Skeletal muscle contraction is a process that requires energy. In order to complete the mechanical work of contraction, actin and myosin use the chemical energy of the molecule adenosine triphosphate (ATP). ATP is synthesized in muscle cells from stored polysaccharide glycogen, a complex carbohydrate composed of hundreds of covalently linked glucose molecules (monosaccharides or simple carbohydrates). In working muscle, glucose is released from glycogen reserves and enters a metabolic pathway called glycolysis. In this process, glucose is decomposed and the energy contained in its chemical bond is used to synthesize ATP. The net production of ATP depends on the level of oxygen reaching the muscle. Under anaerobic conditions (anaerobic conditions), the glycolytic products are converted into lactic acid and produce relatively less ATP. Under aerobic conditions (aerobic conditions), glycolytic products enter the second pathway, namely citric acid cycle, and a large amount of ATP is synthesized through oxidative phosphorylation.
In addition to carbohydrates, fat also provides a lot of energy for muscles. Fat is stored in the body in the form of triglycerides (also known as triglycerides). Triglycerides are composed of three fatty acid molecules (nonpolar hydrocarbon chain with polar carboxyl group at one end) combined with one glycerol molecule. If energy production requires fat deposition, fatty acids will be released from triglyceride molecules, a process called fatty acid mobilization. Fatty acids are broken down into smaller molecules, which can enter the citric acid cycle and synthesize ATP through oxidative phosphorylation. Therefore, using fat to obtain energy requires oxygen.
An important protein of muscle cells is oxygen binding protein myoglobin. Myoglobin absorbs oxygen from the blood (transported by the associated oxygen binding protein hemoglobin) and stores it in muscle cells for oxidative metabolism. The structure of myoglobin includes a non protein group called heme ring. The heme ring consists of a porphyrin molecule bound to an iron atom. Iron atoms are responsible for the binding of oxygen to myoglobin, and there are two possible oxidation states: reduced ferrous form (Fe2 +) and oxidized ferrous form (Fe3 +). In the Fe2 + state, iron can bind to oxygen (and other molecules). However, the oxidation of iron atoms to Fe3 + prevents oxygen binding.
Postmortem muscle
Once the animal's life is over, the process of maintaining life will slowly stop, resulting in significant changes in postmortem (postmortem) muscles. These changes represent the transformation from muscle to meat.
PH change
Usually, after death, the muscle becomes more acidic (pH decreases). When an animal loses blood after slaughter (a process called blood loss), muscle cells no longer have oxygen, and anaerobic glycolysis becomes the only available mode of energy production. Therefore, glycogen storage is completely converted to lactic acid, and then lactic acid begins to accumulate, resulting in a decrease in pH. In general, the pH of living muscle decreases from about 7.2 in physiological pH to about 5.5 in postmortem meat (called final pH).
Protein change
When the energy reserve is exhausted, myofibrillin, actin and myosin lose stretch and the muscle becomes stiff. This condition is often called rigor mortis. The time required for an animal to enter rigor mortis depends largely on the species (for example, cattle and sheep take longer than pigs), the rate at which the body cools at normal body temperature (the process is slower at lower temperatures), and the pressure experienced by the animal before slaughter.
Finally, the hardness of muscle tissue begins to decrease due to the enzymatic decomposition of structural proteins (i.e. collagen) that hold muscle fibers together. This phenomenon is known as the regression of stiffness, which can last for several weeks after slaughter. This process is called meat aging. This aging effect will make the meat more tender and delicious.
Characteristics of meat
Chemical and nutritional composition
Regardless of the animal, lean muscle usually consists of about 21% protein, 73% water, 5% fat and 1% ash (the mineral composition of muscle). These numbers vary with the feeding and fattening of animals. In general, as fat increases, the percentage of protein and water decreases. The table compares the nutritional composition of many meat products.
