P. V. Harris

Electrical stimulation of beef sides

Sides of beef were stimulated with one of three different systems of stimulation-a high voltage system (1100 V), a low voltage system (110 V) and an extra low voltage system (45 V). Warner-Bratzler shear measurements indicated that for muscles removed at 1 or 2 h post slaughter and then subjected to conditions which induce cold shortening, the high voltage system was superior to the other two. Warner-Bratzler shear values and taste panel tenderness scores indicated that, for all stimulation treatments, muscles removed from stimulated sides at 22 h post slaughter were more tender than those removed from control sides at the same time. Muscles from sides which had received the high voltage treatment, but not those from sides which had received the other stimulation treatments, were judged by the taste panel to be less juicy than muscles from the control sides. Lower shear values for individual muscles from stimulated sides were usually accompanied by longer sarcomeres and it is suggested that the major effect of electrical stimulation is the prevention of cold shortening.

INFLUENCE OF ANIMAL AGE ON THE TENDERNESS OF BEEF: MUSCLE DIFFERENCES

Animal age effects on the mechanical properties of cooked samples from muscles able to cold shorten (Mm. longissimus dorsi, LDA, and semimembranosus, SMA from Achilles tendon hung sides) and muscles partially restrained from shortening (LD muscles from pelvic hung sides, LDP) from groups of animals aged 9, 16, 27 and 42 months were studied. Age effects on stretched muscles (SM muscles from pelvic hung sides, SMP) were determined using groups aged 2 and 120 months in addition to those of 9, 16, 27 and 42 months. Taste panel, Warner-Bratzler shear and Instron compression results for stretched SMP muscles indicated that tenderness decreased systematically as animal age increased. In contrast, Warner-Bratzler peak force values of LDA muscles decreased by half; linearly, as animal age increased from 9 to 423 months, a reflection of less postmortem shortening in samples from the heavier carcasses of older animals. It is suggested that results of age/tenderness studies depend on the age range, differences in carcass weight, the muscle(s) chosen and the cooling rate of muscles and hence on chilling conditions, cooking conditions and the method(s) used to assess the mechanical properties of the cooked muscle(s).

Effect of pressure treatments on the mechanical properties of pre- and post-rigor meat

Pressure-heat treatment of beef semitendinosus samples post-rigor gave shear and tensile results similar to those obtained with pressure treatment pre-rigor. Post-rigor pressure-heat treatment did not affect the contraction state, unlike pre-rigor pressure treatment which caused samples to contract by about 40%. Maximum tenderizing effect by pressure-heat treatment (150 M Nm(-2) at 60°C for 30 min) was achieved when samples were heated at 45°C for 45-180 min immediately before application of the treatment. As the pre-pressurization temperature was increased, the duration of heating became more critical until at temperatures ≥ 60°C the effects of subsequent pressure-heat treatment became very small. Pressure-heat treated samples did not show the increase in shear force values for cooking temperatures ≥ 60°C associated with myofibrillar hardening. It was concluded that pressure-heat treatment primarily affected the myofibrillar structure.

Heat-induced tenderisation of meat by endogenous carboxyl proteases

The rôle of carboxyl proteases in tenderising meat was investigated by injecting the inhibitors, pepstatin and EPNP, into pre-rigor muscle. The increase in shear force values induced by these inhibitors provided a minimum estimate of the extent to which endogenous carboxyl proteases normally tenderise meat at 60°C. Gel electrophoresis showed that connectin was hydrolysed to a greater extent than other muscle proteins at this temperature and that breakdown of connectin was inhibited by pepstatin and EPNP. Thus it is likely that, when intact, connectin may contribute to the strength of cooked meat.

Influence of pH on the Warner-Bratzler shear properties of mutton

Initial yield and peak shear force values obtained for stretched muscles cooked at 80°C for different times decreased linearly at a similar rate with increasing pH, which is consistent with the prime effect of pH being on the myofibrillar structure. The tenderising effect of pressure-heat treatment on stretched and cold-shortened muscle decreased rapidly with increase in ultimate pH until, at values near 7, the effect disappeared. Increased ultimate pH effectively eliminated the large increase in shear force values, occurring in cold-shortened muscle of normal pH and attributable to heat denaturation of myosin, as cooking temperature was increased above 60°C.

Comparison of some properties of beef from animals homozygous or heterozygous for muscular hypertrophy

Carcass characteristics and meat properties of fifty-eight steers and heifers heterozygous (M +) for muscular hypertrophy were compared with those of fifteen homozygous (M M) steers and heifers. The M M animals had a greater carcass weight (173 versus 162 kg) and killing-out percentage than M+ animals. Age at slaughter (298 days for M M and 304 for M +) and liveweights (280 kg, M M, versus 287 kg) did not differ significantly. Carcasses from M M animals were significantly shorter, with a greater eye-muscle area and thinner subcutaneous fat cover over the loin, than those from M + animals. M. semitendinosus (ST) muscles had similar sarcomere lengths, ultimate pH values and weight losses during cooking-80°C for 90 min-for M M and M + animals. Adhesion values, both peak force and work done, were less for ST samples from M M than M + animals, indicating that the ST of M M animals was potentially more tender. This, and other work, suggests that the mechanical properties of cooked ST samples of M+ animals are intermediate between those of M M and + + animals with the ST of M M animals being the most tender. No dark-cutting, high ultimate pH (>6·0) samples were found in the fifteen M M animals slaughtered commercially in the present experiment.

Comparison of some properties of meat from normal steers and steers heterozygous for muscular hypertrophy.

Carcase characteristics and meat properties of 19 normal steers and 14 steers heterozygous for muscular hypertrophy (HMH), killed at a liveweight of circa 360 kg and 500 days of age, were compared. Carcases of the HMH animals had less subcutaneous fat and a greater eye muscle area. It was considered that HMH animals were no more stress susceptible than control animals as plasma glucose concentrations at slaughter and ultimate pH values of the M. semitendinosus (ST) did not differ. ST sarcomere length and cooking losses did not differ between control and HMH animals. ST Warner-Bratzler (peak) shear force values and adhesion values were less in the HMH animals; these lower values would indicate that the ST of the HMH animals was more tender. These differences may have resulted from the difference in the mean age of the normal (527 days) and the HMH groups (489 days) at slaughter. However, other workers have found a reduced hydroxyproline content in muscles of homozygous double-muscled animals and the lower adhesion values of animals heterozygous for muscular hypertrophy may reflect the fact that they also have a lower connective tissue content in their muscles compared with normal animals.

The effect of temperature and ultimate pH on the increase in meat toughness resulting from restraint during cooking

Samples of stretched muscle cooked at 50, 60, 70 or 80°C, while restrained at either their original pre-cooking length or further tensioned at about 130% of their original pre-cooking length, had significantly (P < 0·001) greater Warner-Bratzler (WB) peak shear force values for all temperatures than similar samples cooked without restraint except for those restrained at their original length and cooked at 50°C. Restraint during cooking at 80°C increased the peak shear force values of stretched sheep muscles with ultimate pH values in the range 5·5-7·0. This increase, which has been related to connective tissue strength, was not significantly related to ultimate pH. Both initial yield and peak force values, for samples cooked either restrained or unrestrained, decreased significantly (P < 0·001) and at similar (not significantly different) rates with increase in ultimate pH.

A comparison of the meat properties of pasture-fed steers, heifers, pregnant heifers and spayed heifers.

Mean ultimate pH values, sarcomere lengths, cooking losses, Warner-Bratzler peak shear values and compression values for the M. longissimus dorsi (LD) did not differ significantly between heifers, spayed heifers, heifers fitted with a HEIGRO(TM) device(∗) pregnant heifers (2 to 6 months pregnant) and steers, killed at 18 or 22 months of age, with mean carcass weights of 189 and 221 kg, respectively. This result, from animals grazed at pasture, agrees with those of others who used LD samples from lot-fed animals. It would appear that carcasses of non-pregnant heifers, spayed heifers, pregnant heifers, heifers fitted with a HIEGRO(TM) device and steers of similar age and subcutaneous fat depth, do not need to be differentiated on grounds of tenderness.

Pressure-heat treatment of meat: A comparison of beef and buffalo meat

A pressure-heat treatment, which disrupts the myofibrillar structure of meat but leaves the connective tissues essentially intact, was used to compare the connective tissue component of toughness in the Semimembranosus and Longissimus dorsi muscles from nine Brahman cross and nine buffalo steers, 24 to 29 months of age. For assessment of samples, peak force, initial yield force and peak force minus initial yield force values were determined from Warner-Bratzler shear force-deformation curves. In the control, non-pressure-heat treated samples, the only breed difference detected was in peak minus initial yield force value, which was significantly lower for the beef Semimembranosus muscles. However, for the pressure-heat treated samples of both muscles, peak force and peak minus initial yield force values were significantly lower for beef than for buffalo. The pressure-heat treatment could thus be used to detect differences in the contribution of connective tissue to toughness which would otherwise be obscured by the differences in the myofibrillar toughness.