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In this article, you will learn about the tests that are carried out in the laboratory to determine the presence of the biological molecules in the food sample. But before proceeding to discuss the tests, first, let us recall what are biological molecules and why they are needed.
What are biological molecules?
Biological molecules present in our food provide us with energy. These molecules have chemicals that play a vital role in our growth. These biological molecules are composed of long chains that are broken down into simpler ones during digestion. Without these molecules, our body cannot repair itself and our cells cannot function. The three types of biological molecules present in our diet which act as a source of energy are:
- Carbohydrates: Contain carbon, oxygen, and hydrogen atoms
- Lipids: They are composed of carbon, hydrogen, and oxygen atoms
- Proteins: They are made up of carbon, oxygen, hydrogen, and nitrogen atoms
In the next sections of the article, we will discuss what kind of tests can be performed in the laboratories to determine the presence of three kinds of biological molecules.
Tests for Biological Molecules
A number of tests can be conducted in a lab to determine the presence of biological molecules (carbohydrates, lipids, or proteins) in the food sample. These tests are qualitative rather than quantitative. It means that these tests do not provide the quantitative value, i.e. the amount of biological molecule present in the sample.
Iodine Solution to Test for Starch
Introduction
Iodine test also referred to as a starch-iodine test, is a chemical test that is used to differentiate monosaccharides or disaccharides from certain polysaccharides such as dextrin, amylase, and glycogen. Hence, we can say that the primary objective of the iodine test is to:
- Detect the presence of polysaccharides, usually starch present in the food sample
Procedure
Follow the steps below to determine the presence of starch in a food sample:
- Add a few drops of orange or brown iodine in potassium iodide solution to the sample to determine if starch is present in the sample. (Note that we use potassium iodide solution for iodine because iodine is insoluble in water)
- If there is starch in the food sample, then the iodide ions in the solution react with the centre of starch molecules to produce a new complex that has a unique blue-black colour
- This test is quite helpful in experiments to show the presence of starch in the sample that has been digested by the enzymes
Benedict’s Test to Test for Reducing Sugars
Introduction
Benedict test refers to a biochemical test that is performed to differentiate reducing sugars such as monosaccharides and a few disaccharides from non-reducing sugars. There the two main objectives of this test:
- To identify the presence of simple carbohydrates in a solution
- To differentiate between reducing and non-reducing sugars
Procedure
Follow the steps below to conduct this test:
- Add Benedict's reagent which is also known as Benedict's qualitative solution or Benedict's solution to a sample solution in the test tube (Note that Benedict's reagent is a blue-coloured solution because it has copper (II) sulfate ions)
- Bring the water bath or beaker of the water to boil for some minutes and heat the test tube in it
- In the presence of reducing sugar, one can see a coloured precipitate because of the fact that copper (II) sulfate is reduced to copper (I) oxide that is water insoluble
- In case of the positive result, there is a colour change along the colour scale from blue (no reducing sugar) to brown or brick-red (a higher concentration of the reducing sugar)
Remember that this test is semi-quantitative because the degree of the colour change indicates the amount (concentration) of reducing sugar present in the sample. Moreover, sucrose and starch yield a negative result, whereas lactose and maltose yield a positive result under Benedict's test.
Biuret Test for Proteins
Introduction
The Biuret test, which is also referred to as Piotrowski's test, is a chemical test that is used to detect the presence of peptide bonds in the sample. This test can also be used to quantify the proteins that are already present in a solution or are easily soluble in dilute alkali.
The main objectives of this test are:
- To identify the presence of proteins or peptide bonds in a sample item
- To identify the concentration of proteins present in a sample
Procedure
Follow the steps below to test the presence of proteins in a food sample:
- Treat a lipid solution of the sample with potassium or sodium hydroxide to create an alkaline solution
- Add some drops of blue-coloured copper (II) sulfate solution to the sample. (Remember that Biuret's reagent contains copper (II) sulfate and an alkali)
- If one observes a change in colour from blue to purple/lilac, then it signifies that the protein is present in the sample. (It should be kept in mind that the colour variation can be very subtle. Therefore, one should hold the test tubes against a white tile while making the observation. This will help to notice the change in colour easily)
- If there is no protein in the food sample, then it is an indication that there is no protein in the sample
Remember that this test works only if there are at least two peptide bonds in any protein molecule. Hence, if the sample contains dipeptides or amino acids, then the result of this test will be negative.
Ethanol Emulsion Test for Fats and Oils
Introduction
An Emulsion test, which is also referred to as an Ethanol Emulsion test, is a general group test that is used to detect the lipids in a food sample. Hence, the primary objective of this test is to:
- Determine if the lipids are present in the food sample
Procedure
Follow the steps below to conduct this test:
- Add ethanol to the sample item, shake it so that it can mix properly, and then add the mixture to the test tube of water
- Remember that the lipids are non-polar molecules that cannot be dissolved in water, however, they will get dissolved in organic solvents such as ethanol.
- In the presence of the lipids, a milky emulsion will be formed, i.e. the solution will turn cloudy. If there are more lipids present in a sample, then the milky colour of the solution will be quite prominent.
- The solution will remain clear if no lipids are present in the sample.
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