Agriculture Reference
In-Depth Information
lymph vessels. The rapidity of the extension of the putre-
factive process throughout a carcase is greatly influenced
by the condition of the animal before slaughter. In
exhausted animals or in those that have suffered from
fever (especially from a septic cause) where the meat is
alkaline, decomposition sets in very rapidly and quickly
reaches the deeper parts.
The condition known in Britain as heated beef or 'heat
shortening' and in North America as sour side is caused
by inability of the freshly killed carcase to dissipate heat
rapidly when carcases are hung too close to each other,
thus preventing a proper current of air around the sides.
This usually affects the prominent areas of the hindquar-
ters and ribs. In mild cases where hot sides of beef have
not been in close contact for too long, there is merely a
blanching of the surface with no major loss of quality. A
more significant consequence of slow cooling results in
enzymes that enable ageing being destroyed, culminat-
ing in pre-rigour changes which impacts significantly on
meat tenderness and eating quality. The condition is also
observed in rabbits, hares and game which are packed in
hampers or baskets while still warm and is known in the
trade as green struck .
In animals that have died and have not been eviscer-
ated, both external and internal decompositions occur
simultaneously, owing partly to the high blood content
of the meat and partly to the invasion of the abdominal
veins by putrefactive bacteria from the intestines. The
first bacterium to invade the carcase from the bowels
after death is E. coli , which in warm weather may reach
the joints within 24 hours; these bacteria use up the oxy-
gen in the carcase and pave the way for penetration by
anaerobic bacteria, for example, Cl. perfringens , from the
bowel. The presence of a greenish hue, first apparent on
the kidney fat and peritoneal wall, with the diaphragm
soft and flaccid and lying close to the ribs, is a strong
indication that evisceration of the animal has been
delayed and calls for a severe judgement on the carcase.
Lambs coming straight off grass and slaughtered in hot
sultry weather have been known to exhibit evidence of
incipient decomposition within 1 hour of dressing. Pigs
in which evisceration has been delayed, particularly in
the summer months, may show a greenness of the
abdominal fat in 12 hours, while the kidneys, and also
the liver, may exhibit a superficial black colouration; in
the kidneys, this pigmentation is frequently confined to
the anterior poles where blood collects due to hypostasis.
This colouration, known as sulphiding or pseudomelano-
sis , is due to the formation of iron sulphide by chemical
action between hydrogen sulphide from the digestive
tract and iron from the blood haemoglobin.
In the case of shot deer which cannot be gutted imme-
diately, it is customary among sportsmen to incise the
The main types of spoilage organisms on chilled fresh
meats belong to the previous groups, which are responsible
for slime formation during storage. These particular bacte-
ria are found almost everywhere in nature, and it is practi-
cally impossible to avoid their contamination of carcases
during dressing procedures. The time taken for slime to
develop on raw meats is directly related to the initial num-
ber of organisms on the carcase surface. It is thus especially
important to pay attention to efficient methods of hygiene
at slaughter, during carcase dressing, refrigeration, storage
and transportation. Chilling procedures do not prevent
the activity of spoilage organisms, which can grow at about
−7°C; critically, temperatures below 2°C will delay the
onset of slime formation. Control of the relative humidity
in chill rooms, that is, reducing the a w , can reduce bacterial
spoilage but results in a loss of carcase weight and liability
to spoilage by psychrotrophic bacteria ( Pseudomonas ,
Brochothrix , Lactobacillus , Moraxella ) and some moulds
( Cladosporium , Thamnidium , Sporotrichum ). A reduced
oxygen partial pressure ( p O 2 ) in the vicinity of stored meat
is of value in curtailing spoilage, as is increased p CO 2 .
The spoilage process is initially fuelled by the break-
down of carbohydrate. As time passes, however, protein
molecules are broken up into simpler substances by
acids, alkalis, endogenous enzymes and bacteria, the
degree of decomposition varying greatly with the differ-
ent agencies. Of these agencies, the putrefactive bacteria
carry the process further, breaking up the protein mole-
cule into proteoses; then peptones, peptides and amino
acids; and finally indole, skatole and phenol, together
with various gases including hydrogen sulphide, carbon
dioxide, methane and ammonia. It is the amino acids,
non-toxic in nature, which furnish bacteria with abun-
dant and available nutritive material, and their break-
down products which give the typical appearance and
odour of decomposed meat. The recognised everyday
signs of decomposition are marked changes of colour to
a grey, yellow or green, a softening in the consistency of
the tissue, a pronounced repulsive odour and an alkaline
reaction caused by the formation of ammonia.
After slaughter of a healthy animal, decomposition
eventually develops in the parts exposed to the air, the
time taken depending particularly on the temperature
and humidity of the environment. The primary surface
growth is initiated by aerobic bacteria, among these
being Pseudomonas , Achromobacter and some coliforms.
These organisms extract oxygen from the meat surface
and produce conditions suitable for the growth of anaer-
obic bacteria, for example, Cl. sporogenes , which can also
grow within the deeper tissues where there is no oxygen.
After surface putrefaction of meat has commenced, the
process spreads gradually by way of the nerve and con-
nective tissue sheaths and along the surfaces of blood or
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