Of all the MSA criteria, specification of Bos indicus content is obviously the most contentious. A number of researchers (both overseas and Australian) have reported that as Bos indicus percentage increases, tenderness or palatability of the meat decreases. The magnitude of the breed effect tends to vary considerably between studies and as suggested by Hearnshaw et al. (1998) may interact with processing conditions.
Results from the Bos indicus content experiment (BICE) showed that even when overlaid with a variety of post-slaughter treatments Bos indicus content up to 75% has little impact on palatability. This contrasts with other studies from the US, where some experiments have shown that greater than 25% Bos indicus content can have an impact on palatability.
A number of US studies have also reported an increase in variability of meat quality as Bos indicus content increases. However in a number of CRC studies a change in variance of eating quality with increased Bos indicus has not been evident.
In many instances the actual Bos indicus content of the animal is not known. To accommodate this situation MSA has developed guidelines for assessing Bos indicus content from the phenotype of the animal.
7.2.2.1 The interaction of breed effects with processing conditions
A number of studies (both Australian and US) have shown that the effect of Bos indicus content may interact with processing conditions. In a recent experiment conducted by Hearnshaw et al. (1998) this interaction was particularly evident with a large breed effect being evident in non-stimulated carcases, which contrasted the stimulated carcases where the breed effect was not evident. It is interesting that this effect cannot be reliably repeated in other experiments (e.g. the BICE experiment in Figure 7.1). One possible mechanism by which the interaction is occurring is that the Brahman carcases are less fat and therefore without stimulation tend to cold shorten whilst shortening does not occur in the fatter Bos taurus carcases.
However from a commercial perspective, Bos indicus content was generally confounded with harsher environments, poorer nutrition, less handling and greater transportation distances to slaughter and the results from the MSA database are perhaps more relevant to the importance of the Bos indicus content as a grading criteria. The MSA database shows that for cattle that are being processed under commercial conditions Bos indicus content has a large effect on palatability of the striploin (Table 7.4). Certainly for the carcase pathways scheme where non-negotiable hurdles were used as thresholds to the grades, the early results showed that Bos indicus content had a large effect on palatability. Based on striploin palatability data from the database, those carcases that were classed as having greater than 75% Bos indicus content (based on phenotype) had a palatability failure rate (i.e. a CMQ4 score of < 48) of 63%. For low Bos indicus content carcases the palatability failure rate was reduced to 11%. Based on this information the pathways carcase system initially had a threshold of 25% on Bos indicus content, although this was later raised if the carcases were tenderstretched. Therefore under present processing conditions breed is an important component of the MSA criteria. However given the interaction with processing conditions, the importance of breed needs to be continually reassessed as new or modified processing conditions are introduced.
7.2.2.2 The interaction of breed effects with cuts
Shackelford et al. (1995) examined the interaction between Bos indicus content and cut on palatability. They showed that the Bos indicus effect was significant for the Mm. triceps brachii, longissimus dorsi, supraspinatus, biceps femoris and quadriceps femoris and not for a number of other muscles that were tested. Given that Bos indicus effect had been shown to be so important in the MSA carcass pathways scheme, which was based on striploin palatability, it was important that the effect of Bos indicus content on other cuts be quantified, as background for the cuts based grading model.
Therefore as part of the MSA research program 50 milk-fed vealers and 40 heavy pasture-fed steers that ranged in Bos indicus content from 0 to 100% were sourced from the Grafton crossbreeding program. Animals were slaughtered, the carcasses stimulated using a low voltage system directly after slaughter and one side was tenderstretched. At boning 10 cuts from each side were collected and tested using a range of cooking techniques by the MSA consumer panels (Polkinghorne et al. 1999). Table 7.5 shows the regression coefficient for CMQ4 scores as a function of percentage Bos indicus content. The results clearly showed that a Bos indicus x cut interaction, with a decline in palatability with increased % Bos indicus content most evident for the muscles surrounding the spinal column, (i.e. Mm. longissimus and psoas which comprised the cube roll, striploin and tenderloin cuts). These muscles showed nearly a 10-point decrease in palatability over the range of 0 to 100% Bos indicus content. The concentration of Bos indicus effect in the loin muscles was more evident in the MSA study than in the experiment conducted by Shackelford et al. (1995), as in their study they reported little difference in the M. psoas major. In addition, they also reported a breed x cut x cooking interaction which was not evident in the MSA study.
These results indicate that a Bos indicus effect based on the striploin was not appropriate to apply across all muscles of the carcass, rather the magnitude of the Bos indicus effect was muscle dependent. Further taste panel tests are currently underway to validate the coefficients for a wider range of muscles and determine whether there is an interaction between Bos indicus content and ageing rate.
7.2.2.3 Possible mechanisms for the breed effects
Mechanisms, which contribute to the Bos indicus effect on palatability, have been investigated in several studies. Johnson et al. (1990) reported breed differences in the proteolytic enzymes, with the high Bos indicus content animals having higher levels of calpastatin (an inhibitor of the ageing enzymes). Shackelford et al. (1992) estimated that the heritability of post-rigor calpastatin activity to be of the order of 0.70. Gursansky, (pers com) reported Bos indicus differences in compression values, which would implicate the heat stability of the connective tissue matrix. More recent studies by the CRC (Ferguson pers com) suggest that there are breed differences in the composition of the phospholipid component of the cell membranes, which give rise to differences in calcium permeability and consequently impact on muscle shortening. This is consistent with the interaction between Bos indicus effect and processing, whereby stimulation can sometimes minimise breed effects in palatability (Hearnshaw et al. 1998). In addition, the recent MSA results indicate that the breed effects on palatability are not constant across all cuts. It is likely therefore that a number of mechanisms may contribute to the Bos indicus effect, rather than one simple mechanism.