such as humidity and the presence of fat. Polyvinylidene chloride,
PVDC, is a material commonly used as a gas barrier; the oxygen
permeability of a 25mm thick film is 10 cm^3 m^2 (24h)^1 atm^1 compared
with values of 8500 and 1840 for low density and high density polythene
respectively. Higher rates of transfer occur with CO 2 for which the
permeability values are about five times those for oxygen. If a film is
required to exclude oxygen transfer completely, then a non-plastic ma-
terial such as aluminium foil must be included. This is seen for example
in the bags used to pack wines. In addition to the sealing- and gas
barrier-layers, laminates may also contain an outer layer such as nylon
which gives the pack greater resistance to damage.
Invacuum packingthe product is placed in a bag from which the air is
evacuated, causing the bag to collapse around the product before it is
sealed. Residual oxygen in the pack is absorbed through chemical
reactions with components in the product and any residual respiratory
activity in the product and its microflora. To achieve the best results, it is
important that the material to be packed has a shape that allows the
packaging film to collapse on to the product surface entirely – without
pockets and without the product puncturing the film.
Vacuum packing has been used for some years for primal cuts of red
meats. At chill temperatures, good quality meat in a vacuum pack will
keep up to five times longer than aerobically stored meats. The aerobic
microflora normally associated with the spoilage of conventionally
stored meats is prevented from growing by the high levels of CO 2
which develop in the pack after sealing (see Section 3.3.3) and the low
oxygen tension. The microflora that develops is dominated by lactic acid
bacteria which are metabolically less versatile than the Gram-negative
aerobes, grow more slowly and reach a lower ultimate population (see
Section 5.3).
In recent years vacuum packing has been increasingly used for retail
packs of products such as cooked meats, fish and prepared salads. It has
been used less often for retail packs of red meats since the meat acquires
the purple colour of myoglobin in its unoxygenated form. This does not
appeal to consumers even though oxygenation occurs very rapidly on
opening a vacuum pack and the meat assumes the more familiar bright
red, fresh meat appearance of oxymyoglobin. Cured meats, on the other
hand, are often vacuum packed for display since the cured meat pigment
nitrosomyoglobin is protected from oxidation by vacuum packing.
The expanding range of chilled foods stored under vacuum and the
availability of vacuum packing equipment for small-scale catering and
domestic use has prompted concern about increasing the risk from
psychrotrophicClostridium botulinum. A number of surveys have been
conducted to determine the natural incidence ofC. botulinumin these
products and the concensus is that it is very low. In one recent example,
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