Turning muscle into meat – the key factors influencing quality

Muscle tissue is composed of bundles of elongated cells, called muscle fibres that are densely packed and responsible for the contraction of the muscle in the live animal and for a short time after slaughter. In the living animal, a chemical compound called adenosine triphosphate (ATP) is produced with and without oxygen within muscle cells and acts as a source of energy.

Changes in the muscle after slaughter

a. Rigor mortis

Skeletal muscle remains ‘alive’ after the slaughter of the animal until the process known as rigor mortis is complete. Following slaughter and exsanguination, the synthesis of ATP in the muscle initially happens and ultimately the ATP, reduced as glycogen (the tissue’s energy store), is depleted. Freshly slaughtered ‘pre-rigor’ meat is tender and pliable, but becomes stiff and tough at the onset of rigor mortis, and then increases in tenderness again as rigor mortis passes and the ageing process begins. The rate at which rigor occurs and is completed is dependent upon a number of factors, such as species, muscle fibre type, temperature and slaughter procedure.

b. Post-mortem acidification

pH is a value used to represent the acidity or alkalinity of muscle and is recorded on a scale of 0 to 14, with values less than 7 referred to as acidic and greater than 7 as alkaline. The pH of muscle in the live animal is approximately 7.3, but this declines rapidly following slaughter to about 5.5 in normal meat, following rigor. This meat acidification is the result of an accumulation of lactic acid. It is generally accepted that a higher ‘ultimate pH’ (that is, the pH measurement taken after rigor mortis) is associated with increased tenderness and juiciness.

Stress induced ‘DFD’ meat

Stress induced by poor pre-slaughter handling can cause a limited pH fall, resulting in what is called DFD (dark, firm and dry) meat. DFD is a muscle defect seen in beef carcases but rarely in lamb. It is usually associated with long term stress or prolonged feed withdrawal. An indication of DFD in a carcase is a muscle pH above 6. In addition to its apparent abnormal colour, DFD meat has reduced keeping qualities and is prone to bacterial spoilage.

Quality Meat Scotland’s scheme ensures that both the animals and the meat are conscientiously handled to minimise adverse effects and stress

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