The hardening or stiffening of muscle in animals shortly after slaughtering is called rigor mortis. Though the stiffening begins throughout the body at the same time, the rigor is first seen in the smaller muscle of the body followed by the bigger muscles, depending upon the animal, its age and its size, rigor mortis can take from 0-12 hours to completely occur. For beef it takes 6- 12 hours, for pork it takes 0-6 hours and for lamb or mutton it takes 6-10 hours.
Science behind Rigor Mortis
When the animal is slaughtered, the body muscle of the animal loses Adenosine Triphosphate (ATP), it is a substance which is responsible for the flow of energy in the muscle which helps in muscle contraction during life. This is followed by accumulation of lactic acid and fall of pH value, which increases the acidity level of the muscle and this leads to the stiffness of the muscle which is called as rigor mortis. This much is the only science, we are concerned here and no point going in deep
Effect of Temperature on Rigor Mortis
Temperature is an important factor affecting rigor mortis, at normal or at room temperature, complete rigor in beef will occur after 8-12 hours but if the temperate is higher, then the rigor will take less time and if the temperature is lower, then the rigor will take more time to occur.
Effect of physical activity or stress on Rigor Mortis before slaughtering of the animal
The rigor mortis occur quickly in the stressed animal which has gone through physical activity such as fight, running or long grazing before the death affecting the quality of the meat. Whereas as the animal which is well rested and well fed before slaughtering produces fine quality meat because the rigor is slow in the muscle. Other factor which accelerates the rate rigor mortis is hot environment, Electrocution or Shocking, Convulsion, high body temperature of the animal due to fever or medicine. Factors which decreases the rate of rigor mortis are Numbing the brain (as in Ike Jime in Fish), Cold Environment, Cold water shocking or freezing before rigor or Hypothermia.
Timeline and Stages of Rigor Mortis
There are four stages of rigor mortis –
At first stage (0-8 hrs.), the muscle is still relaxed and flexible due to its storage of ATP which is yet to be lost. This is called as absent stage or delay phase.
At second stage (8-12 hrs.), the stored ATP is all used up and the muscle starts to loose its flexibility but it’s just the beginning. This is called as present stage or onset phase.
At third stage (12-24 hrs.), all the muscle energy is depleted and there is no way to regenerate it, at this stage full rigor mortis sets in the muscle making it completely stiff. This is called as complete stage.
At fourth stage (24-36 hrs.), the stiffness dissipates and the muscle loses its stiffness and is ready for fabrication. This is the final stage called as passed stage or resolved stage.
Effect of Rigor Mortis on Cooking
If the meat is frozen before the complete rigor mortis happened, then the meat becomes extremely tough when cooked and takes longer time, this is called as “Cold Shortening”. The toughness caused by this cannot be corrected and this affects the tenderness and quality of the meat. Cold shortening only takes place if the meat is frozen immediately below 10°C not if the temperature decreases gradually but not below 10°C before the onset stage of rigor mortis. Once the rigor is achieved at the ambient temperature, then the meat is rapidly frozen, this process is called as “Conditioning”.
If the meat is cooked when the muscle are still in rigor, then it becomes extremely tough and takes longer time to cook and hence loses some of its nutrients.
Aging or Ripening
This is actually the fourth stage of rigor mortis also called as resolved stage. The aging of the meat is achieved by storing the meat at ambient temperature until the muscle gradually recovers from rigor mortis due to its enzymatic breakdown. The meat used after aging is tenderer leading to less cooking time and improvement in flavor.