B. McGEEHIN

Factors affecting the pH decline in lamb after slaughter.

Early postmortem pH has been linked to meat tenderness, especially in electrical stimulation and/or rapid chilling applications. In order to evaluate factors that affect pH, a trial was carried out in which 96 lambs were slaughtered over a period of 12 months. Carcass traits including age, sex and weight were recorded and the pH of M. longissimus thoracis et lumborum was measured at 0.5, 1.5, 4 and 24 h postmortem. Significant differences in pH were present over the 12 months but these differences could not be attributed to seasonal or age factors. The sex of the animal had no effect on early pH (0.5 h, 1.5 h) or final pH (24 h), but the 4 h pH of female lambs was lower than that of male lambs (P<0.001). Regression analysis, using 0.5 h pH and other measured traits, showed that individual male and female prediction models were better (R(2)=0.80 and RSD=0.10) for predicting 4 h pH than a combined model (R(2)=0.74 and RSD=0.11).

The effect of ultra-rapid chilling and subsequent ageing on the calpain/calpastatin system and myofibrillar degradation in lamb M. longissimus thoracis et lumborum.

The effect of ultra-rapid chilling (-20°C) on the proteolytic degradation of myofibrillar proteins during ageing, by the calpain/calpastatin system in lamb was investigated. Carcasses (36) were ultra-rapidly chilled (-20°C for 3.5 h followed by 20.5 h at +4°C) or chilling at +4°C for 24 h. After chilling all carcasses were stored at +4°C for a further 4 days. Samples of M. longissimus thoracis et lumborum were taken for Warner-Bratzler shear force measurement, sarcomere length, SDS-PAGE and enzyme activity analysis. Ultra-rapid chilling (-20°C) had no effect on the proteolytic degradation of myofibrillar proteins by the calpain/calpastatin system compared to conventional chilling (+4°C). However, no difference in tenderness was found between the two chilling regimes at 1 or 5 days postmortem so differences in myofibrillar degradation or enzyme activity would not be expected.

Optimising a rapid chilling system for lamb carcasses

Abstract 12 rapid chilling treatments using different air speeds, temperatures and durations of the initial rapid chilling stage were tested to optimise rapid chilling of lamb in terms of weight loss and tenderness. Results showed that there was a linear increase in weight loss with chill temperature. Shear force values increased with chill duration. There was a linear increase in day 5 tenderness scores with chill temperature. Overall the results showed that rapid chilling for 2.5 h using air speed of 0.5 m s −1 and a temperature of −20°C gave the best results overall in terms of reducing evaporative weight losses and maintaining tenderness.

Effect of Temperature and Air Speed on the Rapid Chilling of Lamb

Rapid chilling of lamb has major economic advantages for the meat industry. Rapid chilling can reduce weight loss from approximately 2% in normally chilled lamb to approximately 1.2%. These weight loss reductions are a major financial incentive for rapid chilling. With rapid chilling, the requirement for overnight chilling can be eliminated and carcasses can be dispatched to commercial markets on the day of slaughter. When properly carried out, rapid chilling results in lamb carcasses that are not tougher than normally chilled lamb. During the work, considerable variation in shear force values for individual lamb carcasses was encountered irrespective of the chilling treatment. This variation in tenderness appears to be due mainly to animal variation rather than external environmental factors. Continuous monitoring of carcass weight during chilling and subsequent storage indicated that rapidly chilled carcasses gained weight appreciably when moved at the end of the rapid chilling period to the storage chill (4 °C). This increase in weight was the main cause of decreased weight loss associated with rapid chilling. The differences found in weight loss between rapidly chilled carcasses and normally chilled carcasses can be maintained over 5 days of chilled storage when the carcasses are stored at 4 °C and > 90% relative humidity.