Agriculture Reference
In-Depth Information
accumulates. In the absence of water and air, bacterial
multiplication is significantly reduced, and shelf life can
be maintained for up to 3 months, provided that the
meat is of good microbiological standard. In the absence
of air, the meat assumes a bluish discolouration of myo-
globin but on re-exposure to air regains its normal red
colour on oxygenation to oxymyoglobin.
quickly stripped down, the freezer tunnel can be rap-
idly and thoroughly cleaned.
Mobile meat containers are now nitrogen cooled using
either the gas, at −196°C from a tanker via a towed distri-
bution unit, or the liquid as a coolant. The latter can
reduce the temperature of the container more quickly
but requires heavy storage vessels to be carried and is
thus relatively expensive. There is also a tendency
towards discolouration of the carcase and condensation
on unloading; exposure to the air may restore the colour,
while the use of loading hatches with an awning can
reduce condensation. The cost, however, is some four
times that of mechanical refrigeration.
Modified atmosphere packing
Packing meat in a
modified atmosphere packaging
(MAP)
which utilises sealed high-barrier packages in which the
air has been replaced with a mixture of gases which
reduces the rate of deterioration of the meat and permits
the preparation of packs for retail centrally. Most often,
these gases include 10-50% carbon dioxide, which
inhibits the growth of many micro-organisms that cause
spoilage of refrigerated meats. For fresh red meats, the
gas mixture often contains 20-50% oxygen so that the
myoglobin will be in the oxygenated cherry-red form.
The meats must be sealed in high-barrier films which
will keep the air out and prevent the modified atmos-
phere from escaping.
The process depends on the ability of carbon dioxide
gas, when present in high concentration, to prevent the
growth of moulds. Facultative bacteria may or may not
be suppressed by carbon dioxide, while lactic acid bacte-
ria and anaerobes are virtually unaffected. On the other
hand, the highly aerobic bacteria and yeasts and moulds
are selectively inhibited by carbon dioxide, and the stor-
age of meat in an appropriate concentration of this gas
will therefore retard surface decomposition, though it
will not prevent deep-seated anaerobic spoilage. Mould
growth can be arrested completely at 0°C if 40% CO
2
is
used, but any concentration over 20% rapidly produces
methaemoglobin on the exposed muscle and fat, and the
bloom is lost. Experiments have shown that in 10% CO
2
,
the storage life of meat at 0°C is double that of meat
stored in ordinary air at a similar temperature, and in
this way, the storage life of chilled meat can be extended
to 60-70 days.
Freeze-drying or lyophilisation
This is the process of removing water from frozen foods.
The food products must be in comminuted form (sliced
or diced), and packaging must be completely moisture-
proof since the dried products are hygroscopic. At the
present time, beef, pork, chicken, shellfish and other
foods such as mushrooms, fruits, peas and vegetables are
preserved by this process. Since meat has a relatively
high moisture content, it is relatively expensive to pre-
serve by freeze-drying methods, which at this stage of
development can be regarded only as a supplement to
traditional methods of refrigeration.
Storage of fresh meat
Meat cutting in supermarkets is labour consuming, and
as the cost of labour continues to increase, means to
reduce these costs are being sought. Deboned and
trimmed meat, in the unfrozen state, offers cost savings
in labour and transportation and requires less energy for
storage than does frozen meat. This has resulted in a
shift to more centralised meat cutting at packing plants
where the carcase is reduced to smaller sections called
primal
and
subprimal cuts
. These cuts are vacuum packed
in high-barrier bags placed in cartons or baskets for
shipment to retailers or processors, where they are fur-
ther reduced to retail-sized cuts or products.
Vacuum packing
This is a process in which primal cuts of meat are placed
in a gas-impermeable form of plastic (polythene, nylon/
polythene, polyester/polythene) laminate bags at 2-4°C
and a pH of 5.5-5.8. Two basic systems are used: (1)
'Cryovac, in which the air is sucked out and then the
pack is passed through either a water dip or a hot air tun-
nel, and (2) drawing a vacuum without heat shrinkage.
The advantage of (1) is that drip is reduced and there is
less possibility of the package being torn. The packs are
stored at between −18 and 1°C. The residual oxygen is
consumed by tissue respiration, and carbon dioxide
Refrigerated meat transport and storage
Depending on the type of trade and length of journey,
meat may be transported by road in properly insulated
and refrigerated (mechanical or liquid nitrogen) vehicles
or insulated or non-insulated non-refrigerated vehicles.
Only refrigerated transport can be considered adequate;
all other modes are totally ineffective, especially the non-
insulated type, particularly for chilled meat.
A well-designed
refrigerated road vehicle
should have
the following qualities: high standard of insulation, good
internal lining, airtight door seals, watertight flooring,
rigidity of construction, efficient refrigeration unit,
