By Harold McGee
For a brief period after the animal’s death its muscles are relaxed, and if immediately cut and cooked will make especially tender meat. Soon, however, the muscles clench in the condition called rigor mortis (“stiffness of death”). If cooked in this state, they make very tough meat. Rigor sets in (after about 2.5 hours in the steer, 1 hour or less in lamb, pork, and chicken) when the muscle fibers run out of energy, their control systems fail and trigger a contracting movement of the protein filaments, and the filaments lock in place. Carcasses are hung up in such a way that most of their muscles are stretched by gravity, so that the protein filaments can’t contract and overlap very much; otherwise the filaments bunch up and bond very tightly and the meat becomes exceptionally tough. Eventually, protein-digesting enzymes within the muscle fibers begin to eat away the framework that holds the actin and myosin filaments in place. The filaments are still locked together, and the muscles cannot be stretched, but the overall muscle structure weakens, and the meat texture softens. This is the beginning of the aging process. It becomes noticeable after about a day in beef, after several hours in pork and chicken.