NO. 2 (1921) REMARKS ON CANNING 93 



(in a dosed coil) turned on, till the required pressure is reached ; 

 as the water expands slightly by heat, it should not do more than 

 nearly cover the cans. This method not only prevents scorching 

 of the cans by the hot jets of steam playing on the outside but the 

 convection currents in the water cause rapid heating; moreover, 

 not only does this water bath permit more gradual heating and 

 consequent more gradual increase of pressure, but also at the close 

 of the operation when steam is turned off and the escape opened, 

 the water, remaining at boiling point, allows a more gradual 

 lowering of the temperature within the cans and minimises the 

 chances of leaks by the sudden cooling of the cans which is 

 caused when the cool air from outside suddenly enters the retort. 

 Similarly, in autoclave processing it is possible that instead 

 of placing only a small quantity of water in the autoclave, enough 

 should be introduced so as nearly to cover the cans when inserted 

 in their cage, so that the cans would be processed by superheated 

 water rather than steam, at the required pressure ; this water would 

 surround the cans when the operation is over and prevent the 

 sudden access of cold air. If the kettle is steam heated the water 

 bath is of course heated by a closed coil so as to avoid excess of 

 water from the condensation of open steam. 



Where, however, as is most general, the water in the autoclave 

 only suffices partly to fill the autoclave, or the cans in a retort 

 are exposed naked to steam from an outside generator, various 

 precautions must be observed — 



(1) to cause circulation of the steam ; 



(2) to prevent injury to the soldered seams by the sudden 

 pressure of high temperature steam at the beginning of the 

 operation ; 



(3) to minimise the occurrence of leaks or development of 

 'springers' by undue and sudden internal pressure when the steam 

 is let out and the retort opened. This point refers only to soldered 

 cans packed comparatively loosely, as with mackerel, which 

 contain a good deal of air and moisture ; it does not refer to solder- 

 less cans or cans with locked seams, and is of no material 

 importance to cans of fish such as sardines when packed in oil. 



119. On the first point it is only necessary to keep an escape 

 cock slightly open (' cracked ') or the safety valve slightly blowing 

 throughout the operation ; this causes rapid movement instead of 

 comparative stagnation of the steam, and both hastens and equal- 

 ises processing ; without such precaution '\\. is found that cans 



