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| Classification of food |
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| On the basis of nutrients food can be classified into seven different groups: |
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 Carbohydrates |
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 Proteins |
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 Fats |
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 Vitamins |
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 Minerals |
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 Roughage |
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 Water |
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| Table 3. Composition of some common foods |
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| Carbohydrates are compounds of carbon, hydrogen and oxygen. According to their chemical structure they may be simple or complex. Sugar and starch are complex where as glucose is simple. |
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| In the process of digestion, cooked starch, by the action of enzymes, breaks down into glucose. Glucose is very easily assimilated by the intestine and sent to every part of the body. On reaching the cells, glucose gets oxidized to release energy. One gram of carbohydrate yields 4.2 calories of energy. Hence the function of carbohydrates is to supply energy to the body for work and other physiological activities. |
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| Example: Rice, wheat, maize, bajra, ragi, potato, sweet potato, tapioca, banana, sugarcane etc. |
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| Test for the presence of carbohydrates. |
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| Starch: Place a slice of potato, a few grains of cooked rice and a small piece of boiled egg in a petri dish. Add a few drops of iodine solution to each of them. You will notice that the potato and cooked rice turn blue black, proving the presence of starch where as the boiled egg does not turn blue black, proving the absence of starch. |
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| Sugar: Take small pieces of ripe banana, potato and lemon. Crush them separately, strain their juice and put 10 drops of their juices into different test tubes. Add a few drops of Benedict's solution to the juices and heat them. You will notice that the test tube containing the ripe banana juice will show the formation of a red precipitate. This proves that ripe banana contains sugar. The test tubes containing the crushed potato and lemon do not form a red precipitate. This proves that sugar is absent in these food items. |
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| Proteins are complex compounds made up of chains of nitrogen containing building blocks called amino acids. Besides nitrogen they also contain carbon, hydrogen and oxygen. The proteins present in living organisms contain more than 20 amino acids and each of them have a specific function. Table 4, has a list of some of the important types of proteins and their functions. |
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| Table 4. Functions of some proteins |
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| Proteins, being highly complex compounds, cannot be used by our body in this form. By the action of digestive enzymes proteins are broken down into a simpler form i.e. amino acids which are assimilated into the blood in the intestine. The blood carries these free amino acids to the various body cells where they are regrouped to form specific proteins such as skin, muscles blood and bones. Thus proteins are essential for body growth and functioning. Deficiency of protein causes retardation of physical and mental growth. The two PEM (Protein Energy Malnutrition) diseases are Kwashiorkar and Marasmus found in children. Food items which are rich in proteins are meat, eggs, fish, milk, cheese, pulses (dal), nuts etc. |
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| Test for the presence of proteins |
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| Put pieces of cheese, hard boiled egg white and apple in three separate test tubes. Add a few drops of dilute nitric acid to cover the food in each of the test tubes. Heat them slightly pour out the acid. You will notice that there is a yellow colouration on the egg white and cheese but not on the apple. This shows that the egg white and cheese are rich in protein. Now add ammonium hydroxide solution to the test tubes. The egg white and cheese will turn orange in colour confirming the presence of protein in them. |
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