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Warner-Bratzler shear force is an objective measure of tenderness and describes the force required, in Newtons or kilograms, to shear a cooked meat sample with a Warner- Bratzler shear blade (Hopkins et al., 2010). Whilst objective measurements of meat quality such as shear force and compression have the advantage of being cheaper than sensory panel testing for tenderness, they are rather simplistic one-dimensional measures of a complex set of interactions which occur when cooked meat is chewed and masticated in the mouth (Perry et al., 2001; Watson et al., 2008). Furthermore, they are destructive to the carcass and time consuming (Hwang et al., 2008; Watson et al., 2008). Despite this, Huffman et al. (1996) reported that consumer ratings were consistent with Warner–Bratzler shear values in beef and (Safari et al., 2001) advocated the use of shear force as a criterion for determining consumer acceptability of loin from lambs.

Sensory scores are rated out of 100 by consumers. They are subjective measurements of meat eating quality and have a negative correlation with shear force. Increasing shear force is associated with a reduction in tenderness, flavour, juiciness and overall liking (Hopkins et al., 2006; Pannier et al., 2014a; Safari et al., 2001). In the Pannier et al. (2014a) study sensory scores for tenderness, overall liking, juiciness and flavour reduced by 11.6, 8.5, 7.6 and 7.4 respectively across the 37 N range in shear force.

The phenotypic correlation between intramuscular fat and shear force after 5 days of ageing in lamb is -0.30 (Mortimer et al., 2014), suggesting selection for one will also influence the other. There are similar findings in pigs, where there is also a negative

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correlation between shear force and intramuscular fat (Karlsson et al., 1993). As shear force is correlated with intramuscular fat both can be used as indirect predictors of eating quality. Indirect predictors are useful as the cost of measuring sensory parameters with taste panels is often prohibitive.

2.6.2.1 Shear force and growth

Growth rate can be difficult to directly quantify and carcass weight, average daily gain and nutritional restriction are commonly used as proxies for growth rate to investigate the impact of growth on shear force. In general, shear force does not vary with carcass weight, average daily gain or growth rate, although there are some exceptions.

Lambs slaughtered at either medium or light live weights had no differences in shear force, despite differences in carcass fatness between the two groups (Vergara et al., 1999). This aligns with the findings of Kemp et al. (1980) which also demonstrated no relationship existed between shear force and slaughter weight in lambs. Conversely, there was a low negative correlation between hot carcass weight and shear force of - 0.15 found by Vergara et al. (1999), however this was not supported by a phenotypic association in the same study. There are conflicting reports in beef with a reduction in shear force seen in crossbred steers as carcass weights increased with age (Bouton et al., 1978) but no relationship between shear force and carcass weight Japanese Black steers (Ozawa et al., 2000).

Studies of average daily gain and shear force in cattle had varied results. A group of 48 bulls with average daily gains ranging from between -0.55 kg and 1.36 kg showed no relationship between shear force and growth rate (Calkins et al., 1987). This aligns with the findings of Perry and Thompson (2005) of no change in shear force with variations in average daily gain and Greenwood et al. (2006a) of no influence of pre weaning growth on shear force.

45 Meanwhile, Fishell et al. (1985) assigned 36 steers to one of three feeding regimes for 120 days pre slaughter. The unrestricted group had an average daily gain of 1.42 kg, the moderately restricted had an average daily gain of 0.77 kg and the highest restricted group had an average daily gain of 0.34 kg. In both the longissimus and

semimembranosus shear force was significantly lower in unrestricted animals with

faster growth rates compared to animals with restricted nutrition and slower growth rates. This relationship is further evidenced by the negative phenotypic correlation found between average daily gain and shear force in cattle (Shackelford et al., 1994). The difference is in the Calkins et al. (1987) and Fishell et al. (1985) studies is unlikely to be related to the effects of testosterone between bulls and steers with Moloney et al. (2000) reporting no difference in shear force with growth rate in steers.

Evidence for the association between shear force and growth is not clear. While there was a small negative correlation between carcass weight and shear force in lambs there was no phenotypic association (Vergara et al., 1999). In cattle there was also a negative correlation, between average daily gain and shear force (Shackelford et al., 1994), however the phenotypic association was not present in all studies discussed above.

2.6.2.2 Production factors which influence shear force

Shear force varies with production factors including age, sex, and genotype with shear force being increased in older sheep (Hopkins et al., 2007) and cattle (Shorthose and Harris, 1990) demonstrating an increase in sheer force with age at slaughter in cattle due to an increase in collagen crosslinking.

Unlike age, sex and genotype do not appear to have a significant impact on shear force in lambs with Kemp et al. (1980) demonstrating no difference between the sexes.

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Shear force in 60 lambs did not vary between common Australian genotypes for lambs born to Texel, Poll Dorset, Border Leicester and Merino rams (Safari et al., 2001). Likewise there was no relationship between shear force and genotype in the Hopkins and Fogarty (1998) study which involved a total of 436 lambs nor the Speck et al. (1997) study which examined shear force in New Zealand lambs of different genotypes. In cattle, shear force does not vary with breed with Wegner et al. (2000) showing no difference between shear force in German Angus, Galloway and Holstein Friesian cattle.

While shear force varied with age, there was no variation with sex or genotype. As sex and genotype can represent differences in growth rates these findings align with the lack of a phenotypic association between shear force and growth in lambs.

2.6.2.3 Shear force and fibre type

Shear force did not correlate with muscle fibre traits (Wegner et al., 2000) of Holstein- Friesian, Belgian Blue, German Angus and Galloway which have very different growth potential.

Muscles with a large fibre size, especially glycolytic type IIB fibres, exhibit tougher meat than muscles with a narrower fibre diameter in cattle (Renand et al., 2001) and in pigs (Karlsson et al., 1993). Thus shear force is likely to be reduced as the proportion of smaller oxidative fibres increases.

Considering the relationship between fibre type and intramuscular fat, as discussed in Section 2.6.1.3., it follows that as intramuscular fat increases and the oxidative capacity of muscle increases, the muscle fibre diameter will decrease and shear force is likely to reduce. This aligns with the findings of Calkins et al. (1981) who showed that muscle fibre type composition has a stronger relationship with marbling than with shear force.

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