Honor Calnan

Brown discolouration of lamb meat on retail display reduces consumer appeal, limiting the shelflife and value of the product. The rate of change in colour from red to brown, known as colour stability, is increased in short aged meat with high intramuscular fat (IMF) content (Calnan et al 2014). Therefore genotypic selection for IMF to improve sensory appeal may reduce lamb meat colour stability. Extended aging of lamb meat also reduces colour stability of lamb meat (Jose et al, 2008), a concern for chilled meat shipped for 35 - 70 days to distant markets. High intramuscular vitamin E (a-tocopherol) concentration, achieved by dietary supplementation, slows the loss of redness in lamb aged 5 - I 0 days (Jose et al, 2008). Given that colour stability worsens with aging, the impact of a-tocopherol may be greater in long-aged and high IMF meat. We hypothesised that high muscle a-tocopherol concentration in lambs will retain redness during display of the longissimus, particularly in long-aged and high IMF meat.

Meat colour was measured in 7732 lambs produced at 8 sites across Australia over a 5 year period (2007-2011) and slaughtered in 125 groups as pari of the Sheep Cooperative Research Centre's information nucleus flock experiment. Lambs were the progeny of sites of different types (merino, maternal and terminal) selected for a diverse range in Australian Sheep Breeding Values for post weaning eye muscle depth (PEMD). 24 hours post slaughter them. longissimus was cut at the 12th rib, a probe was used to measure pH and the meat surface was allowed to bloom/oxygenate for 30 minutes before fresh colour measures of lightness (L*), redness (a*) and yellowness (b*) were captured using a Minolta colorimeter. These measures were analysed in a multivariate analysis before least square means were produced for L *,a* and b* using a linear mixed effects model in SAS. The base model included fixed effects for site, year of birth, slaughter group, sex and dam breed within sire type as well as random effects for sire and dam by year. In a second analysis sire PEMD estimates were included in the model as a covariate. Increasing sire PEMD across a range of -2 to 4 was associated with an increase (P<0.01) in the predicted means for L*, a* and b* by 1.15, 0.38 and 0.55 units respectively. When pH measured 24 hours post-m01iem was accounted for in the model, the impact of sire PEMD estimates on L *, a* and b* were halved. This suggests that selection for sires based on PEMD may impact meat colour via changes in muscle metabolism that affect the extent of glycolysis and pH decline post slaughter. Our findings suggest that using sires with high PEMD will improve the lightness and redness of the meat produced.

Meat colour data was collected from 4,953 lambs produced at 5 sites across Australia over a 5 year period (2007-2011) as part of the Sheep Cooperative Research Centre’s information nucleus flock experiment. Longissimus muscle samples were collected 24 h post-mortem, vacuum packaged, aged for 5 days and then placed under simulated retail display conditions for 3 days. At the end of this period light reflectance of the meat surface was measured with a Hunterlab reflectometer and a ratio was calculated (630 nm/580 nm reflectance) to represent redness, with higher values redder and hence more desirable. These ratios were analysed using linear mixed effects models. The base model included fixed effects for site, year of birth, kill group, sire type and dam breed. In a second analysis pH measured 24 h post-mortem was included in the base model as a covariate. Of the dam breeds, Merino progeny had 0.19 units lower redness than those of Maternal dam breed. Similarly the Merino sire type produced lower redness values than Maternal or Terminal sired lambs, with 0.39 units difference seen between lambs of Merino and Terminal sire types. pH was negatively associated with meat redness, with a 0.92 unit decrease (P<0.01) in redness across the pH range of 5.4 to 6. With pH included in the model, the effect of sire type was not significant, suggesting that differences in post-mortem muscle pH between sire types may underpin the observed variation in retail meat colour. Our findings suggest that by reducing the pH of lamb loins we can improve the redness of the meat whilst on retail display.

The brown discolouration of lamb meat reduces its appeal to consumers, costing the Australian Lamb industry considerably due to the discounting of product. Lamb meat browning is caused by the oxidisation of myoglobin pigments into the brown metmyoglobin form. The rate of this oxidisation is greater in more aerobic muscles (O'Keeffe and Hood 1982), with aerobicity quantified by the measurement of isocitrate dehydrogenase activity (ICDH). High intramuscular fat percentages (IMF) have been associated with high ICDH levels, so if industry moves to increase IMF we can hypothesise that there will be an ICDH linked increase in the brownness of lamb meat after 3 days of retail display..