Facts of LifeWhy does meat still move after the animal is killed? [Archives:2005/823/Health]

March 10 2005
Anatomy of skeletal muscle. Source: Kimballs Biology Pages
Anatomy of skeletal muscle. Source: Kimballs Biology Pages
Lateral view of the carcass. Source FAO
Lateral view of the carcass. Source FAO
By Dr. Khaled M. Nsour
[email protected]

Have you ever thought why does meat still move even after the slaughtering of the animal? Or why do some meat look watery while others seem to be stiff and dry?

These questions and more are answered in the following article

What is meat?

Meat is mostly the muscle tissue of an animal. Animal's muscle is mostly consistent of 75% water, 20% protein, and 5% fat and carbohydrates. Muscles are made of bundles of cells called fibers. And each cell is crammed with filaments made of two proteins: actin and myosin.

In a live animal, these protein filaments make muscles contract and relax, through sliding action between muscles (see fig 1). This requires enormous amounts of energy, which muscles get from the energy-carrying molecule called ATP (adenosine triphosphate). The most efficient way to generate ATP requires oxygen, which muscles get from circulating blood. And this is why our breathing increases while exercising or exerting ourselves.

How does muscle turn into meat?

After an animal is slaughtered, blood circulation stops, and consequently muscles exhaust their oxygen supply. Muscle can no longer use oxygen to generate ATP and alternatively turn to another method called anaerobic glycolysis, a process that breaks down sugar without oxygen, to generate ATP from glycogen (sugar stored in muscle).

The breakdown of glycogen produces just enough energy to contract the muscles for a short while until the glycogen is exhausted from the muscle. This process produces lactic acid and with no blood flow to carry the lactic acid away, the acid builds up in the muscle tissues. If the acid content is too high (which is dependent on the amount of glycogen in the tissue) the meat loses its water-binding ability and becomes pale and watery. This happens because the acid drives away the water out of the cells to the surface of the meat. Whereas if the acid is too low, the water remains within the muscle and the meat appears stiff and dry.

Lactic acid buildup releases calcium, which causes muscle contraction. As glycogen supplies are depleted, ATP regeneration stops, and the actin and myosin remain locked in a permanent contraction called rigor mortis (see fig2&3). Freezing the carcass too soon after death keeps the proteins all bunched together, resulting in very tough meat. However, if the carcass is not frozen the enzymes in the muscle cells break down the overlapping proteins and make the meat tender after a period of time.