Rigor Mortis

To know how muscle turns into meat, we must first understand what is rigor mortis.

When an animal dies, aerobic glycolysis stops since no oxygen is available. As the muscle attempts to maintain homeostasis, muscle glycogen is metabolised via anaerobic glycolysis, thus phosphorylating ADP to supply ATP.

Anaerobic glycolysis generates lactate that accumulates, lowering the intracellular pH, so that by 24 hours post mortem the pH has fallen to an ultimate pH of about 5.4–5.7.
Muscle is highly sensitive to both ATP and Ca2+, which are both involved in the contraction–relaxation process. Consequently, as ATP levels are reduced and Ca2+ levels rise, irreversible cross bridges form between myosin head and actin. This formation is associated with an increase in toughness.
It was also reported that meat with low ph is likely to be poorer eating quality as the enzymes involved in post mortem tenderization are inhibited by the acidification. (Maltin et al, 2003)

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