Part III
Novel packaging and particular products
17.1 Introduction
Preservative packagings for fresh meats should maintain acceptable appearance
odour and flavour for product, while allowing the development of desirable
characteristics associated with ageing, and retarding the onset of microbial
spoilage (Taylor, 1985). Such effects can be achieved by packaging meats under
various atmospheres of oxygen, carbon dioxide, carbon monoxide and/or
nitrogen. The atmosphere within a pack may alter during storage, because of
reactions between components of the atmosphere and the product, and/or
because of transmission of gases into or out of the pack through the packaging
film (Stiles, 1991). Packagings of that type are termed Modified Atmosphere
Packs (MAP), which are distinguished from Controlled Atmosphere Packs
(CAP) within which invariant atmospheres are maintained throughout the time
of storage (Brody, 1996).
Both MAP and CAP can take various forms, depending on the type of meat
that is packaged, the form of the meat, and the commercial uses for the product.
Obviously, a commercial user of preservative packagings would usually seek the
simplest, and presumably least expensive packaging that would give a storage
life and organoleptic quality suitable to the trading envisaged for a particular
product. Thus, the optimum packaging for a product can be decided only with
knowledge of how the qualities of the particular meat are affected by the various
atmospheres to which it might be exposed, and the conditions the packaged
product will have to tolerate during commercial storage, distribution and
display.
17
Active packaging in practice: meat
C.O. Gill, Agriculture and Agri-Food Canada
17.2 Control of product appearance
The appearance of raw meat has major effects on the purchasing decisions of
consumers (Cornforth, 1994). For red meats, consumers much prefer bright, red
muscle tissue and white rather than yellow fat. When bone is present in a retail
cut, consumers prefer that any exposed spongy bone appears bright red also. For
poultry, bright, white flesh and skin are preferred.
The colour of muscle tissue in red meat is determined by the quantity and
chemical state of the muscle pigment myoglobin (Fig. 17.1). The deoxy form is
a dull, purple colour that consumers consider unattractive. The function of
myoglobin is to transfer oxygen from blood to the muscle tissue cells.
Myoglobin therefore reacts rapidly and reversibly with oxygen to give the bright
red form oxymyoglobin. The fraction of pigment in the oxymyoglobin form is
dependent on the partial pressure of oxygen to which the pigment is exposed
(Livingston and Brown, 1981). Myoglobin can also react with oxygen to give
the stable, oxidised form metmyoglobin (Faustman and Cassens, 1990). Meat
with the dull, brown colour of metmyoglobin is considered undesirable by most
consumers (Renerre, 1990).
Although metmyoglobin is stable, it is slowly reduced to deoxymyoglobin by
enzymic reactions involving reduced co-enzymes (Echevarne et al., 1990).
Those reactions are termed metmyoglobin reduction activity. Muscle tissue with
high metmyoglobin reduction activity can generally maintain a bright red colour
when exposed to oxygen for longer than tissue with little or none of the activity,
although high respiratory activity tends to accelerate discolouration (O’Keefe
and Hood, 1982). Different muscles vary considerably in their metmyoglobin
reduction and respiratory activates, and so vary in their colour stabilities during
Fig. 17.1 Reactions of myoglobin with oxygen and carbon monoxide.
366 Novel food packaging techniques
the first days after slaughter. For example, the longissimus dorsi usually has
good colour stability while the colour stability of the psoas major is poor (Hood,
1980). However, enzymic activities in muscle tissue decay with time, so after
storage for several days all muscle tissue has similar, low colour stability
(Ledward, 1985). The colour stability of ground meat is similarly low because
both respiratory and metmyoglobin reduction activates are rapidly lost when
meat is ground (Madhavi and Carpenter, 1993).
Both deoxymyoglobin and oxymyoglobin can oxidise to metmyoglobin.
However, the rate of the oxidation reaction is considerably faster with deoxy-
than with oxymyoglobin (Ledward, 1970). Consequently, when oxygen tensions
are low, and most of the myoglobin is in the deoxy form, oxidation of the
pigment occurs rapidly; while oxidation is retarded when the oxygen tension is
high and most of the pigment is in the oxy form. Haemoglobin visible in cut,
spongy bone reacts similarly with oxygen. Thus, increasing the oxygen in a pack
atmosphere above atmospheric concentrations will stabilise the desirable red
colours of muscle tissue and cut spongy bone surfaces. In addition, high
concentrations of oxygen will increase the depth of the oxymyoglobin layer at
the tissue surface, and so enhance the red colour of the muscle tissue (Young et
al., 1988).
Although high oxygen concentrations will retard pigment oxidation they do
not prevent it. Pigment oxidation is prevented only if oxygen is stripped from the
pack atmosphere and subsequently prevented from entering the pack (Gill,
1989). When a pack is first filled with a gas or gases other than oxygen, at least
some traces of oxygen will be present in the atmosphere (Penney and Bell,
1993). The residual oxygen will react with the muscle pigment to form
metmyoglobin. However, provided that the metmyoglobin reduction capacity of
the muscle tissue is not exceeded, the metmyoglobin will be reconverted to
deoxymyoglobin during the first few days of storage (Gill and Jones, 1994a).
After that, the pigment will remain in the deoxy form until it is exposed to air or
a high oxygen atmosphere (Table 17.1). Then, the tissue will bloom to the bright
red colour of freshly cut meat as oxymyoglobin is rapidly formed at tissue
surfaces. Such a desirable colour will, however, be maintained for a relatively
short time if the tissues have little if any metmyoglobin reduction activity to
counteract the unavoidable oxidation of the pigment.
In addition to the discolouration of the muscle tissue, exposed spongy bone in
cuts that have been stored under anoxic atmospheres tend to darken and finally
blacken relatively rapidly when the cuts are exposed to air. That intense
discolouration appears to be due to the accumulation of haemoglobin at cut bone
surfaces during storage (Gill, 1990). In air, the pigment oxidises as it would in
freshly cut tissue, but because the amount of pigment is so much greater, the
final colour is dark brown or black, rather than the lighter brown colours that
spongy bone will develop after meat is cut when fresh.
As an alternative to using high oxygen concentrations to stabilise meat
colour, or oxygen depleted atmospheres to prevent discolouration, red colours
for muscle and bone tissues can be maintained by exposing the tissues to carbon
Active packaging in practice: meat 367
monoxide. Carbon monoxide reacts with myoglobin to form the cherry red
pigment carboxymyoglobin, which is stable and oxidises only slowly (Lanier et
al., 1978). Therefore, exposure of meat to low concentrations of carbon
monoxide in a pack atmosphere will result in the tissues developing persistent
red colours.
The above comments about the colour of red meats are not wholly applicable
to poultry muscle. Poultry muscle generally has low concentrations of
myoglobin and high rates of oxygen consumption. Consequently, little
oxymyoglobin is formed when poultry muscle is exposed to air and consumers
are accustomed to the tones imparted to poultry meat by muscle pigment in the
deoxy- and metmyoglobin forms (Millar et al., 1994). Therefore, the colour of
poultry meat is not enhanced by storage under high oxygen atmospheres, while
the appearance of the meat is not grossly degraded by its exposure to low
concentrations of oxygen that would rapidly discolour red meats.
17.3 Control of flavour, texture and other characteristics
Other undesirable, non-microbiological changes that can occur during the
storage of raw meats are oxidation of lipids that impart stale and rancid odours
and flavours to the product; loss of exudate from the muscle tissue; and loss of
texture and development of liver-like flavours as results of the breakdown of
proteins. A desirable change is the increase of tenderness with ageing of the
muscle tissue.
In the absence of oxygen, lipids will not oxidise. Thus, rancidity does not
develop when meat is packaged under an oxygen depleted atmosphere.
Oxidation will occur with meat in air or oxygen enriched atmospheres.
Although it would be expected that the rates of lipid oxidation would increase
Table 17.1 Fractions of metmyoglobin in the muscle pigment of beef steak surfaces
after display in air for 1 h, following storage at 1.5oC under N
2
, CO
2
or, 67% O
2
+33%
CO
2
(Gill and Jones, 1994a)
Storage Metmyoglobin (%)
time Storage atmosphere
(days) N
2
CO
2
O
2
+ CO
2
1 7 60 4
2 25 25 7
4 23 14 0
6 0 2 3
8 8 6 9
12 0 0 17
16 0 6 10
20 7 6 23
24 8 0 42
60 0 0 –
368 Novel food packaging techniques
with increasing oxygen concentration, it has been reported that rates of oxidation
in air and oxygen enriched atmospheres are similar (Ordonez and Ledward,
1977). Antioxidants naturally present in or added to raw meats will retard the
development of rancidity and oxidation of myoglobin, but grinding of meat can
greatly accelerate lipid oxidation (Sanchez-Escalante et al., 2001). Lipid
oxidation is also accelerated by iron and iron containing compounds.
Consequently, when mechanically separated meats, which contain relatively
large amounts of iron, are included in comminuted products the oxidative
stability of the products is greatly reduced (Cross et al., 1987).
Loss of exudate from meat is undesirable, because of ill-effects upon the
appearance and handling qualities of cuts, and because of loss of saleable weight
when cuts must be divided and repackaged. Exudate losses are unavoidable, but
tend to be less with muscle tissue of higher than normal pH. Exudate losses are
exacerbated by cutting of meat to smaller portions and pressure on the product
(Offer and Knight, 1988). Therefore, in practice, the only options for containing
the adverse effects of exudate loss are the avoidance of pressure on product and
the inclusion in packs of absorbent pads or wraps of sufficient capacity to hold
all the exudate that may be released.
Unlike most changes that occur in meat with age, increased tenderness is
generally desirable (Jeremiah et al., 1993). The rate of tenderisation declines
approximately exponentially with time of storage. For beef stored at 2oC, 80%
and 100% of maximum tenderisation have been reported to be achieved after
about 9 and 17 days, respectively (Dransfield et al., 1992). Rates of tenderisation
are seemingly affected little if at all by the compositions of pack atmospheres,
and with most meats, as with beef, tenderising apparently does not continue
indefinitely. Even so, the breakdown of proteins can continue, with the
accumulation of peptides and free amino acids that impart liver-like flavours to
the meat (Rhodes and Lea, 1961). Consumers may find such flavours
objectionable (Gill, 1988a). With lamb it has been observed that tenderising
can continue until the fibrous texture of muscle tissues is lost. That undesirable
loss of texture and development of liver-like flavours do not occur when lamb is
stored under an atmosphere of carbon dioxide (Gill, 1989). No other effects of
carbon dioxide on tenderising processes have been reported.
17.4 Delaying microbial spoilage
Spoilage bacteria will grow on meat that is not frozen under both aerobic and
anaerobic conditions (Lowry and Gill, 1984). When the initial numbers of bacteria
are relatively low, the spoilage flora will be dominated by those species of bacteria
that grow most rapidly in the environment provided by the meat and the surrounding
atmosphere (Gill, 1986). When initial numbers are high, slower growing species
may persist as substantial fractions of a flora, as the maximum numbers may be
approached before they are overgrown by the usually dominant species. The meat
will be spoiled when the metabolic activities of the spoilage bacteria cause changes
Active packaging in practice: meat 369
in the appearance, odour or flavour of the product that are unacceptable to the
consumers (Gill, 1981). The stage of development of the spoilage flora at which such
changes occur depends on both the composition of the spoilage flora and the intrinsic
qualities of the tissues on which the bacteria are growing.
When fresh meat is stored in air, the spoilage flora is dominated by species of
Pseudomonas, which are strictly aerobic (Gill and Newton, 1977). Those
organisms preferentially utilise glucose, which is present in small quantities in
muscle tissue of normal pH (5.5) and usual higher values. When glucose is
exhausted the bacteria metabolise amino acids and produce offensive by-
products such as ammonia, amines and organic sulphides (Nychas et al., 1988).
Thus, on normal pH muscle tissue the onset of aerobic spoilage occurs abruptly
when bacterial numbers are about 10
8
/cm
2
. However, on muscle tissue of high
pH (> 6.0) and moist fat tissue, little or no glucose may be available (Gill and
Newton, 1980). Then, aerobic spoilage will occur when bacterial numbers are
about 10
6
/cm
2
.
The pseudomonads grow at their maximum rates when oxygen concentration
in the atmosphere is as low as 1% (Clark and Burki, 1972). Therefore, increasing
the oxygen concentration in a pack atmosphere to preserve meat colour does not
accelerate microbial spoilage. However, if the storage life of meat is to be
extended the rapid growth of pseudomonads must be suppressed.
Growth of pseudomonads is inhibited by carbon dioxide. The growth rate
decreases with increasing concentrations of carbon dioxide in the atmosphere up to
about 20% (Gill and Tan, 1980). Further increases in carbon dioxide concentration
do little more to slow the rate of growth. Thus, with an aerobic atmosphere, a
doubling of the time before the onset of microbial spoilage is the most that can be
achieved by the inclusion of carbon dioxide in a pack atmosphere.
When growth of pseudomonads is inhibited by carbon dioxide, the flora of
meat in an aerobic atmosphere is usually dominated by lactic acid bacteria, with
more or less large fractions of strict aerobes, such as pseudomonads and
acinetobacteria, and facultative anaerobes, such as Brochothrix thermosphacta
and enterobacteria (Gill and Jones, 1996). If meat is held in air after storage
under a modified atmosphere the lactic acid bacteria, which are of low spoilage
potential, may continue to predominate in the flora. However, the fractions of
the strict aerobes and facultative anaerobes will usually increase as the flora
proliferates; and spoilage will develop as a result of the activities of those latter
organisms (Gill and Jones, 1994b).
Under anaerobic conditions, the strictly aerobic pseudomonads cannot grow
and again the spoilage flora is usually dominated by lactic acid bacteria (Egan,
1983). Those bacteria can grow to maximum numbers about 10
8
/cm
2
without
spoilage of the meat. Thereafter, spoilage will develop only slowly as the by-
products of the lactic acid bacteria’s metabolism impart acid, dairy flavours to
the meat (Dainty et al., 1979). The spoilage process can differ if the tissue pH is
>5.8 or the atmosphere contains traces of oxygen. Then, facultative anaerobes
such as B. thermosphacta, enterobacteria and Shewanella putrefaciens may grow
to spoil the meat as the flora approaches maximum numbers (Blickstad, 1983;
370 Novel food packaging techniques
Grau, 1983). However, in a controlled atmosphere of carbon dioxide alone, the
growth of some of those organisms is inhibited or prevented when temperatures
are at the lower end of the chill temperature range (Gill and Harrison, 1989).
Inclusion of small amounts of carbon monoxide in anaerobic atmospheres does
not affect development of the spoilage flora (S?rheim et al., 1999). If meat is
held in air after storage under an anaerobic atmosphere, spoilage by facultative
anaerobes or strictly aerobic organisms is likely to occur although lactic acid
bacteria continue to predominate in the flora (Gill and Jones, 1996).
17.5 The effects of temperature on storage life
All changes that occur in chilled meat during storage are likely to be accelerated
by increasing temperature. As most changes are deleterious, it follows that the
optimum temperature for storing chilled meats is the minimum that can be
maintained indefinitely without freezing the muscle tissue. In practice, that
temperature is found to be 1.5 0.5oC (Gill et al., 1988).
When red meats are displayed in aerobic atmospheres, discolouration rather
than microbial spoilage is likely to limit the useful life of the product. The rate at
which discolouration develops in muscle tissue exposed to air appears to
increase linearly with temperature for all muscles, but the rate of increase differs
between muscles (Hood, 1980). The rate of increase seems to be less for colour
stable than for colour unstable muscles, as discolouration of the colour stable
longissimus dorsi and the colour unstable psoas major muscles are reported to
be, respectively, twice and five times as rapid at 10oC than at 0oC. The effect of
temperature on the rate of discolouration of meat stored in modified atmospheres
rich in oxygen does not appear to be well identified in the literature, but it seems
likely that discolouration with increasing temperature accelerates much as for
meat stored in air.
When meat is stored anaerobically, the colour stability of muscle tissue
increases at first, and then declines (O’Keefe and Hood, 1980–81). The initial
increase of stability is probably related to the relatively rapid loss of respiratory
activity, while the subsequent decrease in stability reflects the decay of
metmyoglobin reduction activities. The rate at which colour stability degrades is
reported to be twice as fast at 5oC and four times as fast at 10oC as at 0oC
(O’Keefe and Hood, 1982).
Rates of lipid oxidation in air and oxygen enriched atmospheres are
apparently similar, but the effect of storage temperature on the rate of
development of rancidity does not seem to have been established. Exudate losses
are reported to be about 30% and 100% more, respectively, at 5oC and 10oC than
at 0oC (O’Keefe and Hood, 1980–81). The rate at which muscle tenderises is
over twice as fast at 10C as at 0C (Dransfield, 1994).
Spoilage bacteria will grow on meat that is not frozen at temperatures down
to 3oC under both aerobic and anaerobic conditions. Thus, storage at chiller
temperatures can delay but not prevent the ultimate onset of microbial spoilage.
Active packaging in practice: meat 371
Although the rates of growth of different species of spoilage bacteria differ
considerably the rates of all increase rapidly with small increases in temperature
above the optimum for storage of chilled meat (Gill and Jones, 1992; Gill et al.,
1995). The proportional loss of storage life for the same increase in storage
temperature is then broadly similar for all types of spoilage flora. Thus, it is
found that the storage life of meat in any or no packaging at 0, 2 and 5oC is about
70, 50 and 30%, respectively, of the storage life that would be obtained for the
product stored at 1.5oC (Fig. 17.2).
In view of the substantial effects of small increases in temperature on rates of
discolouration and bacterial growth, it is apparent that any storage life ascribed
to a fresh meat product must be accompanied by a statement of the storage
temperature if the storage stability of the product is to be properly understood.
17.6 MAP technology for meat products
Modified atmosphere packagings may be used for bulk or retail ready product.
Several trays of retail ready product may be placed in a master pack which is
filled with the modified atmosphere, or individual, sealed trays may contain the
modified atmosphere. Modified atmospheres invariably contain substantial
fractions of carbon dioxide to retard the growth of aerobic spoilage organisms.
In addition, atmospheres used with red meats will usually contain a high
concentration of oxygen to preserve the meat colour or the initial atmosphere
may contain a small amount of carbon monoxide to impart a stable red colour to
Fig. 17.2 Effects of storage temperature on the storage life of chilled meat limited by
microbial spoilage.
372 Novel food packaging techniques
the product. An atmosphere may also contain a more or less substantial fraction
of nitrogen, to prevent pack collapse.
The materials used to form modified atmosphere packs must provide a barrier
to the exchange of gases between the pack and the ambient atmosphere.
However, the gas barrier properties of the packaging materials differ for
different types of packaging and differing commercial functions of the packs.
Bulk and master packagings which are expected to contain product for only a
day or two are often laminates composed of a strong material with limited gas
barrier properties, such as nylon, and a sealable layer of a material such as
polyethylene. Such materials may have nominal oxygen transmission rates of
more than 100 cc/m
2
/24h/atm under stated conditions of humidity and
temperature. However, films used for modified atmosphere packs usually have
oxygen transmission rates between 10 and 100cc O
2
/m
2
/24h/atm, while
packagings designed to contain product for the longest possible times are likely
to be composed of materials with oxygen transmission rates less than 10 cc/m
2
/
24/atm (Jenkins and Harrington, 1991).
Carbon dioxide, the essential component of any effective modified
atmosphere for meat is highly soluble in both muscle and fat tissues (Gill,
1988b). The solubility in muscle tissue decreases with decreasing pH and
increasing temperature but, within the chill temperature range, solubility in fat
increases with increasing temperatures (Fig. 17.3). Because of the dissolution of
carbon dioxide in the product, the initial atmosphere in a pack should contain a
higher concentration of carbon dioxide than the 20% that it is desirable to
maintain after equilibration for maximum inhabitation of the aerobic spoilage
bacteria. The smaller the volume of the atmosphere in relation to the product
mass, the higher the carbon dioxide concentration needed in the input gas, and
the greater the decrease in the volume of the atmosphere as carbon dioxide
dissolves in the tissues after the pack is sealed.
Fig. 17.3 Effects of temperature on the solubility of carbon dioxide in normal pH
muscle tissue (a72) and fat tissue (?) of beef.
Active packaging in practice: meat 373
Unlike carbon dioxide, the solubility of oxygen in muscle and fat tissues is
low. However, oxygen is converted to carbon dioxide by the respiratory
activities of both muscle tissue and bacteria. Although both gases are lost
through packaging films when both are at concentrations above those of air, the
carbon dioxide dissolved in tissue buffers decreases in carbon dioxide
concentrations. Thus, with modified atmospheres rich in oxygen it is usually
found that oxygen concentrations decline with time of storage, but that carbon
dioxide concentrations alter little after the initial dissolution of the gas in the
tissues (Nortje and Shaw, 1989). If packs with oxygen-rich atmospheres are to
be stored for relatively long times, the volume of the pack atmosphere should be
about three times the volume of the product, to avoid excessive decreases of
oxygen concentrations (Holland, 1980).
The solubility of nitrogen in tissues is low, and the gas is metabolically inert.
Thus, the only function of nitrogen in a pack atmosphere is to buffer against
changes in the volume of the atmosphere that could lead to pack collapse, with
crushing of the contained product. If carbon monoxide is included in a pack
atmosphere it is at concentrations less than 1%. The gas will be removed from
the pack atmospheres as it reacts rapidly and essentially irreversibly with
myoglobin. The changes in pack atmosphere volumes as a result of the binding
of carbon monoxide are trivial in comparison with the volume decreases arising
from dissolution of carbon dioxide. Modified atmosphere packagings for bulk
meats are usually intended only to enhance stability for short times during the
distribution of product from slaughtering or carcass breaking facilities to retail
packing facilities. Protection of the product from crushing by its being in a
pillow-pack is often considered to be as important as any effects of the
atmosphere on the colour or microbiological condition of the product.
Bulk packs are usually formed using equipment with two flattened tubes
(snorkels) that are inserted into the mouth of each bag. Sprung guides at each
side of the mouth prevent bunching. The mouth is held closed around the
snorkels by padded jaws. Air is evacuated from the bag through the snorkels.
The evacuation may be timed, or terminated when the pressure within the
snorkels falls to a pre-set value. If evacuation is controlled by pressure and the
snorkel orifices are not sealed by the bag collapsing around them, then the bag
will collapse around the product and residual air in the pack will be minimised.
After evacuation, the pack is filled for a set time with the selected gas mixture.
The evacuation and gassing cycle may be repeated if it is considered that the
pack atmosphere may be excessively contaminated with residual air after a
single cycle. When the bag has finally been filled with gas, the snorkels are
withdrawn from between the closed pads, and the bag is heat sealed.
With poultry meats the input gas may be a carbon dioxide/nitrogen mixture
with the former gas at concentrations between 40 and 60%. However, 5%
oxygen may be included in a mixture because of concerns about the possible
growth of Clostridium botulinum if the atmosphere should become anaerobic. In
fact, the inclusion of oxygen in the atmosphere will not prevent the growth of
botulinum organisms, as anaerobic niches that could permit the growth of such
374 Novel food packaging techniques
organisms exist in any package of raw meat, irrespective of the surrounding
atmospheres (Lambert et al., 1991). For red meats the input gas would
preferably be 70% oxygen and 30% carbon dioxide. However, nitrogen is often
included in a mixture although that gas will serve no useful function when, as in
these circumstances, the pack is flexible and the volume variable, and any
undesirable pack collapse may be countered by simply increasing the volume of
input gas.
Snorkel type equipment is also used for master packaging of retail ready
product, with master packs being filled with the same gas mixtures that are used
with bulk product. Retail ready product that is master packaged is usually in
conventional, expanded polystyrene trays, which are overwrapped with a
clinging film of oxygen permeability between 5,000 and 10,000 cc/m
2
/24h/atm.
The trays usually contain plastic covered paper pads, to absorb exudate from the
meat.
Because collapse of the bag around the trays when the master pack bag is
evacuated could easily lead to crushing of the trays, evacuation is usually timed.
Evacuation of the bag is then highly uncertain, as the amount of air in the bag
when the mouth is closed around the snorkels can vary greatly. Moreover, the
overwrapped trays will contain more or less large amounts of air that cannot be
removed during evacuation. Consequently, the master pack atmospheres are
diluted with air to varying extents. Carbon dioxide and oxygen concentrations in
master pack atmospheres are then often much below the concentrations optimal
for preservation of the product. However, irrespective of the gas atmosphere,
master packs provide mechanical protection for filled trays during their
distribution from central cutting facilities to retail outlets.
Various types of equipment have been developed for preparing different
forms of lidded trays that each contain a modified atmosphere. The atmosphere
used for such trays is typically 60% oxygen, 30% carbon dioxide and 10%
nitrogen. Storage/display lives of up to two weeks are often claimed for product
in such trays. However, to attain such useful life, temperature during display as
well as during storage must be well controlled, and the volume of the pack
atmosphere must be large in relation to the amount of product in the pack.
Control of product temperatures during display is often uncertain (B?gh-
S?rensen and Olsson, 1990), and many retailers consider that small quantities of
product in large packs are unattractive to consumers. Therefore, retailers often
select modified atmosphere packs to provide an attractive packing in which a
high concentration of oxygen, and thus an enhanced meat colour, are maintained
for a limited time. Adequate display stability for the product is obtained by
control of product temperatures near 1.5oC and by frequent, often daily
delivery of freshly packaged product to retail outlets (Gill et al., 2002a). The
success of many current distribution systems for master packed product is
achieved similarly.
The use of carbon monoxide in modified atmosphere is not permitted in most
countries, because of the highly poisonous nature of that gas. Despite that, the
risks to consumers from the presence of small amounts of carboxymyoglobin in
Active packaging in practice: meat 375
raw meat appear to be small, and carbon monoxide is a common component of
the modified atmospheres used with raw meats in Norway (S?rheim et al.,
1997). As carboxymyoglobin confers a red colour on meat irrespective of the
presence of oxygen, a modified atmosphere with carbon monoxide need contain
no oxygen. The input gas then typically contains 60% carbon dioxide and 40%
nitrogen, with carbon monoxide at 0.3 to 0.5%. The major components of the
input gas are at concentrations that will give the maximum carbon dioxide
concentration after equilibration without the risk of pack collapse. Thus, the
carbon dioxide concentration can be maintained at levels above that required for
maximum inhibition of aerobic spoilage organisms for relatively long times,
without resort to volumes of atmosphere much greater than the volumes of
product. Therefore carbon monoxide/high carbon dioxide atmospheres stabilise
meat colour and delay microbial spoilage, and so preserve the product in an
acceptable condition even when delivery is relatively infrequent and display is
prolonged.
17.7 Controlled atmosphere packaging for meat products
The only types of controlled atmosphere packagings currently used with raw
meats are those in which an anaerobic atmosphere is maintained indefinitely.
Controlled atmosphere packagings may be used for bulk product or items of
irregular shape, such as whole lamb carcasses, or as master packs for retail-ready
product. Controlled atmosphere packaging is not suitable for individual trays of
retail-ready product because of the undesirable colour of anoxic meat, and
because packaging materials that are impermeable to gases are mostly opaque.
Readily available films that are essentially gas impermeable are laminates that
incorporate a layer of aluminum foil, laminates with two layers of a metallised
film, or laminates with unusually thick layers of plastics with high barrier
properties (Kelly, 1989).
Controlled atmospheres may be of carbon dioxide or nitrogen, or mixtures of
the two gases. Nitrogen can provide an anaerobic atmosphere, but does not
otherwise affect the muscle tissue or the microflora. Thus, the storage life of
meats in a controlled atmosphere of nitrogen is similar to that of meats in
vacuum pack; although in a gas impermeable, controlled atmosphere pack there
is no oxidation of myoglobin in exudate or muscle tissue, which eventually
become evident with meat in vacuum packs as the result of small quantities of
oxygen permeating the packaging films (Jeremiah et al., 1992).
Atmospheres of carbon dioxide have inhibitory effects on some organisms of
the anaerobic spoilage flora, and can apparently retard the excessive tenderising
of at least lamb. The inhibiting effects of carbon dioxide on the microflora
appear to reduce rapidly with reducing concentrations of carbon dioxide in the
atmosphere, so an atmosphere of or near 100% carbon dioxide is required if the
storage stability of the product is to be substantially increased over that
attainable with a nitrogen atmosphere (Gill and Penney, 1988). When an
376 Novel food packaging techniques
atmosphere rich in carbon dioxide is used, the high solubility of the gas in meat
tissues must be taken into account. In an atmosphere of 100% carbon dioxide,
meat will absorb approximately its own volume of the gas. Thus, the initial gas
volume must exceed the required final volume by the volume of the enclosed
meat.
Specialised equipment for forming controlled atmosphere packs is available.
With such equipment, snorkels are inserted in the mouth of a filled bag, and the
mouth is closed around them; then a hood is placed over the bag, with enclosure
of the snorkels and bag sealing elements of the equipment. Air is withdrawn
from the bag through the snorkels while the hood is simultaneously evacuated.
The bag inflates in the evacuated hood, which ensures that no part of the bag
collapses to entrap air. Some air is then admitted into the hood to give a low
pressure which will collapse that bag around the product without crushing it.
Thus, the volume of the bag is minimised before it is filled with gas. A pack may
be flushed with the input gas one or more times before it is sealed. That
relatively elaborate filling procedure is adopted to minimise the amount of
residual oxygen in the pack. Even so, residual oxygen concentrations after pack
sealing are usually about 100ppm (Penney and Bell, 1993).
Snorkel equipment without a hood and even tray gassing equipment have
been used, at least experimentally, for the production of controlled atmosphere
packs. The residual oxygen in such packs is apparently often about 1%, which
can have grossly adverse affects upon the colour of red meats. Even 100 ppm of
oxygen can result in discolouration of product. However, in those latter
circumstances discolouration is usually transient, as the metmyoglobin is
reduced to myoglobin, usually within four days, as anoxic conditions are
established and maintained (Gill and Jones, 1994a).
Various studies have been conducted to determine if oxygen scavengers
might be used to prevent permanent discolouration of red meats in atmospheres
with initial concentration about 1%, or transient discolouration of meats in
atmospheres with very low concentrations of residual oxygen. Although some
success with the atmospheres of the former type have been reported (Doherty
and Allen, 1998), the general utility of such an approach must be doubted
because the muscle tissue itself acts as a very efficient oxygen scavenger (Table
17.2). Certainly, findings with the use of oxygen scavengers in atmospheres of
very low initial oxygen concentration have been that numerous, fast reacting
oxygen scaverages must be employed if transient browning is to be prevented
(Tewari et al., 2002).
17.8 Future trends in active packagings for raw meats
In most developed countries, sales of raw meat at supermarkets have tended to
increase at the expense of sales at specialised butchers’ stores (Mannion, 1995).
The maintenance of butchering facilities at supermarkets is increasingly seen as
undesirable, both because of the use of costly floor space that might otherwise
Active packaging in practice: meat 377
be used for selling foods, and because of difficulties with obtaining staff skilled
in butchery. Therefore, supermarket operators have been for some years
generally inclined to move toward the preparation of display ready product at
central butchering facilities (Lazar, 2001). Although modified atmosphere
packagings of various types have been used with mixed results in central cutting
operations, most successful operations now rely on the frequent preparation of
retail packs, with frequent and speedy delivery of product held at temperatures
near the optimum for chilled meat, rather than the preservative capabilities of
modified atmospheres.
Simultaneously there has been a trend towards consolidation of slaughtering
facilities so that in some regions, such as North America, most animals are now
slaughtered at relatively few large plants. Given the move towards central
preparation of retail ready meat, it would seem economically advantageous to
prepare retail packs at slaughtering plants. That would avoid the double
handling, and double packaging of product that now occurs with the
consignment of vacuum packaged primal cuts or bulk packed product from
slaughtering plants to central cutting facilities. The retail packaging of product
of compromised colour stability, and loss of product weight as exudate after
prolonged storage in vacuum pack could also be avoided.
Although retail preparation of product at slaughtering plants is increasing,
particularly with poultry, the need for frequent and speedy delivery limits the
area of distribution. Thus, for the largest plants from which product is widely
distributed preparation of retail product is a minor activity at most.
A general conflation of slaughtering with preparation of retail-ready product
at a few large plants would seem to be practicable only if the useful life of retail-
ready product reliably exceeds the storage and display times usual in current
commercial practice (Fig. 17.4). When meat is stored and displayed at normal
commercial temperatures, such storage stability can be attained by master
packaging meat under controlled, anoxic atmospheres, as has been demonstrated
with commercial systems for the distribution of lamb in the USA or by use of
carbon monoxide in modified atmospheres, as has been demonstrated with
commercial systems in Norway.
Table 17.2 Half life of oxygen in packs containing 4L of atmospheres with < 1%
oxygen when packs contained either four trays of ground beef or 32 oxygen scavengers
each with a capacity of 200 mlg oxygen (Gill and McGinnis, 1995)
Temperature O
2
half life (h)
(C) With meat With O
2
scavengers
1.5 4.7 0.6
0 3.8 0.6
2 2.9 0.5
5 1.4 0.5
10 1.6 0.5
378 Novel food packaging techniques
Despite the commercial advantages of, and the trivial risks associated with,
the use of carbon monoxide, it is unlikely that many countries will sanction meat
being treated with a recognised poison. A trend towards increasing use of
controlled atmosphere packaging for retail ready product might then be
anticipated. However, the complexities of meat trading are likely to ensure that
such a trend develops only slowly.
Although controlled atmosphere packaging could be used for continental
distribution of retail-ready meat, it is unlikely to be used for global distribution
of such product. Storage life would not necessarily constrain global distribution
but the low packing density of retail packed product as compared with bulk
product could render shipment of meat by sea uneconomical. Thus, controlled
atmosphere packing is unlikely to replace vacuum packing in trading of chilled
meats to distant markets, unless there is a move to retail portioning but not retail
packaging at exporting plants. Otherwise, use of controlled atmosphere packing
for trading meat over long distances is likely to remain restricted to products that
cannot be successfully vacuum packaged, such as whole lamb carcasses.
17.9 References
BLICKSTAD E (1983), ‘Growth and end product formation of two psychrotrophic
Lactobacillus spp. and Brochothrix thermosphacta ATCC 115009 at
different pH values and temperatures’. Appl Environment Microbiol, 46,
1345–50.
B?GH-S?RENSEN L and OLSSON P (1990), ‘The chill chain’, in Gormley T R
Chilled foods: the state of the art, London, Elsevier, 245–67.
BRODY A L (1996), ‘Integrating aseptic and modified atmosphere packaging to
fulfil a vision of tomorrow’, Food Technol, 50 (4), 56–66.
CLARK D S and BURKI T (1972), ‘Oxygen requirements of strains of
Pseudomonas and Achromobacter’, Canad J Microbiol, 18, 321–26.
Fig. 17.4 Median temperatures of all ( —), the coldest 2% (
a73 a73 a73
) and the warmest 2%
(--.--) of vacuum packaged beef distributed in Canada (Gill et al., 2002b).
Active packaging in practice: meat 379
CORNFORTH D. (1994), ‘Color-its basis and importance’, in Pearson A M and
Dutson T R, Quality attributes and their measurement in meat, poultry and
fish products, Glasgow, Blackie, 34–78.
CROSS H R, LEU R and MILLER M F (1987), ‘Scope of warmed-over flavour and its
importance to the meat industry’, in St. Angelo A J and Bailey M E,
Warmed-over flavour of meats, New York, Academic Press, 1–18.
DAINTY R M, SHAW B G, HARDING C O and MICHANIE S (1979), ‘The spoilage of
vacuum packaged beef by cold tolerant bacteria’, in Russell A D and
Fuller R, Cold tolerant microbes in spoilage and the environment,
London, Academic Press, 83–100.
DOHERTY A M and ALLEN P (1998). ‘The effect of oxygen scavengers on the color
stability and shelf-life of CO
2
master packaged pork’, J Muscle Foods, 9,
351–63.
DRANSFIELD E (1994), ‘Tenderness of meat, poultry and fish’, in Pearson A M
and Dutson T R, Quality attributes and their measurement in meat, poultry
and fish products, Glasgow, Blackie, 289–315.
DRANSFIELD E, WAKEFIELD D K and PARKMAN I D (1992). ‘Modeling post-mortem
tenderization. 1: Texture of electrically stimulated and non-stimulated
beef’, Meat Sci, 31, 57–73.
ECHEVARNE C, RENERRE M and LABAS R (1990), ‘Metmyoglobin reductive
activity in bovine muscle’, Meat Sci, 27, 161–72.
EGAN A F (1983), ‘Lactic acid bacteria of meat and meat products’, Antonie van
Leeuwenhoek, 49, 327–36.
FAUSTMAN C and CASSENS R G (1990), ‘The biochemical basis for discoloration
in fresh meat: a review’, J Muscle Foods, 1, 217–43.
GILL C O (1981), ‘Meat spoilage and evaluation of the potential storage life of
fresh meat’, J Food Prot, 46, 444–52.
GILL C O (1986), ‘The control of microbial spoilage in fresh meats’, in Pearson A
M and Dutson T R, Advances in Meat Research, vol. 2, Westport, AVI
Publishing pp. 49–88.
GILL C O (1988a) ‘Microbiology of edible meat by-products’, in Pearson A M
and Dutson T R, Edible meat by-products, Barking, Elsevier, 47–82.
GILL C O (1988b), ‘The solubility of carbon dioxide in meat’, Meat Sci, 22, 65–
71.
GILL C O (1989), ‘Packaging meat for prolonged chilled storage: the Captech
process’, Brit Food J, 91, 11–15.
GILL C O (1990), ‘Meat and modified atmosphere packaging’, in Hui Y H, The
encyclopedia of food science and technology, New York, Wiley, 1678–83.
GILL C O and HARRISON, J C L (1989), ‘The storage life of chilled pork packaged
under vacuum or carbon dioxide’, Food Microbiol, 26, 313–24.
GILL C O and JONES T (1992), ‘Assessment of the hygienic efficiencies of two
commercial processes for cooling pig carcasses’, Food Microbiol, 9, 335–
43.
GILL C O and JONES T (1994a), ‘The display life of retail-packaged beef steaks
after their storage in master packs under various atmospheres’, Meat Sci,
380 Novel food packaging techniques
38, 385–96.
GILL C O and JONES T (1994b), ‘The display of retail-packs of ground beef after
their storage in master packs under various atmospheres’, Meat Sci, 37,
281–95.
GILL C O and JONES T (1996), ‘The display life of retail-packaged pork chops
after their storage in master packs under atmospheres of N
2
, CO
2
or
O
2
+CO
2
’, Meat Sci, 42, 203–13.
GILL C O and NEWTON K G (1977), ‘The development of spoilage flora on meat
stored at chill temperatures’, J Appl Bacteriol, 43, 189–95.
GILL C O and MCGINNIS J C (1995), ‘The use of oxygen scavengers to prevent the
transient discolouration of ground beef packed under controlled, oxygen-
depleted atmospheres’, Meat Sci, 41,19–27.
GILL C O and NEWTON K G (1980), ‘Development of bacterial spoilage at adipose
tissue surfaces of fresh meat’. Appl Environ Microbiol, 39, 1076–7.
GILL C O and PENNEY N (1988), ‘The effect of the initial gas volume to meat
weights ratio on the storage life of chilled beef packaged under carbon
dioxide’, Meat Sci, 22, 53–63.
GILL C O and TAN K H (1980), ‘Effect of carbon dioxide on growth of meat
spoilage bacteria’, Appl Environ Microbiol, 39, 317–19.
GILL C O, PHILLIPS D M and HARRISON J C L (1988), ‘Product temperature criteria
for shipment of chilled meats to distant markets’, in Refrigeration for food
and people, Paris, International Institute of Refrigeration, 40–7.
GILL C O, FRISKE M, TONG A K W and MCGINNIS J C (1995), ‘Assessment of the
hygienic characteristics of a process for the distribution of processed
meats, and of storage conditions at retail outlets’, Food Res Int. 28, 131–8.
GILL C O, JONES T, RAHN K, HOUDE A, MCGINNIS J C, CAMPBELL S, HOLLEY R A and
LEBLANC DI (2002a), ‘Control of product temperatures during the
distribution of retail ready beef to stores and vacuum packaged beef to
restaurants’, Dairy Food Environ Sanit, 22, 422–8.
GILL C O, JONES T., RAHN K, CAMPBELL S, LEBLANC D I, HOLLEY R A and STARK R.
(2002b). ‘Temperatures and ages of boxed beef packaged and distributed
in Canada’, Meat Sci, 60, 401–10.
GRAU F H (1983), ‘Microbial growth on fat and lean surfaces of vacuum
packaged beef’, J Food Sci, 48, 326–9.
HOLLAND G C. (1980), ‘Modified atmospheres for fresh meat distribution’, Proc
33rd Meat Ind Res Conf, Chicago, Am Meat Sci Asn, 1980.
HOOD D E (1980), ‘Factors affecting the rate of metmyoglobin accumulation in
pre-packaged beef’, Meat Sci, 4, 247–65.
JENKINS W A and HARRINGTON J P (1991), ‘Fresh meat and poultry’, in Packaging
foods with plastics, Lancaster, Technomic Publishing, 109–22.
JEREMIAH L E, PENNEY N and GILL C O (1992), ‘The effects of prolonged storage
under vacuum or CO
2
on the flavour and texture profiles of chilled pork’,
Food Res Int, 25, 9–19.
JEREMIAH L E, TONG A K W, JONES S D M and MCDONELL C (1993), ‘A survey of
Canadian consumer perceptions of beef in relation to general perceptions
Active packaging in practice: meat 381
regarding foods’, J Consum Stud Home Econom, 17, 13–37.
KELLY R S A (1989), ‘High barrier metalized laminates for food packagings’, in
Findlayson K M, Plastic film technology, vol. 1, Lancaster, Technomic
Publishing, 146–52.
LAMBERT A D, SMITH J P and DODDS K L (1991), ‘Effects of initial O
2
and CO
2
and
low-dose irradiation on toxin production by Clostridium botulinum in
MAP fresh pork’, J Food Prot, 54, 939–44.
LANIER T C, CARPENTER J A, TOLEDO R T and REAGAN J O (1978), ‘Metmyoglobin
reduction in beef systems as affected by aerobic, anaerobic and carbon
monoxide-containing environments’, J Food Sci, 43, 1788–92.
LAZAR V (2001), ‘You’ve got it!’ Meat Proces, 40 (10), 22–31.
LEDWARD D A (1970), ‘Metmyoglobin formation in beef stored in carbon dioxide
enriched and oxygen-depleted atmospheres’, J Food Sci 35, 33–7.
LEDWARD D A (1985), ‘Post-slaughter influences on the formation of
metmyoglobin in beef muscle’, Meat Sci, 15, 149–71.
LIVINGSTON D J and BROWN W D (1981), ‘The chemistry of myoglobin and its
reactions’, Food Technol, 35(5), 244–52.
LOWRY P D and GILL CO (1984), ‘Mould growth on meat at freezing
temperatures’, Int J Refrig, 7, 133–6.
MADHAVI D L and CARPENTER C E (1993), ‘Aging and processing affect color,
metmyoglobin reductase and oxygen consumption of beef muscles’, J
Food Sci 58, 939–47.
MANNION P (1995), ‘Meat retail: major change’, Meat Int, 3 (4), 10–13.
MILLAR S, WILSON R, MOSS B W and LEDWARD D A (1994), ‘Oxymyoglobin
formation in meat and poultry’, Meat Sci, 36, 397–406.
NORTJE G L and SHAW B G (1989), ‘The effect of ageing treatment on the
microbiology and storage characteristics of beef in modified atmospheres
packs containing 25% CO
2
plus 75% O
2
’, Meat Sci, 25, 43–58.
NYCHAS G J, DILLON V M and BOARD R G (1988), ‘Glucose, the key substrate in
the microbiological changes occurring in meat and certain meat products’,
Biotechnol Appl Biochem, 10, 203–31.
OFFER G and KNIGHT P (1988), ‘The structural basis of water holding in meat;
part 2, drip loss’, in Lawrie R A, Developments in meat science, vol. 4,
London, Elsevier, 173–243.
O’KEEFE M and HOOD D E (1980–81), ‘Anoxic storage of fresh beef. 2: Colour
stability and weight loss’, Meat Sci, 5, 267–81.
O’KEEFE M and HOOD D E (1982), ‘Biochemical factors influencing
metmyoglobin formation on beef from muscles of differing colour
stability’, Meat Sci, 7, 204–28.
ORDONEZ J A and LEDWARD D A (1977), ‘Lipid and myoglobin oxidation in pork
stored in oxygen and carbon dioxide-enriched atmospheres’, Meat Sci, 1,
41–8.
PENNEY N and BELL R G (1993), ‘Effect of residual oxygen on the colour, odour
and taste of carbon-dioxide-packaged beef, lamb and pork, during short
term storage at chill temperatures’, Meat Sci, 33, 245–52.
382 Novel food packaging techniques
RENERRE M. (1990), ‘Factors involved in the discolouration of beef meat’, Int J
Food Sci, Technol, 25, 613–30.
RHODES D N and LEA C M (1961) ‘Enzymic changes in lamb’s liver during
storage’, J Sci Food Agric., 12 211–27.
SA
′
NCHEZ-ESCALANTE A, DJENANE D, TORRESCANO G, BELTRAN J A and RONCALE
′
S
P (2001), ‘The effects of ascorbic acid, taurine, carrosine and rosemary
powder on colour and lipid stability of beef patties packaged in modified
atmosphere’, Meat Sci, 58, 421–9.
STILES M E (1991), ‘Modified atmosphere packing of meat, poultry and their
products’, in Oriakul B and Stiles M E, Modified atmosphere packagings
of foods, West Sussex, Ellis Herwood, 118–47.
S?RHEIM O, AUNE T and NESBAKKEN T (1997), ‘Technological, hygienic and
toxicological aspects of carbon monoxide use in modified-atmosphere
packaging of meat’, Trend Food Sci Technol, 8, 307–12.
S?RHEIM O, NISSEN H and NESBAKKEN T (1999), ‘The storage life of beef and
pork packaged in an atmosphere with low carbon monoxide and high
carbon dioxide’, Meat Sci, 52, 157–64.
TAYLOR A A (1985), ‘Packaging fresh meat’, in Lawrie R, Developments in meat
science, vol. 3, Barking, Elsevier, 89–113.
TEWARI G, JEREMIAH L E, JAYAS D S and HOLLEY R A (2002), ‘Improved use of
oxygen scavengers to stabilize the colour of retail-ready meat cuts stored
in modified atmospheres’, Int J Food Sci Technol, 37, 199–207.
YOUNG L L, REVIERE R D and COLE A B (1988), ‘Fresh red meats: a place to apply
modified atmospheres’, Food Technol, 42, 65–9.
Active packaging in practice: meat 383