94 MADRAS FISHERIES BULLETIN VOL. XIII, 



especially in corners of the steam retort or in the centres of close 

 masses of cans, are unequally steamed. 



120. The second difificulty is turned by letting the steam on 

 gradually, so that the pressure gauge mounts slowly ; this not only 

 prevents the hammer-like blow of high pressure steam impinging 

 suddenly on the naked cans, but tends to bring up the internal 

 pressure within the can pari passu with the external pressure, and 

 thus minimises strain. 



121. The third matter is more difficult and complicated. It has 

 been shown that there is always, except in tins closely packed and 

 filled with oil, a certain amount of air even after exhausting — as 

 well as some moisture in the fish ; hence on the application of a 

 temperature above 212° F. there is considerable expansion of air 

 and vapour within the can ; cans when removed from steam 

 kettles or retorts are visibly and even heavily bulged. The 

 expansion and consequent internal pressure due to air plus steam 

 may be many pounds to the square inch when the temperature has 

 been raised to 240^" F. with its corresponding steam pressure of II 

 lb. ; hence if at the close of the processing the steam is turned off 

 and rapidly blown off by the vent cock and safety valve, the cans 

 in which, ex-hypothesi, the same temperature and even higher 

 pressure (due to internal air) have been reached, remain at that 

 pressure, and this being no longer partly balanced by the external 

 pressure causes sudden and considerable bulging which the joints 

 may not be able to stand, or which, especially with very thin tin 

 plate, may so buckle the plate as to permanently distort it and cause 

 ' springers.' Supposing the retort temperature to be 240° F. or 

 II lb. pressure, this is consequently the minimum pressure within 

 the can when fully processed, to which must be added the pressure 

 from any expanded air ; if the can is not completely exhausted 

 (which it never is and should not be, since there would then be an 

 unduly high internal vacuum, see supra paragraphs 103 — 109) this 

 adds considerably to the internal pressure. 



122. Hence the tendency to leaks and distortions. This danger 

 can only be slightly overcome by blowing off the external steam 

 gradually, for the cans, in bulk, retain their full heat for a long 

 time and consequently the internal pressure must eventually exert 

 itself; all that can be said is that the effect is gradual and not 

 sudden, but this is counterbalanced by loss of time and by pro- 

 bable overcooking of the goods in remaining too long at a very 



