the can is sealed. To 

 some products, particularly meat and fish, heat is applied, and the 

 can vented to drive out the air; essentially the same result is accom- 

 plished in home canning in that the caps are not tightened until the 

 contents are thoroughly heated. A recent procedure is to cap the 

 cans and seal the vent in a strong vacuum chamber, instead of heating 

 and later venting. The tin can with a good vacuum always shows 

 the ends drawn in when cooled. Cans packed and sealed cold will 

 have the ends flush or upon becoming warm they may spring out, 

 producing so-called " flippers " or " springers," and while not spoiled, 

 they have the appearance of swells and may not be merchantable. 

 While the role of the vacuum as an aid in preserving is not to be 

 underestimated, it has not the importance in canning that was 

 attributed to it. 



The theory of the arrest of chemical activity was early advanced 

 as being effected by hermetic closure. The French Government ap- 

 pointed Gay-Lussac to investigate the cause of the preservation as 

 introduced by Appert. Gay-Lussac reported that decomposition 

 was a series of oxidation changes in a substance, and that driving out 

 the air and preventing its ingress prevented these changes from 

 taking place. This explanation was satisfactory until microorgan- 

 isms were discovered and their relation to spoilage proved. 



