Agriculture Reference
In-Depth Information
3
Areas of discolouration
may be caused by bruising or
blood splashing. This can be very obvious in the
cooked product.
4
Rancidity in frozen bacon
may be identified by pro-
nounced yellowing of the fat. Although all bacterial
growth stops when bacon is frozen, certain chemical
reactions can proceed at −8°C.
5
Browning
. The cured meat pigment nitrosomyoglobin
changes to the brown metamyoglobin owing to
dehydration caused by low humidity, high tempera-
ture and oxidation caused by prolonged exposure to
air, excessive nitrate and poor packaging.
6
Greening
may be caused by excessive nitrate and by
bacterial contamination.
the increased drip does not influence the eating quality
after cooking.
In the process of refrigeration, heat is extracted from
the carcase in the chill rooms, two basic laws of physics
being involved in this process. First, the boiling point of
a liquid - the temperature at which it is turned into
vapour - depends on the pressure. At normal atmospheric
pressure (1 atm = 760 mm Hg; 29.92, 1013.25 N/m
2
at sea
level), water boils at 100°C, but at a pressure of 0.1 atm, it
boils at 46°C. Conversely, water vapour at 50°C and
0.1 atm can be condensed back to water by increasing the
pressure to 1 atm. When a liquid passes into a vapour, it
absorbs heat which is given off again when it condenses.
Refrigerants are liquids or liquefied gases with low
boiling points, for example, ammonia or hydrofluoro-
carbons (HFCs), which, in the refrigeration cycle, extract
heat at low temperature when evaporated and give off
this heat to the outside air when recondensed.
The preservative action of refrigeration is based on
the prevention of multiplication of harmful bacteria,
yeasts and moulds by the artificial lowering of the tem-
perature. The failure of bacteria to grow at or below
freezing depends mainly on the removal of the availa-
ble water as ice; about 70% is removed at −3.5°C and
94% at −10°C. A further factor is the inhibition of the
life processes of spoilage organisms at low temperatures,
though the actual lethal effect is small. At a temperature
of −8°C, the multiplication of all micro-organisms
stops and only resumes when the temperature is raised
later to a suitable level. Neither fast (cryogenic) nor
slow (blast) freezing completely destroys all the bacteria
commonly found in beef carcases; frozen meat which is
thawed yields an abundant supply of water and forms
an excellent medium for bacterial growth. In addition,
the pH of muscle, which remains constant while the
meat is frozen, falls rapidly after thawing but then rises
rapidly to create an environment which favours bacte-
rial multiplication.
The surface growth of
mould
on meat is controlled not
only by the temperature but by the relative humidity of
the atmosphere. Some moulds are capable of growing on
the surface of meat at several degrees below freezing
point, but they require the presence of water in the sur-
rounding atmosphere as otherwise they lose water by
evaporation and wither. For the prevention of mould, the
temperature and relative humidity must therefore be
kept as low as possible.
Micro-organisms on cured product
The most common form of spoilage found on cured
meats is mouldiness, which may be due to
Aspergillus,
Alternaria, Fusarium, Mucor, Rhizopus, Penicillium,
Cladosporium
and other moulds.
Micrococci
are resistant to salt and consequently are
most common where salt levels are high, especially
the fat.
Lactobacilli
are less resistant to salt but more
resistant to smoke. These, together with
Acinetobacter,
Bacillus, Pseudomonas
and
Proteus
, may result in the
fermentation of sugars in the product to produce
sours
of various types. Pickling cannot be relied upon to
destroy parasitic infections, for example,
cysticerci
in
beef or pork.
Refrigeration
The modern meat industry is based on efficient refrig-
eration. Carcases of freshly slaughtered animals have
surfaces that are warm and wet and thus provide a per-
fect substrate for the growth of pathogenic and spoilage
organisms. Chilling immediately post-slaughter reduces
the surface temperature to a value below the minimum
growth temperature for many pathogens. The combina-
tion of low temperatures and surface drying inhibits the
growth of spoilage bacteria. To provide a long, safe, high-
quality shelf life, the temperature of the meat needs to be
kept at a temperature close to its initial freezing point.
Combining a high standard of hygiene and packaging
with a temperature of −1 to +0.5°C during storage, trans-
port and display can routinely extend shelf life to 12
weeks. If longer periods are required, then freezing the
meat will extend the storage period into years. Scientific
studies show that freezing has little if any effect on the
eating quality of red meat. Overall, the studies indicate
that the meat may be slightly more tender after freezing.
Freezing does increase the ultimate amount of drip from
meat, and this makes the meat less attractive. However,
Mechanical refrigeration
Carbon dioxide and sulphur dioxide were at one time
commonly used as cooling liquids (refrigerants), but
carbon dioxide is uneconomical, and sulphur dioxide is
corrosive and toxic.
