LPT 321: Meat Science (1+1)

CONVERSION OF MUSCLE TO MEAT

• Meat is the post-rigor aspect of muscle and the conversion of 'Muscle' to 'Meat' is the result of series of biochemical biophysical changes initiated in muscle at the death of the animal due to stoppage of the blood circulation.
• The most immediate change caused due to bleeding is the cessation of the oxygen supply to the muscle resulting in inhibition of the cytochrome system & therefore, oxidative phosphorylation.
• The sarcoplasmic ATPase depletes the ATP levels that increase the inorganic phosphate, which in turn stimulates the breakdown of glycogen to lactic acid.
• The ineffectual resynthesis of ATP by anaerobic glycolysis cannot maintain the ATP level and as it drops, actomyosin forms and the inextensibility of rigor mortis ensue.
• The loss of extensibility, which reflects actomyosin formation proceeds slowly at first (the delay period) then with great rapidity (the fast phase), extensibility then remains constant at low levels.
• The time to the onset of the fast phase of rigor  at a given temperature depends most directly on the level of ATP.
• Synthesis of ATP from creatine phosphate and ADP or by glycolysis cannot maintain the ATP level for too long.
• Higher glycogen content of the muscle can prolong that time to some extent but not indefinitely.
• The onset of rigor is accompanied by a lowering in water holding capacity (WHC).
• This is not solely due to drop in pH and the consequent approach of the muscle proteins to their isoelectric point or due to denaturation of sarcoplasmic proteins.
• It has been shown that even when rigor mortis occurred at high pH, there was a loss of water holding due to the disappearance of ATP and to the consequent formation of actomyosin.
• The lowered ATP levels make it difficult to maintain the structural integrity of proteins.
• The lowering in pH, due to accumulation of lactic acid also makes liable to denature.
• Denaturation is accompanied by loss of power to bind water and the fall in pH causes the myofibrillar proteins approach their isoelectric point.
• Both events cause exudation.
• Denaturation of sarcoplasmic proteins also makes them liable to attack by cathepsin.
• The breakdown of proteins to peptide and amino acids and the accumulation of various metabolites from glycolytic and other processes afford a rich medium for bacteria.
• Resolution of rigor is a term used glibly to denote the decrease in tension generated in the muscle, during actinomyosin formation and muscle contraction.
• The  resolution of rigor, is not on account breakdown of actinomyosin, but takes place due to decreasing tension, which is due to proteolytic degradation of specific myofibrillar proteins that lead to dissolution of Z discs, loss of ultrastructral integrity .
• Muscle, post the resolution of rigor is referred to as meat.