Evaluation of the nutritional value of protein in food
Since the protein content and amino acid composition of various foods are different, their nutritional values are also different. It is very important to evaluate the nutritional value of protein in food. It can identify food quality, have important guiding significance for the research and development of new food resources, and guide people to eat reasonably. The nutritional value of food protein can be evaluated from three aspects: protein content, digestion and absorption rate, and human body utilization rate.
1. Protein content
Protein content is the basis of the nutritional value of food protein. If the protein content in food is too low, even if the energy intake exceeds the body's needs, it will not be able to meet the body's needs. The protein content in food is usually determined by the Kjeldahl method, that is, the nitrogen content in the food is first measured, and then multiplied by the conversion factor to obtain the protein content. Since the protein content in food is generally 16%, the conversion factor is 6.25 (100/16), then there is
Protein content (%) = Nitrogen content (%) × 6. 25
2. Protein digestibility
Protein digestibility refers to the degree to which food protein is broken down by the body's digestive enzymes. The higher the protein digestibility, the greater the possibility of being absorbed and utilized by the body, and the higher the nutritional value. The digestibility of protein is often expressed as the ratio of nitrogen absorbed by the body to nitrogen intake. Usually there are apparent digestibility (AD) and true digestibility (TD).
Protein apparent digestibility (%) = (food nitrogen - fecal nitrogen)/food nitrogen × 100
True digestibility of protein (%) = (food nitrogen - (fecal nitrogen - fecal metabolic nitrogen)) / food nitrogen) × 100 Among them, fecal metabolic nitrogen refers to the nitrogen content in the feces when the test subject does not consume protein at all. Adults Fecal metabolic nitrogen within 24 hours is generally 0.9 g to 1.2 g. Since the apparent digestibility measurement method is simple, and the measured value is lower than the true digestibility, which is used to estimate the nutritional value of protein on the low side and has a large safety factor, so "apparent digestibility" is often used to express the digestibility of protein.
Due to the different forms and structures of proteins in food, and the influence of other factors that are not conducive to protein absorption, etc., different foods or different processing methods of the same food have different digestibility of proteins. Because the protein of plant foods is wrapped in cellulose and has poor contact with digestive enzymes, its digestibility is lower than that of animal foods. The digestibility of common food proteins is shown in Table 3-3. However, the digestibility of plant foods can be improved by destroying or removing cellulose through processing and cooking. For example, when soybeans are eaten whole, the protein digestibility is only 65%, but after being processed into tofu, it increases to more than 90%.
Table 3-3 Digestibility of common foods according to protein
Food true digestibility Food true digestibility Food true digestibility
Egg 97±3 Rice 88±4 Soy flour 86±7
Milk 95 Soil 3 Flour 96 ± 4 Beans 78
Meat, fish 94±3 Oats 86±7 Peanut butter 88
Corn 85±6 Millet 79 Chinese mixed diet 96
3. Protein utilization
There are many indicators to measure protein utilization, such as biological price, net protein utilization, protein efficacy ratio, amino acid content, etc. Various indicators reflect the degree of protein utilization from different perspectives. The following mainly introduces the biological value of protein.
The biological value of protein is an indicator that reflects the extent to which food protein is utilized by the body after digestion and absorption. The higher the biological value, the higher its utilization by the body. The maximum value is 100. The biological values of common food proteins are shown in Tables 3 and 4. The calculation formula of biological price is biological price = nitrogen storage/nitrogen absorption × 100%
Table 3-4 Biological values of common food proteins
Food protein biological value Food protein biological value
Flour 52 Rice 77
Millet 57 Pork 74
Broad beans 58 Beef 76
Peanut 59 Shrimp 77
Corn 60 Egg white 83
Soybeans 64 Fish 83
Potato 67 Skim milk 85
Lentils 72 Whole eggs 94
Sweet potato 72 Egg yolk 96
In the complementary role of protein, when two or more food proteins are mixed and eaten, the biological value of the protein can be increased by complementing each other's strengths and weaknesses among the amino acids, thus increasing the nutritional value of the protein. Tables 3 to 5 compare and list the changes in the biological value of protein when different foods are mixed. There are many similar examples in daily life, such as hybrid noodles, rice and mung bean porridge, eight-treasure porridge, bean paste buns, tofu mixed with sesame sauce, etc. If a small amount of animal food is added to plant food, the biological value of protein will increase.
Excerpted from Nutrition and Food Hygiene, People's Medical Publishing House, Sixth Edition