No. 2 (1921) REMARKS ON CANNING 95 



high temperature. That this overcooking is no slight difficulty is 

 shown by the fact that it is usual to cool the cans by jets of cold 

 water after removal from the retort so as to prevent overcooking ; 

 this of course does not get rid of the difficulty now under 

 consideration, viz., the necessary internal pressure which exists as 

 soon as the external pressure is removed by blowing off the steam. 



123. It has consequently been sought to cool the cans while in 

 the retort and still in the steam by introducing cold water ; this 

 seriously aggravates the difficulty, for the cold water condenses 

 the external steam and causes a partial vacuum, so that the internal 

 steam pressure is now II lb. plus internal atmospheric pressure due 

 to the existence of a partial vacuum in the retort ; assuming this to 

 be 7 lb., the total internal pressure tending to burst the seams, will 

 be (11 plus 7) 18 lb., a strain much too heavy for many soldered 

 seams. 



124. A feasible method would seem to be that of supplanting 

 the steam in the retort by air condensed to the same (ll lb.) 

 pressure ; the pressure in the cans is thus balanced by external air 

 pressure, and as the air does not condense like steam it is then 

 possible to introduce jets of water to cool the cans while within 

 the retorts ; this not only reduces the internal pressure to normal 

 but prevents overcooking. 



125. In all the above remarks both as to exhausting and its 

 reasons and to processing and its difficulties, only such cans as 

 normally contain considerable air and moisture and which have 

 soldered and lapped seams, are contemplated, the objects being to 

 minimise leaky joints and buckled plates in such cans. Cans 

 which are normally filled perfectly full of oil and fish and com- 

 pressed by well fitting dished lids, or which are made with locked 

 seams and double seamed instead of being soldered, are not 

 seriously affected by these particular processing troubles, especially 

 if the cans are, as at Beypore, solid drawn and have only the one 

 seam, viz., that joining body and cover. Sanitary cans, double 

 seamed at top and bottom, and with locked and soldered side seam 

 are probably safe from all but extraordinary strains, but when in 

 fish canning these cans are tall cylinders comparatively loosely 

 packed with chunks or slices of fish or whole fish such as herrings or 

 mackerel, good exhausting is highly necessary to avoid processing 

 troubles and to ensure the required concavity in tops and bottoms. 



126. Where steam is not available but it is desired to process at 

 temperatures above 212® F., it is necessary to heat the cans in 



