Preservation of meat - Gracey's Meat Hygiene - page 81

quite harmless, the routine methods employed in the
examination of canned goods render it impossible to dis-
tinguish between tins blown owing to hydrogen swell
and those blown as a result of deleterious changes due to
bacteria or yeasts. All blown tins, whether fruits, meats,
vegetables or condensed milk, must be regarded as unfit
for food, and leakers, springers and flat sours, together
with tins whose contents show evidence of mould,
should likewise be condemned.
Purple staining on the inner surface of cans in which
sulphur-containing foods are packed may occur with all
fish and meat products, especially the liver, kidneys and
tongue. It is due to the breakdown of sulphur-containing
proteins in high-temperature processing by the thermo-
philic Clostridium nigrificans ('sulphur stinker'); hydro-
gen sulphide is liberated, and a thin layer of tin sulphide
is formed on the inside of the can. This discolouration
does not involve the foodstuff itself and varies from a
light pink to a dark purple, but it may be accompanied by
a blackening of both the inside of the can and the surface
of the foodstuff if the hydrogen sulphide attacks the steel
base-forming iron sulphide. It is of more serious import
than the deposition of tin sulphide, as it may lead to pit-
ting of the steel and disfigurement of the surface of the
meat pack. Discolourations of both types may be pre-
vented by a sulphur-resisting lacquer, the basis of which
is copal gum dissolved in a suitable solvent to which are
added substances capable of uniting with the volatile sul-
phur gases released while the food is being processed.

Flat souring in canned goods produces a sour odour of
the foodstuff but the can is not blown. Canned foods
susceptible to flat souring are those containing sugar or
starches and meat products such as sausages or pastes
containing cereal. True flat sours are caused by thermo-
philic organisms ( B. coagulans, B. stearothermophilus, B.
circulans ) which are exceptionally heat resistant and
attack carbohydrates, producing acid but not gas.
Sourness in canned foods may also arise due to leaking
cans, or it may have developed in the foodstuff before
processing. This latter form of spoilage is most likely in
packs cold filled in warm weather, particularly if the cans
are open for even short periods prior to processing.
Flat souring of canned goods due to thermophilic
spore-forming organisms cannot be detected until the
can is opened and its contents are examined, but is
unlikely to occur in temperate climates unless storage
conditions have been exceptionally hot; it is, however,
comparatively common in tropical and subtropical
countries or in cans imported from them.
Ham spoilage may be caused by faecal streptococci, for
example, Streptococcus faecalis var. liquefaciens , which may
liquefy jelly and cause off-colour, off-flavour souring.

Chemical spoilage
Hydrogen swell may occur quite independently of fer-
mentation or bacterial decomposition and is associated
with the formation of hydrogen gas in the can following
internal corrosion . Imperfections or scratches on the
inner tin coating may expose small areas of steel, and,
where the contents are acid, an electric couple may result,
the reaction producing hydrogen gas. Electrolytic action
is accelerated by oxygen and by the colouring matter
(anthocyanins) of red fruit. Cracks in the inner lining of
lacquer serve to concentrate electrolytic action on the
areas of steel exposed and increase the rate of hydrogen
release. Cans affected with hydrogen swell may show var-
ying degrees of bulging from flipping to blowing. If the
tin is punctured, there is emission of hydrogen gas, which
is colourless and burns on the application of a flame. The
condition is chiefly associated with foods containing
organic acids such as fruits , particularly plums, cherries,
raspberries, blackcurrants and loganberries.
The range of acidity most favourable to the production
of hydrogen swell lies between pH 3.5 and 4.5, and the
less acid fruits therefore give more trouble than those of
higher acidity, but with proper precautions, there should
be very little trouble from hydrogen swells in commer-
cially packed English fruits for at least a year after can-
ning. The condition is seldom encountered in canned
vegetables and is practically unknown in canned meat
foods, but it is sometimes seen in tinned sardines.
Although the contents of a can in hydrogen swell may be

Rust or damage
Cans showing external rust require careful consideration.
It is a condition particularly liable to occur beneath can
labels when the adhesive contains hygroscopic substances.
Cans in which the external surface is slightly rusted with-
out noticeable pitting of the iron may be released for
immediate sale and consumption, but if the rust is
removed with a knife and inspection with a hand lens
reveals the iron plate to be definitely pitted, there is dan-
ger of early perforation and the cans should be con-
demned. Minute perforations of the tinplate, known as
pinholing , permit the entrance of air and lead to spoilage
of the can contents. Pinholing may originate from the
outside but also from the inside of the can where the
tinplating is imperfect or has been fractured during
seaming, and in this case, lacquer lining aggravates the
trouble, as the cracks that occur in the lacquer aid in con-
centrating the chemical action on a small area. A can
which is a leaker or pinholed may occasionally seal itself
by blocking of the holes with the contained foodstuffs
and may then proceed to blow; such self-sealing cans may
blow at any period of their storage life, whereas an under-
processed can will blow early in its life, generally within