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Optimising the concentration of glycogen in lamb meat

Jacob, Robin Henry (2003) Optimising the concentration of glycogen in lamb meat. PhD thesis, Murdoch University.

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The lamb industry is actively seeking to improve the quality of lamb meat produced in Australia. Ultimate pH (pHu) is a key determinant of red meat eating quality although this measurement has not been adopted formally by the Australian lamb meat industry. Muscle glycogen concentration is a major determinant of pHu in red meat. This thesis investigates glycogen concentration in lamb muscle and the ultimate pH (pHu) of lamb meat under commercial industry conditions as well as exploring by experimentation, some of the factors that control muscle glycogen concentration in lamb muscle. The results of this work has contributed to an understanding of the significance of high pHu meat to the lamb industry and will assist with developing new management strategies for lambs that avoid low muscle glycogen concentration at the point of slaughter, thus high pHu in meat derived from lambs.

The first part of the study (Experiments 1 and 2) undertook to determine the ranges of muscle glycogen concentration and lamb meat pHu found under commercial conditions and to measure any changes in these parameters associated with consignment of lambs from farm to abattoir and lairage at abattoirs. This study utilised a new biopsy technique that allowed muscle collection from lambs on farm. Some 16 different consignments of lambs and 3 consignments of lactating ewes were intensively monitored on farm and at abattoirs over a range of lairage times. Sensory evaluation tests were done using meat from 6 of these consignments.

The results showed there to be considerable variation between lamb consignments with some consignments having a very high and other consignments having a very low incidence of meat with a high pHu. On balance 'on farm' factors were concluded to have a greater impact on muscle glycogen concentration at slaughter than 'post farm gate' factors. However, there was evidence that muscle glycogen concentrations decreased during the farm curfew and transport period for some consignments so both 'on farm' and 'post farm gate factors' can be important. Characteristically glycogen loss occurred during the farm curfew and transport period in consignments of Merino lambs that had high muscle glycogen concentrations prior to consignment. Holding lambs in lairage caused no negative effects on muscle glycogen concentration although there was some evidence that very short lairage periods may increase meat pHu without causing a change in muscle glycogen concentration. It was concluded from these experiments that the mean muscle glycogen concentration of a group of lambs needs to be greater than 1.5 g/100g on farm in order for the pHu of lamb meat to be less than 5.7.

Subsequent to this industry study, an experiment (Experiment 3) was done to gain an understanding of muscle glycogen concentration as being an integral part of whole body glucose metabolism. This experiment investigated the effects of exercise on a range of different muscles and tissues of lambs including liver, kidney, skin and gastrointestinal tract. Interactions between glycogen concentrations in the liver and muscle with time after exercise showed that glycogen repletion occurred in the liver before muscle tissue. This effect was a unique finding and could explain in part the slow rate of glycogen repletion in muscle tissue that is characteristic for ruminants. Another major finding was an accumulation of glycogen concentration in skin during the recovery period after exercise. It was postulated that this effect may be due to the supply of glucose to glycolytic tissues being continued even when demand for glucose in the skin was low and the capacity to store glycogen in muscle was very high.

Experiment 3 confirmed the existence of a relationship between metabolisable energy (ME) intake and glycogen repletion in muscle tissues and found a slightly different relationship between ME intake and glycogen repletion in the liver tissue of lambs. Muscle glycogen concentration did not change in fasted lambs and the rate of glycogen repletion in muscle after exercise was dependent on ME intake. Differences were observed between different muscles, particularly between M. longissimus thoracis et lumborum (LTL) and all other muscles, in relation to the change in glycogen concentration with time after exercise. Glycogen concentrations changed less rapidly in the LTL than other muscles. Glycogen concentration in the liver was associated negatively with time after exercise in fasted lambs and positively with time after exercise in fed lambs.

Several experiments (Experiments 4, 5 and 6) were conducted to determine the affects of different nutritional factors on muscle glycogen concentration in lambs, both on farm and after commercial slaughter. These studies showed that short term increases in ME intake will increase muscle glycogen concentration to a maximum level over a period of about 7 days (Experiment 4). Diet composition did not affect the change in muscle glycogen concentration associated with an increase in ME intake although results from this experiment (Experiment 5) were not entirely conclusive. There was evidence that the type of feeding and finishing system may influence the susceptibility of muscle glycogen concentration to change during consignment of lambs to slaughter. Results from these experiments demonstrated that a goal for muscle glycogen concentration in lambs on farm of 1.5g/100g is quite achievable with contemporaneous management systems.

Finally this study highlighted the need for further research in a number of key areas in order that muscle glycogen concentration in lambs to be fully understood. In particular, the role of muscle glycogen turnover in relation to muscle glycogen concentration was noted as an area for which further research is warranted.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Veterinary and Biomedical Sciences
Supervisor(s): Pethick, David
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