90 MADRAS FISHERIES BULLETiM VOL. XIII, 



are poured boiling into a jar or container and then at once sealed 

 up while hot, it is well to turn the container upside down for a time 

 so that any air left in the head space between the contents and 

 the cover, is forced to pass up through the boiling contents and is 

 thus sterilised. 



But in the bulk of canned goods, processing is a separate and 

 final operation. 



113. Processing, then, consists in raising the temperature of 

 the entire contents of a can to above sterilising point. For fruits 

 this may be somewhat above the pasteurising temperature, say I/O 

 to 180^ F. ; over-processing in such cases may break up the fruit 

 unduly. Nearly all bacteria are destroyed by such temperature 

 but spores are not ; since certain of these are liable to be found in 

 fish and meats and vegetables the temperature should always be 

 raised to full boiling point (212° F.), and the higher temperature of 

 pressure steam processing is desirable ; this may rise to 240^ F. 

 (11 lb. pressure) or 250'^ F. (14 lb.). The conductivity of the 

 contents of a can differ greatly, not merely by reason of their 

 nature (e.g., air, water, fish, meat, etc.) but according to their 

 solidity or porosity ; moreover, it obviously requires much longer to 

 raise the centre of a large can to the desired point than of a small 

 one, and unless the very centre of a can is sterilised the whole 

 operation is useless and mischievous. Hence the canner requires 

 great experience and judgment in the time allowed for processing ; 

 for instance, a can of prawns packed wet, i.e., with brine, requires 

 a much shorter time than if packed dry (with consequent large air 

 spaces) because water rapidly heats up owing to the convection 

 currents set up in liciuids on being heated, so that all parts of such 

 lic^uid are continuously on the move and are brought successively 

 into contact with the hotter part (walls) of the can ; hence rapid and 

 equal heating ; air on the other hand is more or less stationary and 

 conducts badly. A 2 lb. requires longer than a I lb. one ; a flat can 

 of shallow depth and large superficies gets heated to its centre 

 more rapidly than a more compact one ; a can of solid paste heats 

 slower than a can of oysters which contains much fluid, and so 

 on. One corollary from this is that in steam processing where the 

 vessel is closed steam tight, it is impossible to process cans of 

 different sizes and even contents (if radically different) at the same 

 time; in an open vat this can be done since cans can be removed 

 at pleasure. But to avoid confusion, it is better to sterilise only 



