food hygiene & safety 25
Gas mixtures help preserve
the quality of packaged meats
Myra Armson discusses modified atmosphere
Whilst the inclusion of CO
2
(20-30 per cent) in
packaging and how gas composition and
packages helps to control the former, the latter means
that rancidity may develop despite the product still
protocol adherence can impact on the
being desirable in terms of appearance. In addition,
appearance and quality of packaged meats.
should polyunsaturated fatty acids be oxidised, textural
and nutritive properties may be adversely affected.
Reductions in tenderness, juiciness and flavour have also
hen raw meat is exposed to air, it been linked with the oxidation of proteins in meat.
W
deteriorates rapidly, even when properly
chilled. In red meats, the bright
Spoilage organisms
red colour associated with freshness With low O
2
MAP, these issues are not a problem. O
2
is
fades to grey-brown as oxymyoglobin is converted to removed from meat packaging and usually replaced with
metmyoglobin. Lipid oxidation may also occur, affecting CO
2
(20-30 per cent) to prevent the growth of aerobic
both aroma and flavour acceptability. Growth of spoilage organisms and limit the oxidation of proteins
spoilage bacteria, such as Pseudomonas spp., exacerbates and lipids.
these effects, whilst also influencing meat texture. The Meat packaged in this way is superior to that
willingness of consumers packaged in high O
2

to purchase meat is greatly atmospheres in terms of
reduced by these changes, both tenderness and flavour.
and so, to ensure commercial However, the exudation
success, manufacturers of fluids from raw meats
must try to postpone them. may be increased by high
Modified atmosphere CO
2
concentrations and,
packaging (MAP) is one way because CO
2
is highly soluble
of doing this. in water and fats, it can
be absorbed by the meat,
Gas mixtures
reducing headspace volume
MAP is the enclosure of foods and resulting in packaging
within packs containing collapse.
gas mixtures that differ in To counteract this, N
2

composition from air. Levels is typically used as an inert
of oxygen (O
2
), nitrogen (N
2
), carbon dioxide (CO
2
) filler wherever CO
2
is used in MAP, particularly when
and sometimes other gases are adjusted to control semi-rigid containers are employed. N
2
also displaces O
2
,
biochemical and enzymic reactions, reduce moisture which is desirable for preventing oxidation, but complete
loss and inhibit the growth of microorganisms. Within exclusion of the gas can have detrimental effects upon
the meat industry, the use of modified atmospheres is consumer appeal.
widespread. The exact gas compositions employed vary Myoglobin will not be found in its unattractive
between products, but there are 2 main types: high O
2
oxidized form, but neither will it be oxygenated to form
and low O
2
. oxymyoglobin.
High O
2
MAP is particularly useful for packaging
raw red meat. It uses a combination of O
Carbon monoxide
2
and CO
2
to
enhance meat colour and inhibit microbial spoilage. The Use of carbon monoxide (CO), a colourless gas that has
elevated O
2
levels (70-80 per cent) prolong the period no flavour or aroma and binds to myoglobin to produce
for which the packaged meat exhibits the bright red carboxymyoglobin, may offer a solution to this problem.
coloration which consumers associate with freshness. Carboxymyoglobin imparts a bright red colour to the
This coloration only occurs when myoglobin is present in surface of meat in much the same way as oxymyoglobin,
its oxygenated form: oxymyoglobin. except that it has greater stability. Thus, modified
In the absence of oxygen, myoglobin is dark purple atmospheres containing CO can maintain desirable meat
and less appealing. The problem with high O
2
levels redness for longer than those simply containing high O
2
.
is that they encourage the growth of aerobic spoilage CO may find application in both low O
2
MAP, where
organisms and promote oxidation. conditions prohibit the oxygenation of myoglobin,
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