No. 2(1921) REMARKS ON CANNING 89 



pressure, and shall when cooled show a permanent but slight 

 concavity in top and bottom. 



The above remarks deal principally with the ordinary exhaust- 

 ing of fish cans. 



1 10. In the case of articles such as jams, poured boiling hot 

 into cans and then sealed, exhaustion is not necessary; the head 

 space between the cover and the surface of the hot jam, etc., is 

 only 1/8 inch and the air and aqueous vapour at, say, 180^ F. are 

 of such tenuity that a sufficient vacuum will be formed when the 

 can cools. Glass jars used for fruit and other preserves which 

 are either poured into the jar at boiling point or are heated in the 

 jar to that point, need no exhaustion as they are full of steam when 

 ready for closing, so that a vacuum is necessarily formed on closing 

 the jars by the usual sealing cap. Moreover, being glass jars, one 

 main object of exhausting, viz., the result on the shape of the tin, 

 does not exist. 



111. (9) Processing. — Here we enter on the final and most import- 

 ant of the actual canning processes. A can under-processed will 

 go wrong ; it may go wrong if internal and external pressures are 

 excessive : it will be unsaleable if over-processed so that the 

 contents are scorched or over-cooked or if the tin plate is swollen 

 or battered. But without processing (cooking) of a correct sort, 

 general canning is impossible, for processing means the complete 

 sterilisation (destruction of the bacteria) of the contents of a can 

 after they have been hermetically sealed in a container in such 

 way that no further access of bacteria is possible ; if the bacteria 

 or the spores of such bacteria already within the container and its 

 contents are not utterly destroyed, the contents of the can must 

 go bad. Moreover, it fortunately happens that the toxins 

 and ptomaines which are the products of bacterial activity are 

 mostly destroyed by strong heat, especially when, as in pressure 

 processing, the temperature much exceeds (rising to 250° F.) that 

 of boiling water. Hence the need for careful and thorough 

 processing. 



112. In the making of certain canned goods such as jams and 

 bottled fruits, the contents are either poured boiling into the 

 containers and then closed up, or they are placed in the containers 

 and brought to the boil and hermetically closed ; in these cases 

 the cooking is not a separate processing. This method will not 

 here be dealt with, but it may be remarked that when the contents 



