Most protein also contains sulfur and phosphorus, and some protein also contains minerals such as iron, copper, manganese and zinc. The contents of four main chemical elements in protein are: carbon 15%~55%, hydrogen 67%, oxygen 2 1%~23.5%, nitrogen 15% ~ 15%. Nitrogen has become a symbol to measure the protein content in the body. Generally speaking, the average nitrogen content in protein is 16%, that is, every 6.25g of protein in human body contains nitrogen 1g, so as long as the nitrogen content in human body is determined, the content of protein can be calculated. The proportion of nitrogen in human body is so large that the demand for nitrogen by animals and plants is also very large. According to information, crops all over the world will be removed from the soil within one year.
From the nutritional value, protein can be divided into three categories:
complete protein
This protein contains all kinds of essential amino acids, and the proportion of various amino acids basically meets the needs of human body, so it is easy to be absorbed and utilized. Complete protein can not only maintain the health of adults, but also have a good health care effect on the growth and development of children and the longevity of the elderly, such as casein and whey protein in milk, ovalbumin and lecithin in eggs, albumin and myosin in meat and fish, and glycinin in soybeans and wheat grains.
Semi-complete protein
All kinds of essential amino acids contained in this kind of protein are basically complete, but the contents are different, and the proportion between them is not appropriate. If it is the only source of protein, it can sustain life, but its function of promoting growth and development is poor. For example, gliadin in wheat and barley belongs to this category.
Incomplete protein
This protein contains all kinds of essential amino acids, and its quality is poor. If it is the only source of dietary protein, it will not promote growth and development, and its effect on maintaining life is also very weak, such as zein in corn, collagen in animal connective tissue and skin, globulin in peas, etc.