60% ~ 70% of the daily heat energy required by human body comes from cereals, and 50% ~ 70% of the required protein is provided by cereals and their products. The main cereals in China are wheat and rice, in addition to corn, millet and sorghum.
Nutritional components of cereals
Because there are many kinds of cereals, their nutritional components are not exactly the same. Even the same species, the same variety, will also be due to geographical, climatic conditions, soil conditions, fertilization, farming methods, processing methods, the content of nutrients is not the same.
Nutritional characteristics of cereals.
1, rich in carbohydrates
The content of carbohydrate in grain is above 70%, and the main form of carbohydrate is starch. Starch expands, splits and gelatinizes in water due to heating during cooking, which is convenient for human digestion and absorption. It is the most ideal and economical source of heat energy for human beings.
2. The biological value of protein is low.
The content of cereal protein is generally between 7%- 16%, and the proportion of amino acids in its protein is far from that in ideal protein. Generally speaking, lysine is deficient and leucine is often excessive, which leads to the imbalance of amino acids in protein, which is the main reason for the low nutritional value of cereal protein.
Except lysine deficiency in protein, threonine deficiency in protein and tryptophan deficiency in protein, their biological values are relatively low, with only 52 wheat flour and 60 corn. Protein amino acids in rice are relatively rich in other essential amino acids except lysine, isoleucine and threonine. Therefore, the nutritional value of rice protein is higher than that of wheat, and its biological value is 77, which is not only the second in the main cereals, but also comparable to many animal proteins, such as fish, shrimp and meat. Although the biological value of wheat protein is lower than that of rice, its crude protein content is about 10% because of its high protein content. The content of corn flour is about 8.5%; Rice is only about 8%, so wheat flour can provide more protein for human body than other grains, and the advantage in quantity is enough to make up for the shortage in quality.
3. The preservation rate and absorption rate of inorganic salts are low.
All grains contain a certain amount of inorganic salts, which is 1.5% ~ 3%. Rice loses 70% of inorganic salts after washing before cooking. Protein content in rice is relatively low, the ratio of calcium to phosphorus is small, and there are no nutrients such as vitamin D that can help the human body absorb calcium, so the absorption and utilization rate of calcium by the human body is low. The content of iron and calcium in wheat is slightly higher than that in rice, and in the process of processing wheat flour into food, it does not need to be washed like rice, and the heating time is shorter.
So that that preservation rate of inorganic salt is high.
Generally, cereals contain phytic acid, which can combine with inorganic salts such as iron, calcium, zinc and other essential elements to generate phytic acid, so the absorption and utilization rate of inorganic salts in cereals is very low. However, because wheat flour is often steamed into steamed bread or baked into bread for human consumption after fermentation, most phytic acid in wheat flour is hydrolyzed and eliminated during fermentation. Because wheat flour is rich in protein, it is hydrolyzed into amino acids when digested, which can form soluble salts easily absorbed by human body with inorganic salts such as calcium, which is beneficial to human body's absorption and utilization. According to the determination, the absorption rate of iron in wheat flour is twice that of corn and five times that of rice.
4. The preservation rate and absorption rate of vitamins are low.
Cereals are an important source of B vitamins in the diet, especially thiamine and nicotinic acid. Generally, vitamins C, D and A are not included, and only yellow corn and wheat contain a small amount of carotene.
Before cooking, rice will lose 29% ~ 60% thiamine and 23% ~ 25% riboflavin. The whiter the rice, the more times it is washed, the higher the water temperature and the longer the soaking time, the more serious the vitamin loss will be. Therefore, in the areas where rice is the staple food in southern China, if we eat rice with high processing precision for a long time and the steaming method is unreasonable, it will easily lead to the lack of B vitamins such as beriberi.
Nicotinic acid in corn mainly exists in the form of compound, and only exists after proper cooking and processing. If treated with alkali, it will become free nicotinic acid, which can be absorbed and utilized by the human body. If left untreated, people who live on corn are prone to niacin deficiency and boils.
5. Content and function of fat
Cereals generally contain a small amount of fat, which is 1% ~ 2%. Mainly concentrated in embryos and shells. Wheat and corn germ contain a lot of oil, and unsaturated fatty acids account for more than 80%, of which linoleic acid accounts for about 60%, which has the functions of lowering blood cholesterol and preventing atherosclerosis.
Although the fat content in grain is very low, it plays an important role in making its products produce unique aroma after steaming. However, during the long-term storage of grain, due to the role of oxygen in the air, fat will be oxidized and rancid, which will make the aroma of grain disappear or decrease, resulting in unpleasant taste change. Therefore, the oxidation of fat is one of the important reasons of grain aging.