Preservation of meat - Gracey's Meat Hygiene - page 80

a blower, and it emits a dull sound when struck, whereas
a blown tin emits a resonant note. The term slack caps
denotes a can which has a movement of one of the ends
similar to that of a can in the early stages of blowing but
is now rarely, if ever, encountered, and the great majority
of cans classed as slack caps are blown and should be
treated as such.
Spoilage of canned goods may be of microbial origin
or chemical origin due to deleterious influences such as
rust or damage.

Non-sporing proteolytic or fermenting bacteria, for
example, Proteus and E. coli , may cause decomposition
of canned foods; no single type of organism is responsi-
ble for microbial spoilage. The problem of spoilage in
canned goods is not the simple one of the presence or
absence of such bacteria, but why in some cans bacteria
of this type decompose the contents, while in others they
remain inactive. Though yeasts and moulds are of great
importance as causes of unsoundness in acid substances
containing sugars, for example, canned fruits, they are of
less importance in canned meats and marine products.
The presence of yeasts, moulds and non-sporing bacteria
in canned meat foods is evidence of leakage after sealing
and can make the food unsound. Canned goods which,
on opening, show such evidence should be condemned.

Microbial spoilage
Bacteria of the decomposing or fermenting type are the
most important as regards canned foods, while spore-
forming bacteria are the most resistant. There are three
main types of spore-forming organisms which can resist
normal processing and may cause spoilage in canned
foods: gas-producing anaerobic and aerobic organisms
with an optimum growth temperature of 37°C, gas-pro-
ducing anaerobic organisms growing at an optimum
temperature of 55°C and non-gas-producing aerobic or
facultative anaerobic spore-forming organisms with an
optimum growth temperature of about 55°C, which pro-
duce flat sours .
Processing is not a substitute for cleanliness and will
destroy a small number of bacteria rather more easily
than a large number. Bacteria subjected to heat or other
harmful influences are destroyed in accordance with a
definite law which prescribes that where two different
suspensions of the same organism are subjected to heat
under uniform conditions, the number of bacteria will
be reduced by the same percentage over equal periods of
time.
Insufficient processing is a cause of unsoundness of
canned goods, though not the all-important factor gen-
erally assumed.
The bacteria found in canned meat or fish are nearly
always secondary invaders gaining access through a
leak. Microbial spoilage may thus result from under-
processing or from leakage through the seam. Leakers
may be detected by the disappearance of the vacuum
from the sides and ends of the can, and bubbles appear
if the can is held under water and squeezed. Another
test for leakage is to heat the can to 38°C in the interior
and allow it to cool slowly; if a leak is present, there
will be no concavity of the sides or ends. The detection
of leakers by striking the suspect can with a mallet has
little value in industrial practice. The most common
form of leaking occurs at the seams and may some-
times be detected by liquid or stain on the can surface.
Mould formation on the surface of canned meats is
also indicative of leakage, but cannot be detected until
the can is opened.

Types of spoilage
Canned goods are classified as spoiled when the food has
undergone a deleterious change or when the condition of
the container renders such change possible. Spoiled cans
may show obvious abnormalities such as distortion, blow-
ing, concave ends or slightly constricted sides; or they may
present a perfectly normal external appearance.
A can with its ends bulged by positive internal pressure
due to gas generated by microbial or chemical activity is
termed a swell or blower . A flipper has a normal appear-
ance, and though one end flips out when the can is struck
against a solid object, it snaps back to normal under light
pressure. A springer is a can in which one end is bulged
but can be forced back into normal position, whereupon
the opposite end bulges. All blown cans pass successively
through the flipper and springer stages, and these two
conditions must be regarded as suspicious of early spoil-
age of the can contents. A change in the appearance of
the gelatin surrounding meat packs is usually associated
with the formation of gas, the gelatin being discoloured
and more liquid in consistency. It should be remem-
bered, however, that in hot weather the gelatin of meat
packs is likely to be of a more fluid nature. These abnor-
mal cans are brought about by imperfect canning
operations such as inadequate exhaustion of air before
sealing, overfilling and the so-called nitrate swell which
arises during thermal processing and is recognised
during subsequent cooling, but whose nature is not fully
understood.
A leaker is a can with a hole through which air or
infection may enter or its contents escape. An overfilled
can is one in which the ends are convex due to overfill-
ing, but filling by weight or accurate measurement has
done much to obviate this condition, and most tins clas-
sified as overfilled are actually in the early stage of blow-
ing. Though an overfilled can cannot properly be
regarded as a spoiled can, it must be differentiated from