Meat is a general term used to describe the edible parts of animal tissues and any processed or manufactured products prepared from these tissues. Meat is usually classified according to the type of food animal. Red meat refers to meat from mammals, white meat refers to meat from poultry, seafood refers to meat from fish and shellfish, and game refers to meat from normally untrained animals. In addition, the most commonly eaten meat is specifically determined by the living animals they come from. Beef refers to the meat of cattle, veal of calves, pork of pigs, lamb of lambs and mutton of sheep over two years old. This section focuses on the latter kind of red meat.
Turn muscle into meat
Muscle is the main component of most meat and meat products. Other components include connective tissue, fat (adipose tissue), nerves and blood vessels, which surround and embed muscles. Therefore, the structure and biochemical characteristics of muscle are the key factors affecting the way animals are treated before, during and after slaughter, and the quality of meat produced during slaughter.
Muscle structure and function
Animals have three different types of muscles: smooth muscle, myocardium and skeletal muscle. Smooth muscle exists in the organ system including digestive tract and reproductive tract, and is often used as the casing of sausage. Myocardium is located in the heart and is often eaten as meat products. However, most meat and meat products come from skeletal muscle, which is usually attached to the bone. In living animals, skeletal muscle helps to exercise and support body weight. Skeletal muscle is the focus of the following discussion.
Skeletal muscle structure
Skeletal muscle is separated by a layer of connective tissue called the adventitia. Individual muscles are divided into different parts (called muscle bundles) by another connective tissue sheath called perimuscle. Clusters of adipocytes, small blood vessels (capillaries) and nerve branches can be found in the area between muscle bundles. The muscle bundle is further divided into smaller cylindrical muscle fibers (cells) of different lengths, which are individually wrapped in a thin connective tissue sheath called the myointima. Every connective tissue sheath in skeletal muscle is composed of collagen, a structural protein that provides strength and support for muscle.
The plasma membrane of muscle cells is called myomembrane, which separates the myoplasm (myocyte cytoplasm) from the extracellular environment. There are about 1000 to 2000 myofibrils in the sarcoplasma of each muscle fiber. Myofibril is composed of actin and myosin. It is the smallest contraction unit in living muscle.
Skeletal muscle contraction
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 he).