Abstract:
Proc. Aust. Soc. Anim. Prod. 1996 Vol. 21 A QUALITATIVE MODEL OF FACTORS INFLUENCING BEEF TENDERNESS W.R. SHORTHOSE CSIRO, Division of Food Science and Technology, Brisbane Laboratory, Tingalpa D.C., Qld 4173 SUMMARY A conceptual model is presented in which the effects of both pre- and post-slaughter factors on tenderness were considered. The pre-slaughter factors were plane of nutrition, breed type and target carcase weight. Post-slaughter factors considered were carcase weight, fatness and cooling rate, the use of effective electrical stimulation, muscle ultimate pH and ageing the meat. Generally, 2 extreme, but commercial, levels of each factor were considered. The relative importance of the contributions of the myofibrillar and connective tissue components to toughness were illustrated by predicting Instron Compression (IC), an index of the connective tissue contribution, and calculated tenderness values; the latter are calculated using IC values and Warner-Bratzler (WB) peak force (PF) values, which are more of an index of the myofibrillar contribution. The modelling was simplistic in that values were predicted by extrapolation and interpolation from a number of experiments involving many animals. Advantages were that the same procedure was used to determine the objective tenderness of muscles of all of these animals and there were at least 40 animals per treatment in the base data. Keywords: model, tenderness, beef, electrical stimulation INTRODUCTION There have been a number of models produced to account for the wide variation in tenderness found in a muscle, across animals within a species (Dransfield 1992). Unfortunately, data used were often from experiments in which only one muscle, Mlongissimus dorsi (LD), was used from animals slaughtered to meet car-case specifications for 1 market and with carcases and cuts being processed in 1 way. This approach is not sufficiently complete to be of maximum benefit in predicting variations in tenderness of muscles of beef animals slaughtered in Australia for export to a large number of countries. Generally, extremes have been chosen and predicted numbers are relative, and not absolute, values. Objective measurements of tenderness will be used; these are the Warner Bratzler (WB) peak shear force (PF) and Instron Compression (IC) values, which can be used together to account for about 80% of the variation in mean tenderness scores of trained taste panels (Shorthose and Harris 1990). WB values are more influenced by contractile proteins and IC values by connective tissue. The 'tenderness' (T) values reported were calculated (T= 1.16 WBPF + 3.24 IC -1.04, Shorthose and Harris 1990) using predicted WBPF and IC values. THE MODEL The outcomes of interpolation and extrapolation of data from a number of experiments, on the effects of a number of treatments on the tenderness of 4 muscles, are presented in tabular form. Limits of the variables in the model The limits were animals of light (LMW) or heavy mature weight breeds (HMW), grown on planes of nutrition either average for Central Queensland pasture (NAP) or average for feedlots in Queensland (NAFL) and slaughtered at either domestic, supermarket (190 kg) or Japanese trade (320 kg) carcase weights. It was presumed that carcases were chilled at either prescribed (PC) (Meat Order 250 = deep butt temperature of 20�C at 20 hours postmortem), or slower (AC), chilling rates after either effective electrical stimulation (EES) or no stimulation (NES). Muscles The 4 muscles chosen were 2 muscles restrained (R) from shortening postmortem by their muscle attachments and 2 muscles free to shorten (F). Within the 2 groups (R and F), 1 muscle had a high connective tissue content (HC) and the other a low connective tissue content (LC). The muscles considered were:- 143 Proc. Aust. Soc. Anim. Prod. 1996 Vol. 21 The muscles were presumed to have an ultimate pH of 5.6 (except in Table 6) and to have been cooked 'well-done' at either 1 day postmortem (no ageing (NA) or after ageing for 4 weeks at 0�C (A4). RESULTS Age at slaughter Age at slaughter (at a target carcase weight) is determined by the interaction of breed type (LMW v HMW) and plane of nutrition (NAP v NAFL). The data (Table 1) are derived from Loxton (1992). Table 1. Age (days) at slaughter of steers of either light mature (LMW) or heavy mature weight (HMW) breeds produced either off pasture (NAP) or after 100 days in a feedlot (NAF) Injluences of the interaction of carcase weight and carcase processing conditions (chilling rate and electrical stimulation) and animal age on 'tenderness' of 4 muscles Chilling rate affects the toughness of muscles free to shorten; the faster the chilling rate the more the muscle shortens and toughens. Carcase weight, therefore, affects toughening in these muscles, as heavier car-cases cool more slowly with consequent less muscle shortening and toughening; subcutaneous fat (related to carcase weight) has an independent, insulating effect. Changes in muscle shortening affect PF values more than IC values and animal age differences affect IC values more than PF values, particularly in muscles with a high connective tissue content. IC and calculated tenderness (T) values of the 4 muscles of the groups of animals described in Table 1 are shown in Tables 2 and 3; results in Table 2 are when carcases are effectively electrically stimulated and in Table 3 with no ES. Table 2. Effects of age differences on tenderness (IC [kg] and T* values in parentheses) of 4 unaged muscles of effectively electrically stimulated steer carcases chilled at an average rate (deep butt temperature of 140 kg side, 20�C at 20 hours postmortem) Note that: the 2 muscles free to shorten (LD and SM) did not shorten due to EES and the PM was always tender (as it cannot shorten and has a low CT content). The SM and ST muscles of the heavier car-cases were tough because of increased animal ages, even though shortening was inhibited (by EES) in the SM and the 144 Proc. Amt. Sot. Anirn. Prod. 1996 Vol. 21 ST cannot shorten. Without EES, the LD and SM would be least in LD and SM muscles from heavier The effects of animal age, carcase weight and muscles chilled at an average rate without EES are would be expected to shorten and toughen; the effect carcases and less in the SM, a deep muscle, than the LD. fatness differences on the tenderness of the 4 unaged shown in Table 3. Table 3. Effects of animal age differences on tenderness (IC [kg] and T values in parentheses) of 4 unaged muscles of non-electrically stimulated carcases chilled at an average rate Faster than average chilling (AC) exacerbates differences in WBPF of muscles free to shorten, especially in superficial muscles of light, lean carcases. Faster chilling has no effect on WBPF or IC values of restrained muscles, therefore only the tenderness of muscles free to shorten (LD and SM) are shown in Table 4 to demonstrate the effects of faster chilling (PC) in non-stimulated &cases. Table 4. Effect of 2 chilling regimes (PC v AC) on tenderness of 2 unaged muscles free to shorten (LD or SM) of non-stimulated carcases of pasture-fed steers of 2 weights (190 or 320 kg) Although the HMW animals have the same carcase weight they would be leaner and their loin muscles would be expected to cool more quickly at any carcase weight or in either chilling regime. In contrast they would be younger; the age difference would increase with increasing target carcase weight. Effects of handling primal cuts (ageing) Ageing rates Muscles that have shortened excessively, by about 40 per cent, do not age, ie WBPF values do not change with the duration of ageing and they remain tough. Similarly, stretched muscles (eg PM) are very tender at 1 day postmortem and do not age much. There is, thus, an apparent curvilinear relationship between the extent of ageing (reduction in WBPF) and the contraction state of the muscle (Bouton et al. 1973). The tenderness of EES muscles with LC does not change much with time (they are already quite tender), and ageing does not influence the connective tissue component of toughness. Therefore, in Table 5, the effect of ageing on the tenderness of a muscle free to shorten (LD) of car-cases chilled PC or AC after EES is shown. 145 Proc. Aust. Sot. Anim. Prod. I996 Vol. 21 Table 5. The tenderness (IC and T values) of loin (LD) muscles of carcases, of pasture-fed steers of an average mature weight breed, chilled PC or AC, with or without EES, unaged or aged for 4 weeks at 0�C Table 6. The tenderness (IC [kg] and T values) of loin muscles with ultimate pH values of 5.4 or 6.0 of carcases of pasture-fed steers, of an average mature weight breed, chilled fast (PC) with or without EES, and unaged or aged for 4 weeks at 0�C DISCUSSION The model presented is only a conceptual model and has not been validated. The tenderness of muscles free to shorten can vary more than that of muscles that are restrained from shortening. In muscles free to shorten, preslaughter stress, car-case processing and meat ageing conditions can have large effects on tenderness over and above the effects of connective tissue toughness. The tenderness of muscles restrained from shortening is largely dependent on the amount and nature of their connective tissue; as a result their tenderness is primarily dependent on animal age. These influences on tenderness mean that, depending on processing conditions, some muscles get tougher with increasing animal age, others become more tender and the toughness of others changes little (Bouton et al. 1978). REFERENCES BOUTON, P.E., FORD, A.L., HARRIS, P.V., SHORTHOSE, W.R., RATCLIFF, D. and MORGAN, J.H.L. (1978). A4eat Science 2: 301-11. BOUTON, P.E., HARRIS, P.V., SHORTHOSE, W.R. and BAXTER, RI. (1973). J. Food Science 38: 9327. DRANSFIELD, E. (1992). Proc. 38th KoMST, Clermont Ferrand, France, 1: 71-8. LOXTON, I. (1992). Final Report DAQ 52 to the Meat Research Corporation. SHORTHOSE, W.R. and HARRIS, P.V. (1990). J. Food Sci. 55: l-8,14. 146