Agriculture Reference
In-Depth Information
most until conditions favourable to their growth appear.
Freezing is therefore of no great value in rendering a
carcase affected with pathogenic bacteria safe for human
consumption, nor are the bacteria commonly found on
beef carcases destroyed by slow or sharp freezing. Anthrax
bacilli can withstand a temperature of −130°C, while
Salmonella
can withstand exposure to −175°C for 3 days,
and tubercle bacilli have been found alive after 2 years in
carcases frozen at −10°C. The virus of foot-and-mouth
disease can remain viable for 76 days if carcases of ani-
mals slaughtered during the incubative stage of the dis-
ease are chilled or frozen immediately afterwards. Under
similar conditions, the virus of swine fever may remain
infective in the bone marrow for at least 73 days and has
also been shown to be viable in frozen pork for 1500 days.
Freezing is, however, a valuable method for the treat-
ment of meat affected with certain
parasitic infections
.
For example, pork affected with
Cysticercus cellulosae
can be rendered safe if held for 4 days at −10.5 to −8°C
as can beef with
Cysticercus bovis
by holding for 3 weeks
at a temperature not exceeding −6.5°C or for 2 weeks at a
temperature not higher than −10.5°C.
Trichinella
cysts in
pork are destroyed by holding the carcase for 10 days
at −25°C, but this is unreliable if the pork is more than
15 cm thick.
lamb or pork, but the better the original quality of a beef
carcase, the less on average will be the drip from the
meat after thawing. Quarters of frozen beef defrosted at
10°C for 3 days and cut into large wholesale joints lose
about 1-2% of their weight during the following day,
while smaller joints of the retail trade lose 1.5-2.5%.
It is claimed that drip is minimised if thawing is very
slow. One method employed for beef is to subject the
meat to a temperature of 0°C with 70% humidity, gradu-
ally increasing the temperature to 10°C and the humidity
to 90%; the forequarter requires 65 hours for complete
thawing and the hindquarter 80 hours. The major effect
on drip is the final temperature on thawing.
It is known that the faster the rate of breakdown of
ATP in muscle, the more rapid is the onset of
rigor
mortis
and the greater the release of fluid from the
muscles. If the rate of breakdown of ATP could be
slowed, that is, rigor mortis delayed, less free fluid
would be available for drip formation on subsequent
freezing and thawing.
Durability of frozen meat
Frozen meat stored too long becomes dry, rancid and
less palatable, the most important change being the
breaking down of the fat into glycerine and free fatty
acids, with the production of
rancidity
. The better the
quality of meat, the less trouble one encounters in its
storage. The storage temperature, the degree of fluctua-
tion in the storage temperature and the type of wrap-
ping (packaging) in which the meat is stored are
generally thought to have the main influence on frozen
storage life.
Temperature fluctuation is of limited importance
when the product is left at a temperature below −18°C
and the variation in temperature is only 1-2°C.
Well-packed products and those that are tightly packed
in palletised cartons are also less likely to show quality
loss. However, poorly packed items are severely affected
by temperature changes.
Heat: Thermal processing
The underlying principle of all food preserving methods
is either the creation of unfavourable environmental
conditions under which spoilage organisms cannot grow
or the destruction of such organisms. In commercial
canning, carefully selected and prepared foods contained
in a permanently sealed container are subjected to heat
for a definite period of time and then cooled. In most
canning processes, the heat destroys nearly all spoilage
organisms, and the permanent sealing of the container
prevents reinfection.
Aseptic canning
, involves the use of high temperatures
for short periods. The food is sterilised at 120°C for 6
seconds to 6 minutes depending on the food, before it
enters a sterilised can which is then closed with a steri-
lised lid. This method is said to improve the flavour and
the vitamin content of the canned product.
Aluminium or coated aluminium may be used as
alternatives to mild steel and tin in the fabrication of
cans
. While it has the advantage of lightness (and thereby
lower transport costs) and freedom from sulphiding and
rust, it buckles fairly easily. Efforts are being made to
produce an alloy strong enough to withstand the stresses
of processing, packing and transport.
Flexible
pouches
made from laminates of thermo-
plastic and aluminium foil are widely used. They will
Freezer burn
This occurs on the outer surface of frozen offals, particu-
larly the liver, hearts and kidneys, and is caused by loss of
moisture from the outer tissues. It may sometimes be
seen where a carcase is stored, unwrapped, close to the
opening of a cold air duct. The meat or offals have a
brown, withered discolouration. This can be prevented
by using suitable packaging or cryogenic freezing.
Effect of freezing on pathogenic
micro-organisms and parasites
Some bacteria are destroyed by freezing, but low temper-
atures merely inhibit the growth and multiplication of
