Q2 Madras fisheries bulletin vol. xiii, 



pan. Moreover by having two pans side by side, the processed 

 cans can be removed from the boiling pan into its neighbour 

 which is filled with cold water; this cools the cans, which is desi- 

 rable, and at the same time heats up the water ready for a processing 

 operation in this second pan, and so alternately. 



116. If steam is available a pan, which may then be of wood 

 or masonry with a steam coil in the bottom, may be usefully sub- 

 stituted for the open fire ; the coil may be closed or open, the latter 

 having the advantage of utilising the whole heat of the steam : 

 moreover, the condensed steam supplies a quantity of pure water 

 to the vat ; in either case the coil should be placed below a stout 

 perforated false bottom on which the baskets of cans rest. The 

 temperature of course can never rise in the open vat beyond 212° F. 



117. But it is often, perhaps generally, desired to process fish 

 (also meat and vegetables) at a temperature above 212° F. so as to 

 kill sporing bacteria and their spores and destroy toxins. More- 

 over, this method saves two-thirds of the time in processing, and 

 consequently needs less space than the open vat system, since a 

 single retort will do the work of several open vats. This rapidity 

 of action is due to the fact that not only is pressure steam of a 

 higher thermometrical temperature than boiling water (the tempera- 

 ture rising according to pressure so that steam at one atmosphere 

 (14— 15 lb.) marks 250° F. on the thermometer), but steam contains 

 a large amount of ' latent ' heat ; water in passing into steam 

 absorbs 966 units of heat, so that while boiling water at 212° F. 

 contains 180 units of heat, steam at 212° F. contains 1 146. Hence 

 when steam is brought into contact with the comparatively cold 

 cans, it condenses and in condensing gives up its contents both 

 of latent and thermometrical heat to the can. Hence the can is far 

 more rapidly heated up by steam at even 212° F. than with water 

 at that temperature, and still more so at 220° F. or higher tempera- 

 tures. Consequently it is usual, where possible, to process cans of 

 fish, meat, and vegetables (which latter frequently contain sporing 

 bacteria) by pressure steam. As mentioned above (paragraphs 

 65—67, etc.) this may be done in autoclaves raising their own steam 

 within the vessel by means of a fire below it, or in a plain retort 

 steamed from an external generator. 



118. There is more than one way of steaming cans; sometimes 

 the vessel is filled with water until the cans are nearly covered ; 

 the cover is then clamped steam-tight and the fire urged or steam 



