Freezing/Thawing 107pre - rigor state. However, pork and beef car-
casses, with their greater insulation of fat,
cool more slowly; thus thaw shortening is
rarely encountered in these meats. Thaw
shortening may be prevented in lamb car-
casses by applying electrical stimulation
prior to freezing, or by tempering the meat at
temperatures between − 2 and − 5 ° C for up to
6 days during thawing (Dransfi eld 1974 ).
When meat is stored at above freezing
temperatures it becomes progressively more
tender. This process, known as “ aging ” (or,
alternatively, as conditioning or maturation),
is traditionally carried out by hanging meat
carcasses for periods of 14 days or longer (in
the case of beef) in a controlled environment
at between − 1 and 5 ° C (so called “ dry
aging ” ). Alternately, the carcass may be
divided into sub - primals and aged in vacuum
packs (usually referred to as “ wet aging ” ).
The rate of aging differs signifi cantly between
animal species (Dransfi eld 1986 ) and neces-
sitates different times for tenderization. Beef,
veal, and rabbit age at about the same rate
and take about 10 days at 1 ° C to achieve
80% of aging. Lamb ages slightly faster than
beef but more slowly than pork. The ultimate
tenderness will depend on the initial “ back-
ground ” tenderness of the meat and the
tenderization that has occurred during chill-
ing. The age of the animal is also
important.
Frozen meat that has been aged prior to
freezing is more tender than that frozen
within 1 or 2 days, and the difference has
been shown to be maintained throughout
frozen storage for 9 months (Jakobsson and
Bengtsson 1973 ). However, there is evidence
that aging shortens the frozen storage life.
Chilled storage of lamb for one day at 0 ° C
prior to freezing can reduce the subsequent
storage life by as much as 25% when com-
pared to lamb that has undergone accelerated
conditioning and only 2 hours storage at 0 ° C
(Winger 1984 ). It has been shown that pork
that has been held for 7 days prior to freezing
deteriorates at a faster rate during subsequentThe meat “ sets ” in the shortened state as rigor
comes on, and this causes it to become
extremely tough when it is subsequently
cooked. If no cooling is applied and the tem-
perature of the meat is above 25 ° C at comple-
tion of rigor, then another form of shortening
rigor or “ heat shortening ” will occur
(Dransfi eld 1994 ), also on cooking.
The severity of cold shortening is highly
pH - dependent. It is much greater if muscle
temperatures below 10 ° C are achieved while
the pH is 6.8 (i.e., exceptionally rapid chill-
ing) than at pH 6.2 (i.e., at an easily attain-
able commercial rate of chilling). To allow a
safety margin, and taking into account the
fact that some carcasses will show high initial
pH values in the eye muscle, it is recom-
mended that any part of a beef or lamb
carcass should not be chilled below 10 ° C
until at least 10 hours after slaughter. In pork,
cold shortening occurs if temperatures
between 3 and 5 ° C are reached before the
onset of rigor (normally 3 to 8 hours); this
will only occur in rapid pork chilling systems
and is not as common. Avoiding cold short-
ening in beef through the use of slow chilling
rates can lead to problems of “ bone - taint ”
(James and James 2002 ). Electrical stimula-
tion of the carcass after slaughter can allow
rapid chilling to be carried out without much
of the toughening effect of cold shortening.
However, electrical stimulation followed by
moderate cooling may affect tenderness in an
unpredictable way and could result in tougher
meat (Buts et al. 1986 ). Electrical stimulation
will hasten rigor and cause tenderization to
start earlier at the prevailing higher tempera-
ture. In beef, meat from carcasses given high -
or low - voltage stimulation and slow cooling
can obtain adequate aging in about half the
time of non - stimulated meat.
If freezing is applied immediately after
slaughter, cold shortening may be prevented.
However, a more severe shortening, thaw
shortening, will occur on thawing (Bendall
1974 ). An entire lamb carcass can be frozen
in 6 hours, thus freezing all the meat in a