86 MADRAS FISHERIES BULLETIN VOL. XIII, 



(5) decomposition from bacterial or enzymic causes even yet 

 unknown or obscure or from action of the contents on the metal, 

 etc. 



It is to be remembered that a hole so minute that the jfinest 

 needle point cannot enter it, is of ample size to admit bacteria 

 readily, and that since a partial vacuum is caused within the can 

 when it eventually cools (as shown by the concavity of good tins) 

 the outside air, laden with germs, is gradually sucked into the can 

 through such hole, and decomposition then sets in. 



103. (8) Exhausting. — This is in general a necessary process 

 and may be made practically continuous with testing by simply 

 raising the temperature of the testing bath ; exceptions to exhaust- 

 ing will be mentioned below. Exhausting is the removal from 

 a can of free air for physical and business, not bacteriological, 

 reasons, viz., to prevent undue pressure within the cans during or 

 immediately after processing ; its object is not sterilising though 

 this partially results. When air is left in a can as must be the 

 case in cans of plain sardines (without oil), or mackerel, or sliced 

 fish or prawns in a cylindrical can, this air expands when heated, 

 and if heated above boiling point so that steam also forms 

 within the can, the internal pressure may easily disrupt minute 

 weak places in the seal or even burst the seal. Moreover, if the 

 tin plate is thin it may be distorted or buckled by the internal 

 pressure and become what is called a 'springer' or 'flipper' 

 simulating a 'swell' or 'blown' tin. For instance, a can sealed 

 at 50° F. and not exhausted, may, when heated to 212° F. have to 

 bear an internal pressure from the expanded air alone, of about 

 18 lb. per square inch, a pressure which is hard for tin plate and 

 for seams to withstand. Moreover if the can is processed under 

 pressure and raised perhaps to 240° F. the pressure inside the can 

 rises still higher both from the expansion of the air and the for- 

 mation of steam ; so long as it is being heated in the retort, the 

 external steam pressure balances the internal steam pressure, but 

 on blowing off the steam this external pressure is removed, and the 

 can has therefore to bear the whole internal pressure due to the 

 contained and highly expanded air and steam. This may easily 

 cause leaks in strained seams, and it is therefore necessary to 

 remove most of the air and thus reduce so much of the internal 

 pressure as arises from the expanded air. Again, even if the can 

 remains sound and normal it will, if full of air, swell and take 

 on a slightly bulged appearance when transported to a place 



