Honor Calnan

The colour of lamb meat on retail display is critical to consumer appeal. Consumers demand a bright red colour in lamb meat and associate dark, pale or browned meat with a lack of freshness and quality. Retailers are forces to downgrade or discount discoloured meat to prevent consumer rejection. Fresh lamb meat may be discoloured due to poor colour development with blooming, or due to browning that develops with time on retail display. The quantity of lamb meat with poor bloomed colour is unknown because lamb carcasses are not routinely graded for meat colour in Australia. The rapid browning of overwrapped lamb meat limits its retail display to only around 2 days. Retailers frequently discount lamb meat to ensure its rapid sale prior to the onset of browning and to thereby limit the economic losses associated with discounting, downgrading or wasting browned meat. Better understanding is required of the factors influencing bloomed colour of lamb meat and its stability on retail display in order to develop strategies to reduce meat discolouration. This thesis evaluates different phenotypic and genotypic factors influencing lamb meat colour in Australia. Bloomed colour (L*, a*, b*, hue angle and chroma) was measured in the loin muscle of over 8000 mixed breed lambs of known genetics produced at 8 sites across Australia over 5 years as part of the Sheep CRC’s INF experiment. The colour stability of the loin muscle was evaluated in over 4000 of these lambs using spectrophotometric measures of meat redness (R630/R580) taken over a 3 day simulated retail display. The first experiment of this thesis examines the influence of production factors and muscle traits on the bloomed colour of lamb loin. Over 40% of the 8165 loin samples were too dark for consumer acceptance, suggesting that dark meat is a substantial problem in the Australian lamb meat industry. Production factors such as lamb slaughter group, production site and year of production had substantial effects on bloomed colour, though these effects could not be attributed to changes in muscle traits such as myoglobin or pH24. Further investigation is required to better understand how production factors influence bloomed meat colour. Of the muscle traits analysed, changes in pH at 24 hours (pH24) had the greatest effect on meat a*, while myoglobin had the greatest effect on meat L*. Increasing lamb age from 140 to 400 days reduced meat L* due to increased myoglobin concentration. These results suggest that industry focus needs to shift to consideration of myoglobin concentration as well as meat pH in order to improve the bloomed colour of lamb meat, particularly meat lightness. The second experiment examines the influence of selection for IMF and lean meat yield on bloomed colour. Increasing IMF from 2 to 8% and shortloin fat weight from 100-500g were positively associated with meat L*, a*, b*, hue angle and chroma. Shortloin muscle weight was negatively associated with these colour parameters, though could largely be accounted for by correlated changes in IMF. The effect of sire breeding values for lamb weight, shortloin muscle depth and fat depth on loin L*, a*, b*, hue angle and chroma were small and varied between lamb sire type, dam breed and sex. Thus selection for increased lean meat yield in lambs will have neutral or positive effects on meat colour, while selection for increased IMF will increase the L*, a*, chroma and thereby the consumer appeal of bloomed lamb meat. The third experiment examines the influence of muscle weight and oxidative capacity on the colour of lamb loin following 72 hours of simulated retail display. Production factors such as slaughter group and site of production had the greatest magnitude effects on meat R630/R580 (redness) after 3 days of display. Increasing loin ICDH activity, reflecting muscle oxidative capacity, reduced R630/R580 at the end of display. Selection for high sire breeding values for shortloin muscle depth increased R630/R580, likely due to an associated reduction in muscle oxidative capacity. Lamb carcass weight also increased R630/R580. Genotypic factors influencing lamb size and growth rate such as sire type and dam breed further support that increased growth rate improves meat colour. These findings suggest that breeding for increased growth rate and muscle weight could improve the colour stability of overwrapped lamb on retail display. The fourth experiment examines the ability to predict the rate of lamb meat browning on display using measures of bloomed colour and information of animal factors that influence colour stability. Simple and partial correlation coefficients between initial boomed R630/R580 and subsequent R630/R580 measures over display were ≤ 0.4, despite incorporation of carcass traits including pH24, lamb age and IMF. Therefore, bloomed colour at the start of display cannot provide a useful prediction of subsequent meat browning. Correlations between 24, 48 and 72 hr measures of R630/R580 were high (> 0.8), suggesting that colour measured from 24 hours of display can provide an accurate prediction of subsequent meat colour. While predicting the rate of meat browning would allow retailers to maximise the display time of lamb, meat colour measured at 24 hrs of display is unlikely to be practical in a retail setting. The predictive ability of 24 hr colour may therefore be limited to use in the development of a breeding value for retail colour. The final experiment examines the use of dietary vitamin E supplementation to improve the colour stability of long-stored lamb meat, particularly meat with high IMF, pH24 and ICDH activity. The capacity of this antioxidant to improve the colour stability of lamb meat was expected to increase in long-stored meat with high IMF, pH24 and ICDH activity due to the anticipated increased oxidative load in this meat. Vitamin E supplementation at 275 mg/kg of feed for 8 weeks prior to slaughter increased R630/R580 throughout the display of lamb loin following 5, 35 and 70 days of storage. However contrary to expectations, vitamin E supplementation had a similar magnitude impact on meat colour stability regardless of storage time prior to the display. Vitamin E did mitigate the negative effect of high IMF on short-stored meat colour, however high IMF did not impact the colour stability of medium or long-stored meat. These results demonstrate that while vitamin E supplementation will improve the colour stability of lamb meat following short, medium or long storage, its greatest impact is on the display colour of short-stored lamb with high IMF content.