How Do We Test For Specific Nutrients in Our Food

Biology Topics encompass a wide range of subjects, from cell biology to ecology.

Tests for Major Nutrients like Protein, Carbohydrates and Fat

The most common carbohydrate in our food is starch. So, we will give test to detect the presence of ‘starch carbohydrate’ in a given food item. This is described below.

Test for the Presence of Starch in Food

The presence of starch in a food item can be tested by using a dilute iodine solution. This test is based on the fact that iodine produces a blue-black colour on combination with starch. So, a food item which gives a blue-black colour on adding a few drops of dilute iodine solution will contain starch. On the other hand, a food item which does not produce a blue-black colour on adding dilute iodine solution, will not contain starch (The solution remains brown in this case).

The test for the presence of starch in a food item can be performed as follows :

1. Take a small quantity of the food to be tested.
2. Add 2 or 3 drops of dilute iodine solution to the food with a dropper.
3. If a blue-black colour is produced, then starch is present in the given food.

The test for starch will become clear from the following activities. In these activities, we will be using ‘potato’ and ‘flour’ as the food items for testing starch.

Activity 1 : To Test The Presence Of Starch In Potato
Cut a potato into two pieces with a knife. Add 2 or 3 drops of dilute iodine solution on the freshly cut surface of the potato piece with the help of a dropper. A blue-black colour appears on the cut surface of potato (see Figure). The appearance of blue-black colour shows that potato contains starch (which is a carbohydrate).

Activity 2 : To Test the Presence of Starch in Flour
Take a small quantity of flour (atta) in a test-tube and shake it with a little of water [see Figure (a)]. Add 2 or 3 drops of dilute iodine solution to the mixture of flour and water. A blue-black colour appears in the test-tube [see Figure (6)]. This shows that starch is present in flour. We can repeat this test with a piece of bread.

If iodine solution is added to a piece of boiled ‘egg-white’, no blue-black colour is produced. This shows that the ‘egg-white’ (white portion of boiled egg) does not contain starch. Actually, egg-white is made up mainly of proteins and hence does not show the test for starch.

Before we go further and describe the test to detect the presence of fat (or oil) in a food, we should know the meaning of the word ‘translucent’. A translucent object is one which allows light to pass through it partially. A translucent object is neither fully transparent nor fully opaque. It is semi-transparent. The greasy patch (chikna dhaba) formed by rubbing butter, ghee or oil on paper is translucent. Keeping this is mind, we will now give the test for fats in food. Please note that the term ‘fat’ which we are using here also includes ‘oil’.

2. Test For Fat In Food

The presence of fat in a given food item can be tested on the basis of the fact that fats produce a greasy patch when rubbed on a clean sheet of paper. The greasy patch produced by a fat on paper is ‘translucent’ which lets some light to pass through it when the paper is held towards a source of light (such as an electric bulb or sunshine). Since some light passes through it, a greasy patch appears to be ‘bright’ as compared to the rest of the paper. Though we can use a white sheet of paper for testing fat in food, but greasy patch appears more prominent on a brown paper. So, we will be using a brown paper for testing fats.

Test for the Presence of Fat in Food

We can test the presence of fat in a given food as follows :

1. Rub a small quantity of the given food in the centre of a brown paper.
2. Hold this paper in front of a source of light and look through it.
3. If the food has left a bright greasy patch (or translucent patch) on paper, then fat is present in the food.

The food item being tested may sometimes contain a little water which may produce a wet patch on paper. So, after rubbing the food item on paper, the paper should be allowed to dry for a while. If the greasy patch remains even when the paper is dried, then the food item contains fat. If no greasy patch remains on paper after drying, then the food item does not contain any fat. We will now test the presence of fat in butter.

Activity 3 : To Test The Presence Of Fat In Butter

Rub a little of butter in the centre of a brown paper [see Figure (a)], Hold this paper towards a source

of light (like a lighted bulb or sunshine). A bright, greasy patch (or translucent patch) is seen in the centre of the paper [see Figure b]. The formation of greasy translucent patch on paper shows the presence of fat in butter. We can also perform this experiment by rubbing a few drops of groundnut oil (or any other oil) in the centre of a paper.

Butter is a soft food which can be easily rubbed on a piece of paper. Some foods containing fats (like groundnuts) are quite hard and difficult to rub on paper as such. We can test the presence of fat in a hard solid food as follows :

1. Take a small quantity of the hard, solid food item and place it on a brown paper lying on a table.
2. Crush the food item by using a small hammer carefully and rub the crushed food on the paper.
3. Remove the crushed food from the paper, and hold the paper towards a source of light.
4. If the food has left a number of bright, translucent spots on paper, then fat is present in the food. Groundnut kernels are a hard solid food. We can test the presence of fat (or oil) in groundnut kernels as follows.

Activity 4 : To Test The Presence Of Fat In Groundnuts

Take some groundnut kernels and place them on a brown paper spread on a table. Crush the groundnut kernels carefully by using a small hammer. Remove the crushed groundnut kernels from the paper and hold the paper towards light. A number of bright, greasy spots (or translucent spots) are seen on the paper. The formation of greasy translucent spots on paper shows the presence of fat (or oil) in groundnut kernels.

3. Test For Proteins In Food

The presence of proteins in a food material is tested on the basis of the fact that proteins give a violet colour with an alkaline solution of copper sulphate. Two solutions are needed for testing proteins : copper sulphate solution and sodium hydroxide solution. Copper sulphate solution is made alkaline by mixing sodium hydroxide solution. Please note that sodium hydroxide solution is also known as caustic soda solution.

In order to test proteins, the food should be in liquid form or in the form of a suspension. So, if the food to be tested is in solid form, we should first prepare its suspension in water as follows : Take a small quantity of solid food and mash it with a pestle and mortar. Put the mashed solid food in a test-tube and add a little water to make a suspension. This suspension of the food can then be used for testing proteins.

The presence of proteins in a given food-stuff can be tested as follows :

1. Take 2 mL of the given food-stuff solution (or suspension) in a test-tube.
2. Add a little of dilute sodium hydroxide solution till the mixture clears.
3. Then add 2 or 3 drops of copper sulphate solution and shake the test-tube.
4. If a violet colour appears in the solution, then protein is present in the given food-stuff.

If, however, the solution remains blue (which is the colour of copper sulphate solution), then protein is not present in the given food-stuff. Milk is a common food item. We will now test milk to find out whether it contains proteins or not.

Activity 5 : To Test The Presence Of Proteins In Milk
The presence of proteins in milk can be tested as follows :

1. Take 2 mL of milk in a test-tube [see Figure (a)],
2. Add a little of dilute sodium hydroxide solution [see Figure (b)].
3. Then add 2 or 3 drops of copper sulphate solution and shake the test-tube.
4. A violet colour appears in the solution. This shows that milk contains proteins [see Figure (c)].
   

If we perform this experiment by using ‘egg-white’ (the white portion of a boiled egg) as food, then we will find that egg-white also contains proteins.