MEATing POINT Magazine: #08/ 2016

FREEZING
Stability of pork during
frozen storage
by Lene Meinert and Hardy Christensen, Danish Meat Research Institute, Denmark
Freezing is a well-known and widespread
preservation method, prolonging the
shelf life of meat and many other
food items. Freezing is popular, as it allows
meat to keep a close-to-fresh quality for a
long time, and it also allows long distance
transports. However, depending on the
time and temperature combinations
during frozen storage, changes in meat
quality can indeed occur. These changes
include lipid oxidation, a major cause
of deterioration of meat generating the
undesired rancid flavour. It is important
to note that freezing, which we use for
food storage and distribution today, is
not an infinite preservation method and
quality reductions, made before freezing,
do not disappear.
The composition of pork
In order to understand which factors
may affect the stability of pork during
frozen storage, it is important to look at
the composition of pork. Pork consists
primarily of water, protein and fat, the
proportion of the three vary greatly
between the different cuts, some being
more fatty e.g. pork belly than others
e.g. pork loin. The composition of fat,
or fat quality, has a huge influence
on shelf life and thereby also storage
time, as the unsaturated fat content
is prone to oxidation. Furthermore,
pork also contains components that
may promote oxidation such as iron.
Finally, the amount of water influences
the freezing process, as high water
contents require increased freezing times.
The freezing process
The freezing process is influenced by several
factors including 1) the size of the meat cuts,
2) temperature differences between the meat
surface and meat core, and as previously
mentioned, 3) the content of water in the
meat. During the freezing process, the water
will gradually change into ice. See figure 1
Figure - 1 The freezing process in a piece of meat.
Figure1 - 2 Temperature curves for the freezing of
pork loins using three different freezing processes.
Figure - 3 Lipid oxidation measured as TBARS in
pork patties as a function of storage time
(Hansen et al., 2004).
The speed by which the meat is frozen is
closely related to the freezing method used.
An example of this is the time used for
reaching -18°C. One of the fastest methods
is freezing with cryogen, in two hours
a pork loin of approx. 3 kg will reach
-18°C. In comparison, using air blast
will take 17 h to reach -18°C, and still
air in a freeze room will take 24 h. That
is, if the loins are not packed together
in cardboard boxes and stacked on
pallets – then the time to reach -18°C
is much longer with worst case scenario
of several days before all meat cuts
are frozen.
Another example of an industrialised
and fast method for freezing meat
is the impingement freezer, in which
meat cuts are placed individually on
a belt conveyer transported through a
tunnel with air blast. The meat is frozen
within a short time e.g. 1 h depending
on the size of the individual cuts.
Effects on shelf life during frozen
storage
Generally, shelf life of meat is limited
by growth of bacteria and chemical
changes in the meat. In frozen
meat, it is the chemical changes that
prevail. However, chemical reactions
are slowed down closely related to
the lowering of the temperature.
Furthermore, the oxidation reactions
are slowed or completely inhibited
by the absence of oxygen meaning
that the packaging materials also
influence the shelf life.
Most bacteria do not grow well at
temperatures below 0°C, and microbial
growth at freezing temperatures is
more commonly related to molds
and yeasts. Some processed products
e.g. bacon are more likely to support
microbial growth below zero degrees, as
these products might contain so-called
cryoprotectants, e.g. sugar compounds,
that depress the freezing point of water
(Jay, 2000). It is important to note, that
if sugar is added to bacon it is because
issue 8 | www.meatingpoint-mag.com
39