3
Effect of refrigeration on texture
of meat
Whilst a number of characteristics affect the overall quality and acceptability
of both fresh and frozen meats, tenderness is the major characteristic of
eating quality because it determines the ease with which meat can be chewed
and swallowed. The tenderness of meat is affected by both chilling/freezing
and storage. Under the proper conditions, tenderness is well maintained
throughout the chilled/frozen storage life, but improper chilling/freezing can
produce severe toughening and meat of poor eating quality.
Some of the factors that influence the toughness of meat are inherent in
the live animal. It is now well established that it is the properties of the con-
nective tissue proteins, and not the total amount of collagen in meat, that
largely determine whether meat is tough or tender (Church and Wood,
1992). As the animal grows older the number of immature reducible cross-
links decreases. The mature cross-links result in a toughening of the colla-
gen and this in turn can produce tough meat. Increasing connective tissue
toughness is probably not commercially significant until a beast is about
four-years-old (Husband and Johnson, 1985).
Although there is common belief that in beef, breed has a major effect,
CSIRO (1992) state ‘although there are small differences in tenderness
due to breed, they are slight and currently of no commercial significance to
Australian consumers.’ However, there are substantial differences in the
proportion of acceptable tender meat and toughness between Bos indicus
and Bos taurus cattle. The proportion of acceptable tender meat decreased
from 100% in Hereford Angus crosses to 96% in Tarentaise, 93% in
Pinzgauer, 86% in Brahman and only 80% in Tsahiwal (Koch et al., 1982).
Toughness of meat increases as the proportion of Bos indicus increases
(Crouse et al., 1989).
There can also be significant differences within a breed. Longissimus
dorsi shear force values for double muscled Belgium Blue bulls were sig-
nificantly higher than those of the same breed with normal conformation
(Uytterhaegen et al., 1994). Calpain I levels at 1 h and 24h post-mortem
were also much lower. It was suggested that the lower background tough-
ness in the double muscled was compensated for by reduced post-mortem
proteolytic tenderisation.
Again, castration appears to have little influence on tenderness. Huff and
Parrish (1993) compared the tenderness of meat from 14-month-old bulls
and steers. Strip loins were removed from carcasses ca. 24 h post-mortem,
vacuum packed and held at 2°C for up to 28 days. No differences were
found between the tenderness of bulls and steers.
Experiments designed to determine the effect of treatments immediately
before or at the point of slaughter appear to show that they have little effect
on meat texture. Exercising pigs before slaughter has been shown to have
no effect on texture parameters, i.e. muscle shortening and shear force
(Ivensen et al., 1995). The use of different stunning methods (both electri-
cal and carbon dioxide) does not seem to have a significant effect on the
quality of pork (Garrido et al., 1994).
Consumers’ surroundings influence their appreciation of tenderness
(Miller et al., 1995). Consumers were more critical of the tenderness of
beef steaks cooked in the home than those cooked in restaurants. The
Warner–Bratzler force transition level for acceptable steak tenderness was
between 4.6 and 5.0 kg in the home and between 4.3 and 5.2 kg in restau-
rants. Warner–Bratzler tests are probably the most uniformly used method
of texture measurement. However, there are many other methods of deter-
mining the mechanical properties of meat (Lepetit and Culioli, 1994). In
cooked meat it is suggested that applying mechanical tests in different strain
directions is likely to produce information that can be more readily related
to perceived texture.
End-point temperature after cooking is crucial to tenderness. Davey and
Gilbert (1974) showed that there was a three- to four-fold toughening
occurring between 40 and 50°C and a further doubling between 65 and
70°C.
Refrigeration has two critical roles in meat tenderness. One is in the
prevention of muscle shortening in the period immediately following
slaughter. The second is in the conditioning of the meat so that the desired
degree of tenderness is obtained.
3.1 Muscle shortening
Chilling has serious effects on the texture of meat if it is carried out rapidly
when the meat is still in the pre-rigor condition, that is, before the meat pH
has fallen below about 6.2 (Bendall, 1972). In this state the muscles contain
44 Meat refrigeration
sufficient amounts of the contractile fuel, adenosine triphosphate (ATP),
for forcible shortening to set in as the temperature falls below 11 °C, the
most severe effect occurring at about 3 °C. ‘Cold-shortening’ first became
apparent in New Zealand, when tough lamb began to be produced routinely
by the improved refrigeration techniques which were introduced after the
Second World War (Locker, 1985). The shortening phenomenon was first
observed scientifically by Locker and Hagyard (1963) and the resulting
extremely tough meat after cooking by Marsh and Leet (1966). The mecha-
nism of cold shortening has been well described by Bendall (1974) and
Jeacocke (1986) and forms the basis of the next sections of this chapter.
3.1.1 Mechanism of shortening
The characteristic pattern of post-mortem chemical change, found in all the
skeletal muscles of the mammals so far investigated, is shown in Fig. 3.1.
The figure has an arbitrary timescale, because although the pattern is
virtually constant its duration is highly temperature dependent. Relative
time scales can be interpolated from the temperature data in Fig. 3.2.
It can be seen in Fig. 3.1 that the supply of contractile fuel (ATP) remains
constant and high for some time. It is kept topped up by two resynthetic
processes that counteract its slow wastage in the resting muscle. The first of
Effect of refrigeration on texture of meat 45
100
50
7.5
7.0
6.5
6.0
5.5
pH
PC
1 2
0
3456789
Time (arbitrary)
PC or
A
T
P
as % initial value
pH
ATP
1
2
Fig. 3.1 Biochemical changes during the course of rigor mortis. Arrow 1 indicates
onset of rapid decline of ATP, and arrow 2 the time for half-change of ATP.The time
scale is arbitrary and highly temperature dependent (see Fig. 3.2) (source: Bendall,
1974).
these processes is the creatine kinase reaction, which resynthesises ATP
from its breakdown product, ADP, and phosphocreatine (PC). The second
is the complex process of glycolysis in which the energy for resynthesis
comes from the breakdown of glycogen to lactate, with concomitant acidi-
fication and fall of muscle pH. The ATP supply remains constant only while
PC is still available, but begins to fall as soon as glycolysis is left on its own
as the sole source of resynthesis. This phase of declining ATP supply is
known as the rapid phase of rigor, because it is then that the stiffening
(rigor) of the muscle sets in. It is shown by the first arrow in Fig. 3.1.
At temperatures above 12 °C the post-mortem muscle remains in a
passive, relaxed state until the ATP supply begins to dwindle at the onset
of the rapid phase of rigor. It then begins to shorten actively. At body tem-
perature (38 °C) the shortening can reach 40% or more of the muscle length
if unopposed by the force of a load. This so-called ‘rigor shortening’ can be
overcome by quite small loads and is incapable of doing much work, even
at 38°C (see Fig. 3.2).
The effect of temperature on the duration of the chemical changes
during rigor is shown in Fig. 3.2, using the time for half-change of ATP as
46 Meat refrigeration
W
o
rk done (mJ
g
–1
) during 'rigor' shortening
11
10
9
8
7
6
5
4
0 5 10 15 20 25 30 35 40
Temperature (°C)
T
ime for half change of
A
T
P
(h)
1
2
3
5
4
3
2
1
0
Fig. 3.2 Time for half-change of ATP during rigor in beef LD muscle, plotted
against temperature. Initial pH = 7.0 in all cases. Curve 1: times for an initial re-
action. Curve 2: observed times. Curve 3: work done during shortening (source:
Bendall, 1974).
the criterion (see second arrow in Fig. 3.1). From 38°C down to 25°C the
duration increases in the manner for a normal chemical reaction (cf. curves
1 and 2). Below this point, however, the experimental points diverge more
and more from the predicted line; in other words, the processes take place
more quickly.At about 10 °C the experimental curve actually inverts, so that
the rate of chemical change at 2°C is greater than at 15°C. Such anomalous
temperature dependence can only mean that new reactions are occurring
with increasing intensity as the temperature is reduced.
The clue to the nature of the new reactions is given by curve 3, which
represents the total work the muscle does during shortening. From 16 °C up
to 38°C the total work increases about 2.5-fold, but even so it is very small.
By contrast, it increases by a similar amount by going only from 16 to 9°C
and eight-fold by going to 2 °C. Quite clearly, therefore, the new reactions
intervening below 9–10 °C are somehow concerned with the increased
muscle shortening.
The shortening occurring below 10 °C is usually described as ‘cold short-
ening’ or ‘cold contracture’. In some muscles, it can develop a force of
between 1 and 2 N cm
-2
, which is between 4 and 8% of the total force devel-
oped in a fully stimulated contraction of living muscle. It is supposed to set
in because the trigger for contraction is itself highly temperature sensitive
and fires spontaneously to an increasing extent as the temperature is
reduced below 10 °C.
This trigger has been shown to be the release of calcium ions, Ca
2+
, from
the sarcoplasmic reticulum (Bendall, 1974; Jeacocke, 1986; Offer et al.,
1988). During use, muscle cells are triggered to contract by calcium ions
(Ca
2+
) liberated from internal stores within the muscle cell. Although the
early stages of activation in muscle contraction in life and cold shortening
in a carcass differ, the final stage, the release of calcium ions, is the same.
In resting muscle, the intrafibrillar level of free Ca
2+
is very low. Most of
the total store of intracellular calcium (about 10
-3
Mole) is locked up in
highly specialised structures which enwrap each of the 1000 or so fibrils
within a muscle fibre (see Fig. 3.3). These structures which are part of the
so-called sarcoplasmic reticulum (SR) have transverse connections (SR(T))
with the outer membrane or sarcolemma (S) of the fibre, so that when a
nervous impulse from the motor nerve (MN) arrives at the motor end-plate
(EP) it travels in both directions along the sarcolemma and invades the
muscle fibre itself via the myriads of these transverse tubules.These tubules
are in contact with the longitudinal elements (SR(L)) of the SR which
enwrap each fibril (see upper fibril in Fig. 3.3). The contact is made via the
triad junctions (TJ) where two dense structures, the so-called lateral cis-
ternae, are closely opposed to the transverse tubules (SR(T)). It is thought
that the lateral cisternae are the storehouse for Ca
2+
in the resting muscle.
In many muscles there are pairs of cisternae at the level of the Z-discs (Z)
of each sarcomere, so that in a fibril that is 10cm in length there are about
40 000 transverse connections and pairs of cisternae.
Effect of refrigeration on texture of meat 47
The effect of an impulse invading the muscle fibres is to cause release of
Ca
2+
from the cisternae of each fibril. The Ca
2+
then diffuses down its elec-
trochemical gradient, finally reaching the microfilaments of actin (thin) and
myosin (thick) shown in the lower, stripped fibril in Fig. 3.3. Here the Ca
2+
is temporarily absorbed and thereby triggers the contractile explosion. This
consists first of the rapid splitting of ATP to ADP and Pi (inorganic phos-
phate) at active centres on the myosin filaments. Then there is transduction
of some of the free energy released into relative movement of the two sorts
of interdigitating filaments. The process is not unlike the explosion of the
petrol/air mixture in a car cylinder, when the sparking plug fires. For a crude
analogy, the cylinder can be likened to the myosin filaments and the piston
to the actin filaments. However, it is the upstroke which resembles a con-
traction and not the downstroke (i.e. the piston is actively pulled or pushed
into the cylinder).
Relaxation is the opposite process during which the status quo at the
sarcolemma is restored, thereby enabling the lateral cisternae of the SR to
re-accumulate the Ca
2+
released during the contraction. They do this by an
active pumping process, using the energy of ATP-splitting to push Ca
2+
up the now adverse electrochemical gradient. Meanwhile fresh ATP has
48 Meat refrigeration
MN
EP
S
SR (T)
SR (L)
TJ
Fibril
Fibril
(stripped)
ZZZZZ
Fig. 3.3 Diagram of part of a muscle fibre in longitudinal section to demonstrate
the effect of a nervous impulse. For abbreviations see text (source: Bendall, 1974).
flooded the microfilaments, thus separating them from each other once
more and enabling them to slide freely over each other in response to any
externally applied force.
Two features of the calcium-pumping mechanism are of special impor-
tance in the present context. First, it is likely that the calcium storage vesi-
cles are somewhat leaky, even in resting muscle, so that the calcium pump
has to operate continuously, albeit slowly, to keep the intrafibrillar Ca
2+
concentration at its low resting level. Second, the calcium pump has an
extremely high temperature coefficient, so that at 10 °C it works at 1/200th
and at 2 °C at only 1/1000th of the rate at the body temperature of about
38 °C (Bendall, 1974). Passive diffusion (leakage) out of the pump would
only be reduced at 10 °C to about half the value at 38°C. Thus, there is an
increasing chance of net Ca
2+
leakage into the myofilaments as the tem-
perature falls, the effect becoming dramatic below 10 °C. Such leakage
stimulates the contractile ATP-ase, bringing about the shortening charac-
teristic of cold shortening and increasing the production of ADP. The latter
in its turn would then stimulate the reactions of ATP synthesis mentioned
earlier, so that the timescale in Fig. 3.1 would become shorter and shorter
the lower the temperature. This explains the anomalous temperature
dependence of the time for half change of ATP, shown in Fig. 3.2.
The contracture, which occurs when a rapidly frozen muscle is thawed,
resembles cold contracture in that it sets in while the level of contractile
fuel (ATP) is still high. However, it differs because the amount of work
done and force developed are much higher.With ‘thaw shortening’ the tem-
perature is raised through the ‘calcium release’ danger zone from 0 to
10 °C, whereas in cold shortening it is reduced through this zone. The rate
of contracture depends entirely on the rate of thawing. Rapid thawing of a
freely suspended, unloaded muscle strip causes very dramatic shortening,
often to less than 40% of the ‘frozen’ length.
3.1.2 Preventing shortening
Rapid chilling has many practical advantages but increases the danger of
cold shortening. As discussed in Chapter 2 the breakdown of glycogen to
lactic acid occurs at different speeds in different species. In lamb and beef,
the rate is low and the pH falls slowly. Hence, it is only too easy to cool car-
casses of these animals, at least on the surface, below 10 °C when the pH is
above 6.2 and such carcasses are extremely vulnerable to cold shortening.
In pork, the rate of breakdown of glycogen is more rapid and under
moderate chilling regimes, cold shortening will not occur. However, pig
muscle can cold shorten, and with fast chilling, for example using sub-zero
air temperatures, cold shortening has been clearly demonstrated.
Another point that should be made is that at an early stage, the surface
of the carcass will reach the same temperature as that of the air. Since the
air temperature used in chilling is commonly below 10°C, there exists the
Effect of refrigeration on texture of meat 49
possibility that cold shortening may occur at the surface, even if it does not
occur in the bulk of the meat. Whether or not cold shortening occurs on the
surface will often depend on the amount of fat cover over the carcass.
This leads to the question of whether shortening can be eliminated whilst
retaining high cooling rates? This can be done in two ways: (1) by prevent-
ing the underlying cold contraction or (2) by restraining the muscle suffi-
ciently to prevent the deleterious shortening. The second solution has been
developed with considerable success and has generally involved adopting
novel methods of hanging the carcass, such as from the hip (Taylor, 1996).
The alternative avenue, of prevention, has found favour with the wide-
spread application of electrical stimulation (ES) of the carcass immediately
after death. This procedure greatly accelerates post-mortem metabolism by
stimulating the muscles to contract and relax at a very fast rate, which
quickly depletes glycogen and ATP and thus accelerates rigor. ES of
the carcass after slaughter can allow rapid chilling without much of the
toughening effect of cold shortening. Taylor (1987) and Taylor (1996)
provide details of optimum ES treatments. ES has also been shown to be
effective in reducing cold shortening in deer meat (Chrystall and Devine,
1983; Drew et al., 1988).
Although chilling or freezing pre-rigor produces tough meat caused by
cold shortening or thaw rigor it still has good functional properties (Xiong
and Blanchard, 1993). It is therefore feasible to manufacture good quality
comminuted meat products from hot boned pre-rigor refrigerated beef.
Abu-Bakar et al. (1989) found no differences in eating quality between
Wieners manufactured from either hot boned beef chilled rapidly using
CO
2
or brine, or conventionally chilled cold boned beef.
As a ‘rule of thumb’, cooling to temperatures not below 10 °C in 10h for
beef and lamb (Offer et al., 1988) and in 5 h for pork (Honikel, 1986) can
avoid cold shortening.
3.2 Development of conditioning (ageing)
The terms ‘conditioning’, ‘ageing’, ‘ripening’, ‘maturing’ and ‘the resolution
of rigor’ have all been applied to the practice of storing meat for periods
beyond the normal time taken for cooling and setting, to improve its
tenderness after cooking. Conditioning imposes a severe limitation on
processing conditions because it is a slow process.
The deficiencies in the commercial conditioning of meat were clearly
illustrated by replies to a questionnaire to sections of the trade in the UK
in 1977/8 (Dransfield, 1986). At the time a period of storage for wholesale
meat was often not specified by retailers. When specified the duration of
storage had much to do with distribution and turnover of meat and could
often be shortened by commercial pressures. At retail, beef was kept for
1–4 days and most beef was sold 3–6 days after slaughter (Palmer, 1978).
50 Meat refrigeration
The majority of beef therefore had been only partially conditioned and
tenderness would have been improved if the beef had been stored for a
further week. Many retailers nowadays condition beef for longer periods,
but economic factors often still dictate the time of conditioning.
3.2.1 Mechanism of ageing
The major change, which takes place in meat during ageing, occurs in the
muscle fibre. Little or no change which can be related to tenderness
improvement takes place in the structures which hold the fibres together
(the connective tissue, collagen) (Herring et al., 1967).
Conditioning is caused by the presence of proteolytic enzymes in the
muscle which slowly catalyse the breakdown of some of the muscle pro-
teins. This causes weakening of the muscle so that the meat is more readily
pulled apart in the mouth and is therefore tenderer.Two groups of enzymes
are thought mainly responsible: calpains, which are active at neutral pH
shortly after slaughter, and cathepsins, which are active at acid pH after
rigor (Offer et al., 1988).
Dransfield (1994) states that it is generally accepted that tenderisation
results from proteolysis by endogenous enzymes. The major problem in
identifying the specific enzymes has been that the enzyme activities cannot
be measured in meat since they depend on local in situ concentrations of
cofactors and inhibitors. However, modelling the activation of calpains
shows how tenderness develops and points to methods of optimising its
development. Calpain I is activated first, at low calcium ion concentrations,
and then calpain II is activated as the concentration of calcium ions rises
further. There are enough free calcium ions to activate all of calpain I
but only about 30% of calpain II. Tenderisation therefore begins when
calpain I starts to be activated, normally at about pH 6.3 or about 6 h
after slaughter in beef, and rapidly increases as more calpain is activated.
After about 16 h in beef, calpain II becomes activated and causes a further
tenderisation.
The calpain-tenderness model shows that in beef longissimus dorsi, most
of the tenderisation is caused by calpain I. Approximately 50% of the
tenderisation occurs in the first 24h, after which the rate is exponential.
The model clearly shows that the ultimate tenderness of the meat will
depend on (1) the tenderisation that occurs during chilling and (2) further
tenderisation during storage. In extreme cases, for example dark, firm and
dry (DFD) beef, all the tenderisation will occur in stage 1 and none during
ageing. The incidence of DFD beef is markedly dependent on the sex of
the animal. It occurs in about 1–5% of steers and heifers, 6–10% of cows
and 11–15% of young bulls (Tarrant and Sherington, 1981). Rigor develop-
ment is very rapid in DFD beef and during normal cooling to an ultimate
pH of 7.0, all of the tenderisation occurs before 24 h and no ageing occurs
(Dransfield, 1994).
Effect of refrigeration on texture of meat 51
3.2.2 Prediction of tenderness
There is great interest in the development of any measurement method that
can be applied soon after slaughter, which will predict the tenderness of
meat. Many laboratory techniques (Dransfield, 1986; Dransfield, 1996) have
been used to detect changes in the muscle down to the molecular level.
However, there is no routine test which can indicate how much of the ten-
derising has occurred or, more usefully, how much longer a piece of meat
must be stored.
In 1988 Marsh et al. proposed that the pH in the longissimus dorsi at
3 h post-mortem may have a use as a predictor of tenderness. However, sub-
sequent studies involving large numbers of animals (Marshall and Tatum,
1991; Shackelford et al., 1994) found that it was not highly correlated with
tenderness. Thus it is not a reliable method of identifying potentially tough
or tender meat.
Cross and Belk reviewed, in 1994, all the non-invasive technologies
capable of objectively determining yield or the eating quality of the lean
meat in live animals or carcasses in commercial situations. Technologies
included X-ray, nuclear magnetic resonance, electrical conductivity analy-
sis, near-infrared reflectance, video image analysis, optical fat/lean probes,
optical connective tissue probes, bioelectrical impedance analysis, velocity
of sound and elastography. Elastography, which measures the internal dis-
placement of small tissue elements in response to externally applied stress
using ultrasonic pulses, was thought to have the best potential in the future.
It may be capable of depicting muscle structure at the muscle bundle level,
and of detecting differences in elasticity of muscle bundles, connective
tissue amounts and the quality of intramuscular fat.
3.2.3 Consumer appreciation of ageing
Consumer assessments of unaged beef are variable, ranging from ‘moder-
ately tough’ to ‘moderately tender’ whilst beef conditioned for 9 days at
1 °C receives largely favourable reactions, being scored ‘moderately’ to
‘very’ tender (Dransfield, 1985). Consumer panels, however, have rarely
been used to assess the factors affecting conditioning. They have usually
been measured by laboratory taste panels and mechanical tests. A type of
‘shear’ test is frequently used on cooked meat and the measurements are
usually well related to sensory assessments (Dransfield, 1986).
The results of Dransfield (1986) illustrate the effect of conditioning on
a taste panel’s assessment of texture of 3 beef joints. Tenderness increased
in all 3 joints (Table 3.1) and these changes were reflected in increases in
the overall acceptability.
In further work roasted sirloin joints from a six-year-old cow were com-
pared with an 18-month-old heifer at storage times of 2–15 days at 2°C. By
8 days, the heifer joint showed significant improvement in tenderness (Table
3.2). By 15 days both joints showed significant improvements.
52 Meat refrigeration
3.2.4 Preslaughter factors
Rates of conditioning differ widely between species.The tenderness of meat
improves approximately as the logarithm of the storage time. Most of the
improvement in tenderness therefore takes place in the initial storage
period and tenderness eventually reaches a maximum. Table 3.3 shows the
first order rate constants derived from the exponential decay of toughness
of cooked muscles with time (Dransfield, 1986). Beef, veal and rabbit have
a rate constant of 0.17 per day, which means that 80% of the tenderising
that is theoretically possible occurred in 10 days at 1 °C. Although beef and
veal condition at the same rate, veal is tenderer and therefore can reach an
acceptable tenderness in 5 days at 1°C. Lamb conditions slightly faster than
beef, and pig meat about twice as fast as beef.
There is little information on the influence of breed on the rate of con-
ditioning. Purchas (1972) observed that Friesian Brahman cross bull beef
improves more in tenderness than Friesian bull beef from 5 to 14 days
post-mortem at 4 °C.
There appears to be little difference in rate of conditioning between dif-
ferent muscles. Semlek and Riley (1974) reported that in 18 Hereford bulls,
the longissimus muscle conditioned more during storage at 2 °C than did
Effect of refrigeration on texture of meat 53
Table 3.1 Average taste panel scores for roasted joints from a 30-month steer carcass
aged between 4 and 22 days at 2 °C
Joint Unaged Aged
Day Texture Flavour Overall Day Texture Flavour Overall
tasted acceptability tasted acceptability
Sirloin 4 -2.2 2.5 2.5 22 0.8 3.6 3.2
Silverside 4 -3.0 2.4 1.8 15 1.0 4.2 4.0
Topside 4 -3.4 2.5 1.9 15 1.4 2.4 2.6
Texture: an eight-point scale from extremely tough (-4) to extremely tender (+4). Flavour and overall acceptabil-
ity: a five-point scale from not acceptable (1) to extremely good (5).
Source: Dransfield, 1986.
Table 3.2 Average taste panel texture scores for roasted
loin joints of 6-year-old cow compared with 18-month-old
heifer aged between 2 and 15 days at 2 °C
Day tasted Cow Heifer
2 -3.7 -3.0
4 -3.2 -2.4
8 -1.8 -0.4
15 +0.4 +3.2
Eight-point scale from extremely tough (-4) to extremely tender
(+4).
Source: Dransfield, 1986.
biceps, semitendinosus or triceps. However, it is not clear whether this
represents different rates or whether it was caused by different amounts
of ‘background’ toughness. More comprehensive data on beef muscles was
obtained at Texas A and M on 125 choice beef carcasses (Dransfield, 1986),
the normalised data are shown in Table 3.4. The rate of conditioning did
not differ significantly between the 17 muscles and averaged 0.25 per day
at 1 °C. In pork, the semitendinosus muscle conditions at a similar rate to
that of longissimus (Dransfield et al., 1980b).
There appears to be an interaction between carcass grade, marbling and
animal age in the development of tenderness. Doty and Pierce (1961) found
that unaged meat from ‘Prime’ grade carcasses was tenderer than from
‘Good’ grade, but when the meat was aged the difference in tenderness was
not so pronounced.Tuma et al. (1962, 1963) found that meat from 18-month-
old animals was influenced little by conditioning for 14 days whereas older
animals improved in tenderness. They also found that marbling influenced
the development of tenderness in older animals but not in animals of 18
months. Other studies found that the rate of conditioning was not depend-
ent on the level of finish or marbling in steers, bulls and heifers (Martin et
al., 1971) nor on the weight of steers (Purchas, 1972).
In reviewing the research on marbling and eating quality Rhodes
(1971) concluded that, at the slaughter ages then commonly used in beef
production, marbling did not affect tenderness and consequently was of
no importance in tenderness development during conditioning.
3.2.5 Pre-rigor factors
Over 30 years ago, tenderness of beef was promoted by holding freshly
killed and dressed carcasses at 37 °C for 4–5 h prior to normal chilling
54 Meat refrigeration
Table 3.3 Variation in rate of conditioning among
species
Rate Time for 50% Time for 80%
(day
-1
) tenderising tenderising
(days) (days)
Beef 0.16 (0.04) 4.3 10.0
Veal 0.17 (0.03) 4.1 9.5
Rabbit 0.17 (0.06) 4.1 9.5
Lamb 0.21 (0.05) 3.3 7.7
Pork 0.38 (0.11) 1.8 4.2
Longissimus muscles from four species were stored at 1–4 °C (cf.
Dransfield et al., 1980b) and rates calculated (cf. Dransfield et al.,
1980a). Values are the rate of tenderising with standard errors and
the time taken after stunning for 80% of the complete tenderising
to occur.
Source: Dransfield, 1986.
(Lochner et al., 1980). More recent work (Martin et al., 1983) has confirmed
that toughness decreases in direct relation to the holding temperature over
the range 10–42 °C but a complex mechanism involving muscle shortening
and conditioning appears to take place. The tenderising process also occurs
in the absence of cold shortening (Lochner et al., 1980) but a full explana-
tion of the mechanism has not been made.
The rapid glycolysis produced by these high temperatures can also be
induced by pre-rigor electrical stimulation of carcasses. Electrical stimula-
tion also tenderises meat in the absence of cold shortening (Martin et al.,
1983; George et al., 1980). George et al. (1980) found, however, that the ten-
derising effect decreases with storage, and at completion of conditioning,
stimulated and control meats were equally tender.
To investigate how much of the tenderising effect was due to advance-
ment of conditioning, three factors were needed: (1) the temperature co-
efficient for conditioning, (2) the time of the start of conditioning and (3)
the temperature profile after the start of conditioning.
Effect of refrigeration on texture of meat 55
Table 3.4 Conditioning in different beef muscles
Muscle Conditioning
Cut Rate
a
Ultimate
toughness
b
Longissimus Chuck 0.2 (0.3) 5.0
Spinalis dorsi Chuck 0.3 (0.1) 3.1
Rhomboideus Chuck 0.1 (0.1) 4.6
Latissimus dorsi Chuck 1.3 (5.0) 4.6
Serratus ventralis (steak) Chuck 0.2 (0.2) 2.4
Infraspinatus (steak) Chuck 0.1 (0.1) 2.5
Serratus ventralis (roast) Chuck 0.1 (0.0) 2.5
Infraspinatus (roast) Chuck 0.1 (0.1) 1.5
Complexus Chuck 0.3 (0.2) 3.2
Extensor carpi radialis Shin 0.4 (0.3) 5.0
Triceps brachii Shoulder 0.3 (0.1) 4.3
Deep pectoral Brisket 0.1 (0.1) 4.6
Superficial pectoral Brisket 0.3 (0.1) 4.8
Longissimus Rib 0.3 (0.1) 3.4
Semimembranosus Topside 0.2 (0.2) 4.1
Semitendinosus Silverside 0.2 (0.1) 4.6
Biceps femoris Silverside 0.3 (0.4) 4.4
Rectus femoris Thick flank 0.1 (0.2) 3.7
Vastus lateralis Thick flank 0.2 (0.1) 4.1
Muscles from US choice beef carcasses were stored at 1 ± 1 °C, 87 ± 7% RH for up to 28 days.
Parameters for tenderising were calculated from shear values determined instrumentally.
a
day
-1
, with standard errors in parenthesis;
b
Toughness in kg force predicted at infinite
storage time.
Source: Dransfield, 1986.
Calculations were performed assuming that conditioning started at
90–95% of rigor development although at that time there was little evi-
dence for this. Later evidence suggested that this was correct (Locker and
Wild, 1982) and their conclusion was valid in that most, if not all, of the
tenderising was due to advancement of the start of conditioning at slightly
higher temperatures. Later work (Takahashi et al., 1984) suggests that the
tenderising may have been caused by fracture of fibres brought about by
tetanic contractions induced by high voltage stimulation. It showed that,
sometimes (not always) low frequency (2 Hz) stimulation, which caused less
fracture, made meat less tender than high frequency (50 Hz). However, it
has not been shown how fibre fracture relates to tenderness, and also, if
fracturing affects tenderness, why there is no permanent tenderising. Rapid
glycolysis, with or without electrical stimulation, also causes tenderising
(Martin et al., 1983). It seems then that the tenderising effect of electrical
stimulation in the absence of cold shortening is due to the advancement of
the start of conditioning and acceleration of its rate at higher temperatures.
The extent of tenderising would therefore depend upon the rate of glycoly-
sis, the temperature at the start of conditioning (full rigor), the subsequent
time/temperature profile and the time when tenderness was measured. In
five beef muscles, stimulated and held at 15°C for 7h and then at 1°C, the
tenderness of control non-stimulated muscles at 10 days was achieved in 7
days following low voltage (85V). Only 3 days were required when high
voltage (700 V) stimulation was used (Taylor et al., 1984).
3.2.6 At chill temperatures
The bulk of investigations to determine the time required for the tenderis-
ing changes to take place in beef have been carried out in North America.
Deatherage and Harsham (1947) investigated the changes in beef at
0.5–2 °C and found that the tenderness of cooked sirloin (longissimus dorsi)
increased up to 17 days storage with some additional improvement up to
31 days. They concluded that unless beef is to be aged beyond 4 weeks,
it need be aged only 2.5 weeks. Doty and Pierce (1961) also showed that
conditioning for 2 weeks at 0.5–2°C improved texture and caused very
substantial reductions in the shear strength of cooked meat, but much less
change occurred during the next two weeks.
Most studies have shown that the major improvement in tenderness
occurs in less than 14 days. Larmond et al. (1969) compared beef aged at
1 °C for 2, 9 and 16 days. They found that samples stored for 9 days were
tenderer than those stored for 2 days, but samples stored for 16 days were
not more tender than those stored for 9 days. Steinhauf and Weniger (1966)
showed that the transformations in the muscle necessary to give meat its
highest eating quality were achieved by storing at 2–4 °C for 8–14 days, no
significant improvement taking place between 12 and 19 days. In a study by
Martin et al. (1971) in which more than 500 animals were examined, it was
56 Meat refrigeration
concluded that for beef carcasses, a period of 6 days is sufficient for a con-
sumer product of satisfactory tenderness. Buchter (1970) also showed that
no significant increase in tenderness occurs after 4–5 days for calves and
8–10 days for young bulls at 4 °C.
However, a more recent study by Huff and Parish (1993) shows a
different pattern. Strip loins were removed from carcasses ca. 24 h post-
mortem, vacuum packed and held at 2 °C for up to 28 days. Samples were
removed after 3, 7, 14 and 28 days. Sensory scores for tooth softness and
fibre fragmentation showed little difference between 7 and 14 days of
ageing (Fig. 3.4). However, there were substantial differences between 3 and
7 days and between 14 and 28 days. Average Warner–Bratzler shear force
values decreased with length of ageing with the biggest decrease from 2.8
to 2.3kgcm
-2
occurring between 14 and 28 days.
3.2.7 At frozen temperatures
Increasing the delay period before freezing enhances tenderness because
meat ages at chill temperatures but not at normal freezer temperatures.
Meat which has been conditioned prior to freezing is more tender than that
frozen within 1 or 2 days and the difference is maintained throughout
frozen storage for 9 months (Table 3.5).
Freezing rate affects the rate of tenderising after thawing (Table 3.6) but
not the ultimate tenderness. Freezing at -10°C more than doubles the rate;
freezing in liquid nitrogen almost trebles the rate. Freezing is known to
cause structural damage by ice crystal formation. It seems likely that ice
crystals, particularly small intracellular ice crystals formed by very fast
freezing rates, enhance the rate of conditioning probably by release of
Effect of refrigeration on texture of meat 57
99.23 d
7 d
14 d
28 d
109.7
110.1
121.1
97.7
107.9
109.7
121.7
120.9
123.5
114.1
115.9
Tooth Softness Fragmentation Mealines
140
120
100
80
60
40
20
0
Fig. 3.4 Mean sensory scores for loin steaks after different ageing periods (source:
Huff and Parrish, 1993).
enzymes (Dransfield, 1986). Repeated freeze–thaw cycles using relatively
low freezing rates do not seem to cause any enhanced tenderising (Locker
and Daines, 1973). This enhancement of conditioning resolves the apparent
anomaly, reported by Hiner et al. (1945), that freezing did not affect ten-
derness of aged beef but increased it in unaged or partially aged beef.
3.2.8 At higher temperatures
After the onset of rigor, the muscle temperature has the largest effect on
the rate of conditioning. From zero up to 40°C the rate increases about 2.5-
fold for every 10 °C rise in temperature (Ewell, 1940; Dransfield et al., 1980a;
Davey and Gilbert, 1976).Above 60 °C the rate drops rapidly due to enzyme
denaturation (Davey and Gilbert, 1976). In beef therefore which achieves
80% of the tenderising in 10 days at 0 °C, 4 days would be required at
10°C and only 1.5 days at 20°C.
58 Meat refrigeration
Table 3.5 Effect of conditioning time on the tenderness of beef
Conditioning time (days) Period of frozen storage (months)
prior to freezing
1919
Cooked after Cooked from the
thawing frozen state
4 5.6 4.3 5.7 4.6
14 6.8 6.7 6.9 6.7
Note: Sections of eye muscle (M. longissimus dorsi) were aged at 3 °C, frozen at -20 °C and
thawed at 10 °C. Numbers are the mean taste panel scores (scale: 1, extremely tough to 9,
extremely tender).
Source: Jakobsson and Bengtsson, 1973.
Table 3.6 Effect of rate of freezing on the conditioning rate at 1 and 15 °C after
thawing
Treatment Freezing time Conditioning temperature
(min) from -1 to -7°C
1°C 15°C
Not frozen 0.14 (0.03) 1.01 (0.13)
Frozen at -10 °C 45 0.16 (0.04) 2.59 (0.61)
Frozen in liquid N
2
0.2 0.39 (0.07) 2.99 (1.40)
24 hours after slaughter, slices of beef M. semitendinosus were frozen in air at -10 °C and in
liquid nitrogen, stored for 2 h at -10 °C and then thawed in air at 15 °C for 60 min. Thawed
meat was conditioned for up to 25 days at 1°C or 7 days at 15 °C. Values are rate constants
(day
-1
) with standard errors.
Source: Dransfield, 1986.
3.3 Influence of chilling on texture
3.3.1 Lamb
The small size of a lamb carcass makes very rapid chilling possible in
modern refrigerated installations.
In a study by Bailey (Taylor et al., 1972) two cooling regimes were com-
pared; in the rapid condition the centre of rib-eye muscle and of the leg
muscle reached 10 °C in 3 and 5 h, respectively, and in the slow condition
after 27 and 28 h. Meat from these treatments was cooked and judged in
direct comparison by a trained taste panel using eight-point scales, four
degrees of tender and four degrees of tough, and the mechanical strength
of the cooked meat was measured on a shearing device. The results showed
that a drastic increase in the toughness of the meat resulted from the rapid
cooling. The difference between the rapid and slow treatments was found
directly after chilling and was maintained after 5 days ageing at 0°C. The
lamb treated normally (slow cool, conditioned) was judged to be close to
‘very tender’ by the panel, whereas the rapidly cooled meat was two whole
scale divisions lower at ‘slightly tender’. Before ageing, descriptions were
at ‘moderately tender’ and ‘slightly tough’, respectively. Since the meat
came from young English lambs, a texture score of ‘very tender’ would be
expected, and ‘slightly tender’ would be regarded as the lower desirable
limit. The mean score of ‘slightly tough’ would indicate a high proportion
of definitely unacceptable carcasses. The mean shear values showed a close
correspondence with the panel results.
These results show that chilling of lamb carcasses without freezing can
be sufficiently rapid to produce toughness in the rib-eye muscle. They show
too that such toughening can be considerable, especially if rapidly cooled
carcasses were to be distributed quickly to reach consumers without a con-
ditioning period after processing.
3.3.2 Pork
Although pork carcasses are not much larger than lamb, the problem of
cold shortening is not so acute in this species. First, the rate of fall in pH in
pig muscle is much more rapid than in lamb hence the critical conditions
for cold shortening are less onerous. Second, the pig generally has a much
thicker layer of subcutaneous fat and the retention of this with the skin
on the carcass slows the rate of cooling. Finally, the scalding and scraping
manipulations extend the processing time and delay the start of cooling. It
appeared in the 1970s that little danger could arise in the normal handling
of pork carcasses (Rhodes, 1972). However, studies in the 1980s (James
et al., 1983; Gigiel and James, 1984) clearly showed that rapid chilling could
produce tougher pork.
In pork muscles, cold shortening is observed if temperatures between 3
and 5 °C are reached before the onset of rigor, which in normal glycolysing
Effect of refrigeration on texture of meat 59
pork muscle lasts 3–8 h depending on the muscle and the breed (Honikel,
1990). Taylor et al. (1995a) showed that rapid cooling of pork sides (-20 °C,
1–1.5ms
-1
for 2–3h followed by 1°C, 0.5 m s
-1
) produced tougher pork than
conventionally cooled meat (Table 3.7).
ES, hip suspension and conditioning have been found to alleviate the
toughening affect of rapid chilling of pork. Dransfield et al. (1991) found
that stimulation improved tenderness but increased drip and paleness, but
both effects were reversed by rapid chilling. Pelvic suspension reduced drip
and improved tenderness to the same magnitude as stimulation. Taylor and
Tantikov (1992) found ES (700 V peak at 12.5 Hz for 90 s) 20 min post-
slaughter improved the tenderness of the l. dorsi and to a lesser extent
the semimembranosus of rapidly chilled pig sides. This advantage was
gained without producing PSE pork as shown by the lower drip losses from
the ES sides.
Taylor et al. (1995b) showed that even under conventional chilling con-
ditions a form of toughening that can be avoided by electrical stimulation
occurs.Their data also clearly show that pork can benefit from up to 12 days
of conditioning.
There are potential problems in hot boning of pork. M?ller and Jensen
(1993) reported that pork loins excised 1 hour post-mortem and then cooled
in air at 2–4 °C, 0.2 m s
-1
had a similar texture to conventionally cold boned
loins. Loins excised at the same time but cooled in iced water showed signs
of cold shortening. These results are not surprising since the conventional
cooling treatment used air at -18 °C, 3.0 m s
-1
for 65min before transfer to
a room at 2–4 °C, 0.2 m s
-1
. A temperature of 10°C was reached in the
control loins in ca. 3 h compared with 7 h in the hot-boned loins (cooled at
2–4 °C) and 2.5h for those in iced water. Similarly, Ivensen et al. (1995)
showed that hot boning pork carcasses, 1 h post-stunning, and cooling in
iced water produces significant toughening. After 7 days of conditioning at
2 °C the tenderness was still not acceptable. Boning at 6h post-stunning and
conditioning produced an acceptable product.
60 Meat refrigeration
Table 3.7 Comparison of weight losses and organoleptic
parameters from pig sides electrically stimulated or not
chilled under different regimes
Treatment ES Sarcomere Panel
length (mm) toughness
Rapid cooling Yes 1.85 4.30
No 1.89 3.8
Conventional Yes 1.92 4.57
No 1.99 4.47
Source: Taylor et al., 1995b.
3.3.3 Beef
As in lamb, cold shortening can be readily induced in all muscles of a beef
carcass by rapid cooling before rigor, and the critical conditions of 10 h to
10 °C apply equally (Rhodes, 1972). The cooling of the larger carcass can
not be so rapid, however this is offset by splitting into sides.
When considering the possibility of cold shortening in beef sides it is
important to know the thickness of the layer of tissue below the surface
which has been cooled to 10 °C in 10 h (or less). The thickness of this layer
will, of course, vary over the carcass; it will be thinner where there is a bulk
of underlying tissue, for example on the round, and thicker at the flank. The
presence of interstitial fat layers will also exert a considerable influence.
Measurements made at the centre of the rib-eye muscle have shown that
at an air temperature of 0 °C and air speeds of 2.0 and 3.0 ms
-1
, the cooling
rate is sufficient to expose the whole muscle to cold shortening. Such air
speeds are greater than that usually achieved on average in commercial
practice, which is more closely reflected in the results at 0.5 and 1.0m s
-1
(Table 3.8). These results showed that the time taken for the centre of the
eye muscle to reach 10 °C was just outside the critical limit for cold short-
ening. However, this infers that the remainder of the muscle, which will cool
more rapidly than the centre, was adversely affected, the outer layer most
certainly.
3.4 Influence of freezing on texture
Other sections in this chapter have dealt with the biochemistry of rigor and
the excessive shortening which may occur in meat frozen pre-rigor which
causes toughening. In this section the main concerns are:
1. whether freezing of carcasses can be rapid enough to reduce the eating
quality;
2. the freezing conditions which are required to maintain good eating
quality.
Effect of refrigeration on texture of meat 61
Table 3.8 Cooling rates of beef sides in moving air at
0 °C. Temperature measured at centre of rib-eye muscle
Air speed (m s
-1
) Number Mean wt Time for rib-
of sides of sides eye to reach
(kg) 10 °C (h)
0.5 12 141 10.7
1.0 17 146 10.1
2.0 13 146 8.3
3.0 13 143 7.3
Source: Rhodes, 1972.
3.4.1 Lamb
Smith et al. (1968) found that lamb frozen at about -20 °C had tougher
loin chops and leg roasts but more tender rib chops than unfrozen lamb.
No difference in the palatability of lamb loin, leg and liver has been found
when frozen slowly or rapidly (Lampitt and Moran, 1933; Brady et al., 1942).
Modern freezing plants can freeze the entire lamb carcass in 6 h thus
freezing all the meat in a pre-rigor state. The effect of rapid freezing on the
toughness of the cooked meat can be seen from the results of work carried
out by McCrae et al. (1971) (Table 3.9). Two points are evident: first that
early freezing toughened some parts of the carcass more than others and
second, that the induced toughness decreased as the delay in the time
before freezing was increased. A toughness value of 40 units was stated to
be the minimum acceptable level. It was therefore evident that some parts
of the carcass would be made unacceptable by freezing within 24h after
death if cooked directly from the frozen state.
The effect of storage condition on the toughness of lamb loins cooked
after thawing was investigated at the MRI (Dransfield, 1974). Lamb car-
casses were frozen soon after slaughter and the loins stored at -30°C or
-3 °C. The former temperature was chosen to prevent the progress of
rigor compared with the latter. Meat held at -30°C was tough (Table 3.10).
When held at -3 °C for 6 days before thawing, the loins became acceptably
tender but were still slightly tougher than control unfrozen loins (Table
3.10). Thus toughening caused by thaw rigor was avoided by a holding
period just below 0 °C. Such a storage temperature is not used in normal
commercial handling of frozen meat. However, a sufficient period for rigor
resolution may be attained unwittingly by temperature rises during trans-
portation and handling, and the major toughness caused by thaw rigor
would be avoided.
62 Meat refrigeration
Table 3.9 Effect of delay in the time before freezing on the toughness of lamb
Cuts Muscles Delay time (h)
0 5 10 16 24
Toughness values (0–120 units)
Loin LD 73.4 70.3 56.1 41.3 25.5
Leg/fillet BF, SM, ST, GM 54.1 44.0 46.5 28.0 21.4
Shoulder IS, SS, TB 17.8 14.8 16.3 13.6 13.8
Sides from 60 carcasses were delayed at 18 °C for up to 24 h before freezing at about -18 °C
with air velocity 500 ft min
-1
. Cuts were roasted directly from the frozen state, dissected into
LD, M. longissimus dorsi; BF, M. biceps femoris; SM, M. semimembranosus; ST, M.
semitendinosus; GM, M. gluteus medius; IS, M. infraspinatus; SS, M. supraspinatus; TB, M.
triceps brachii muscles and the average toughness was calculated for each of the cuts.
Source: McCrae et al., 1971.
3.4.2 Pork
The larger pork carcass, with its greater insulation of fat, cools more slowly
than lamb and this, in combination with the faster onset of rigor, means
that the meat is unlikely to be frozen pre-rigor. Toughness caused by thaw
shortening (which produces tough lamb) is not encountered in pork. How-
ever, exposure to freezing temperatures would cool the carcass rapidly
and produce cold shortening and equally tough meat after cooking. Unlike
meat frozen pre-rigor, the toughness due to cold shortening cannot be ade-
quately removed by subsequent storage conditions.
3.4.3 Beef
DuBois et al. (1940) showed that freezing times (from 0 to -10 °C in the
centre of 1.4 kg cuts of meat) of greater than 8h toughened the meat, whilst
shorter freezing times down to 1 hour improved the tenderness. However,
in all these studies, the material used was of good quality and the freezing
rate accounted for only 16% of the variation in tenderness (Hankins and
Hiner, 1940). At about the same time, Lampitt and Moran (1933), Brady
et al. (1942) and others found no differences in the palatability of beef
frozen slowly or rapidly. Similar results were obtained for beef slices frozen
at 0.04, 2 and 13 cmh
-1
which corresponded to freezing times of about 19 h,
23 min and 4 min, respectively (Jakobsson and Bengtsson, 1973). The latter
authors noted that the slowest rate tended to cause toughening (Table 3.11).
It has also been reported that the tenderising effect of faster freezing rates
disappeared after storage for 6 months (Nicholas et al., 1947; Hankins and
Hiner, 1941).
It is generally agreed that there is a progressive toughening of meat
during frozen storage. The cause of this toughening is uncertain but its
development is unaffected by the freezing rate and is retarded at lower
Effect of refrigeration on texture of meat 63
Table 3.10 Effect of storage temperature on the toughness of lamb eye muscle
Pre-rigor cooling
Rapid freezing Slow chilling
Storage temperature ( °C) -30 -3 +1
Toughness (kgf) mean 6.26 3.37 2.13
range 5.12–9.04 1.76–4.78 1.65–3.13
One side of each of four carcasses was frozen at -40 °C with air velocity 5 m s
-1
. After 24 h, the
loins were halved and stored for a further 6 days at -30 °C or -3 °C. Each of the other sides
were slowly chilled and stored for the same period at 1 °C. The sections were thawed at room
temperature before cooking and the toughness determined on seven replicate samples taken
from the M. longissimus dorsi. Only the meat with toughness values of 4 kgf or less was accept-
able to all panellists.
Source: Dransfield, 1974.
temperatures, such that, at -20 °C, beef cuts are still tender after storage for
9 months (Table 3.11).
3.5 Influence of thawing on texture
It might be expected that slow thawing,like slow freezing,would have a detri-
mental effect on eating quality by increasing the time spent at the thawing
temperature, but such an effect has only been demonstrated with very long
thawing times. Beef frozen post-rigor and thawed before cooking has similar
tenderness to that thawed rapidly by cooking directly from the frozen state
(Table 3.5 and Table 3.11). This has been found with beef, pork and lamb
steaks (Brady et al. 1942). A similar study with beef, pork and lamb patties
Causey et al. (1950) found only small differences attributable to the thawing
method. It was noted that judges tended to prefer pork and lamb patties that
were cooked directly from the frozen state. Although cooking directly from
the frozen state may be convenient for steaks, to produce adequate cooking
in larger pieces of meat, cooking times may have to be increased by 50%.
Repeated freeze–thaw cycles of freezing and thawing are unlikely to affect
the subsequent rate of tenderisation (Dransfield, 1994).The process of freez-
ing and slow thawing has been found to improve the tenderness of deer meat
(Drew et al., 1988) by 10–40% when compared with unfrozen.
3.6 Conclusions
1 The rate of cooling, the length of time and the temperature during con-
ditioning are the most important refrigeration factors controlling the
texture of meat.
64 Meat refrigeration
Table 3.11 Effect of freezing rate, frozen storage and thawing conditions on beef
tenderness
Treatment Tenderness scores within treatments
Cooked after Cooked from the
thawing frozen state
Freezing rate (cm h
-1
) 13 6.0 6.0
2 6.0 6.2
0.04 5.6 5.8
Storage time (months) 1 6.2 6.3
9 5.5 5.7
Sections of eye muscle (M. longissimus dorsi) were frozen at -20 °C and thawed at 10 °C.
Tenderness scale: 1, extremely tough to 9, extremely tender.
Source: Jakobsson and Bengtsson, 1973.
2 Rates of conditioning vary widely among species. The time for 80% of
the tenderising at 1 °C for beef is 10 days, lamb 7.7 days and pork 4.2
days. Since the time for distribution is typically only 3 days, a time
should be deliberately allocated to complete the conditioning process
in beef, lamb and pork.
3 It seems, at least for beef, that there are only small, if any, differences
in the rate of conditioning between muscles. Therefore no cut can
be stored for shorter periods and obtain the same proportion of
conditioning.
4 The extent of tenderising varies with muscle length. In cold
shortened meat little or no tenderising occurs. Cold shortening
toughness therefore cannot be reversed by even prolonged
storage.
5 Over the range of 0–40 °C, the rate of conditioning increases about 2.5-
fold for every 10°C rise in temperature. Above 60°C the rate drops
rapidly due to enzyme denaturation and is therefore arrested during
cooking.
6 Cold shortening sets in during the chilling of lamb and beef muscles if
the conditions are such that the temperature has fallen below 11 °C
before the pH has fallen below 6.2. In pork cold shortening occurs if
temperatures between 3 and 5 °C are reached before the onset of rigor
(normally 3–8 h).
7 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
recommended that lamb or beef carcasses should not be chilled
below 10 °C until at least 10 h after slaughter. Only under these condi-
tions can optimal tenderness be ensured.
8 The severity of cold shortening is highly pH dependent, being much
greater at pH 6.8 (i.e. exceptionally rapid chilling) than at pH 6.2 (i.e.
at an easily attainable commercial rate of chilling).
9 Cold shortened muscle is tougher after cooking than unshortened
muscle, the toughness increasing with the degree of shortening (up to
a limit of about 40% of the initial length).
10 Freezing merely delays conditioning. Conditioning stops on freezing,
continuing on thawing. Freezing lamb, pork or beef carcasses shortly
after slaughter can produce very tough meat after cooking. Normal
commercial handling of frozen meats cannot reliably make such meat
acceptable to the consumer.
11 Storing chilled carcasses or cuts of meat for more than one day be-
fore freezing enhances the tenderness. The maximum tenderness is
obtained by conditioning beef for 10 days and lamb for 4 or 5 days
prior to freezing.
12 Freezing cuts of meat over a wide range of conditions has little effect
on the eating quality.
Effect of refrigeration on texture of meat 65
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