Effects of goat ecotype and sex on post-mortem muscle energy status and meat quality

Abstract

The aim of this study was to investigate the effects of muscle energy metabolism and calpain proteolytic system on meat colour and tenderness in different Southern African ecotypes. A total of sixty-five (65) goats representative of different goat ecotypes namely Northern Cape Speckled (NCS), Eastern Cape Xhosa Lob Ear (XL), Mbusi/Nguni (MBZ), Southern African Boer Goat (SAB), Village type semi intensive (VT) and Village type communal veld (VTV), of A-age class (about nine months old) were used. The does and bucks used ranged between 10-14 and 6-12 animals from each group of goat ecotypes respectively. The VTV goats were transported directly from the communal areas the morning of slaughter and thus were exposed to transportation stress (241 km). Goats were slaughtered and dressed down by means of standard abattoir procedures. Each carcass was subjected to step-wise chilling (10-15˚C for 6 hours and then 4°C overnight). Muscle pH and temperature, muscle energy status, water holding capacity, drip loss, cooking loss, thawing loss, myofibril fragment length, sarcomere length, meat colour and forms of myoglobin, calpain systems were all evaluated in samples of the longissimus dorsi (LD) and semimembranosus (SM) muscles of the left sides of each carcass. Live weights of carcasses in goat ecotypes showed that the SAB was significantly heavier (40.88 kg) than MBZ, VT, VTV carcasses which had significantly lighter live weights (22.95 kg, 24.22 kg, 22.00 kg), while the NCS and XL carcasses had moderate live weights (33.13 kg and 29.77 kg) respectively. There were no significant differences in dressing percentage between goat ecotypes (40.8-42.5%). Chilling losses were significantly higher in the NCS and VTV carcasses (8.41 % and 7.80 %) respectively than SAB carcasses (5.46 %). The XL goats of both sexes were the only group that differed in carcasses characteristics. Interaction effects of ecotype and sex did not differ in terms of chemical composition. There were no sex differences in carcass characteristics. Doe carcasses had twice higher percentage of fat and dry matter percentage than that of the buck carcasses (0.27% vs 0.52%). The ultimate pH values were significantly different between goat ecotypes, but they were within an acceptable range (pHu 5.6-5.7), except for those of VT and VTV goats that had slightly higher ultimate pH values (pHu 5.8) for LD. Bucks had significantly higher ultimate pH than does (pHu 5.7 vs 5.6) for the LD. Buck carcasses from VT goats had significantly higher ultimate pH (pHu 5.9) than that of the doe and buck carcasses from MBZ, NCS, SAB, VTV and XL goats that significantly ranged between (pHu 5.5-5.8) for LD. Temperature declines and longer sarcomere lengths were indicative of no cold shortening in all goat ecotypes, either sexes and their interactions, which was facilitated by (step-wise) delayed chilling. Energy metabolites (creatine phosphate and ATP concentration) status decreased with post-mortem time, except for the production of lactate concentration, glucose concentration and glucose-6-phosphate concentration. Muscle samples from all goat ecotypes were associated with a lower glycolytic potential. Environmental effects such as transportation stress and production system influenced biochemical status of different goat ecotypes and sexes. There were no interaction effects between goat ecotype and sex on the water holding capacity, drip loss, cooking loss and thawing loss. Transportation stress in LD of VTV goats significantly increased WHC (0.41) compared to LD of MBZ, NCS, VT, XL and SAB goats but all goat ecotypes caught up after aging as assessed at 4 days post-mortem. Transportation stress lowered drip loss in LD of VTV goats (0.5%) compared to that measured in LD of MBZ, NCS, XL, VT and SAB goats (1.70-1.86%). The SM muscles of MBZ had a significantly higher thawing loss (9.0%) than that of NCS, SAB, VTV, VT and XL that ranged between (6.0-7.2%). Calpain enzymes worked optimally, thereafter causing tenderness and shorter MFL after 4 days post-mortem. Lightness in meat differed significantly at 1 day post-mortem but meat colour of all goat ecotypes caught up at 4 days post-mortem. Oxymyoglobin increased post-mortem giving meat a cherry red colour (favoured by consumers), with only marginal changes in metmyoglobin which eliminated the chances of meat becoming too brown (consumers discriminate). Muscle fibre characteristics in the goat ecotypes were associated with higher percentage of red fibres. Delayed chilling should be considered to help reduce the risk of meat being cold shortened as well as improving the calpain activity, which will ensure the best carcass and meat quality.