No. 2 (1921) RKMARItS ON CANNING 87 



warmer or at a greater elevation above the sea level than the 

 place of canning, so that the retailer or consumer suspects or rejects 

 the can to the detriment of the canner in purse and reputation. 

 See paragraphs 136 — 139 on ' swells ' and ' springers.' 



104. By exhausting the air these risks are largely removed, 

 and since the removal of the air causes a partial internal vacuum, 

 the tops and bottoms when at normal air temperature, assume a 

 concave shape : this is the test of a sound can, and since cans which 

 are unsound speedily bulge and become convex on both sides, 

 exhausting enables one to distinguish at once between a sound and 

 an unsound can. Hence the objects of exhausting are to remove 

 much of the expansible air and thus prevent risks by internal 

 pressure to the soundness of the seams and to the due ' set ' of the 

 tin plate, to cause the cans to become concave at normal tempera^ 

 tures, to prevent temporary convexity ('swelling') at somewhat 

 elevated temperatures (e.g., at the temperature of 120° F. in some 

 places in the hot weather as compared with 85 to 90° F. at the 

 cannery) or at elevations perhaps 7,000 feet above the place of 

 canning, and to enable the instant differentiation of sound and 

 deteriorated cans. 



105. Since the removal of the air is the main point it follows 

 that cans packed full of sardines in oil, need not be exhausted ; 

 there is so little air in them that it may be neglected; so also the 

 solderless cans at Beypore need no exhausting because the 

 mechanical pressure of the dished cover removes practically all 

 air. 



106. On the other hand very large tins of fish (other than tins 

 of sardines in oil filled to the very brim) may easily be exhausted 

 too much ; a kerosine tin of fish may, when highly exhausted 

 assume a battered and ugly appearance and may even collapse 

 under external pressure ; hence large tins, if fully exhausted, must 

 be of strong material and seams. Nor is it necessary or advisable 

 to exhaust cans too thoroughly when they are loosely packed as 

 with salmon, mackerel in cylindrical tins, slices, etc For if too 

 thoroughly exhausted of the air and filled entirely with steam, it 

 follows that on cooling and consequent condensation of the steam 

 there would be a very high vacuum, which would not only en- 

 danger the seams by the ordinary atmospheric pressure, but would 

 facilitate the indraught of air with its attendant bacteria through 

 a hole or porosity in the metal or seam so excessively minute 



