What are Carbohydrates, Proteins, and Fats?
Carbohydrates are naturally occurring sugars, starches and fibre in food. These are the main sources of energy. Carbohydrates are subdivided into several categories on the basis of the number of sugar units and how the sugar units are chemically bonded to each other. Proteins are biomolecules made up of amino acids. They are essential for building msucle mass. Protein is commonly found in animal products, though it is also present in other sources, such as nuts and legumes.
Fats are a class of lipids containing two kinds of monomers, fatty acids and glycerol. It is a subgroup of lipids. Here we’ll discuss tests for carbohydrates, proteins and fats.
Tests for Carbohydrates, Proteins and Fats
Tests for carbohydrates, fats, and proteins confirm the presence of these molecules in the food. Let’s have a look at an experiment to test the carbohydrates’ presence in food:
Aim: To detect the presence of carbohydrates, fats and proteins that are present in the samples of the given food.
Theory: The presence of carbohydrates, fats and proteins in any foodstuff is detected by performing the tests with the extract of the foodstuff. The advantage is these tests do not interfere with each other.
Materials Required:
Molisch’s reagent
Fehling’s reagent
Benedict’s reagent
Tollen’s reagent
Iodine solution
Copper sulfate solution
Sodium hydroxide
Nitric acid
Mercuric sulfate
Sodium nitrite
Alcohol
Chloroform
Filter paper
Potassium bisulfate
Concentrated hydrochloric acid
Furfural solution
Test tubes
Test tube holder
Water bath
Dropper
Stirrer
Bunsen burner
Apparatus setup:
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Procedure:
The extract of the given foodstuff from the sample should be prepared in order to identify the presence of carbohydrates, proteins, fats and oils in the given sample. With the help of a pastel, dry the given foodstuff in the mortar, else, boil the sample with a little amount of water to take out the extract from the food sample. Thus, water will extract a small quantity of an organic solvent after grinding the foodstuff.
Some of the examples of preparing an extract of foodstuff are given below.
For potatoes – cut to slices and boil with water.
For butter – test directly
For grapes – Extract the juice.
For boiled egg – take the white portion grind and shake with water.
Food Test for Carbohydrates
Carbohydrates can be tested by using the following methods:
1. Tollen’s Test
Take the given food sample and prepare its extract. Add the Tollen’s reagent to the prepared food extract. A silver mirror formed on the wall of the test tube confirms the presence of carbohydrates in the foodstuff.
AgNO3 + NH4OH → NH4NO3 + AgOH
2AgOH → Ag2O + H2O
Ag2O + 2NH4OH → 2 Ag(NH3)2OH + 3H2O
2. Molisch’s Test
Take the given food sample and prepare its extract. Add the Molisch’s reagent to the prepared food extract. A purple-violet ring formed in the test tube confirms the presence of carbohydrates in the foodstuff.
3. Benedict’s Test
Take the given food sample and prepare its extract. Add Benedict’s reagent to the prepared food extract. A red precipitate formed in the test tube confirms the presence of carbohydrates in the foodstuff.
CuSO4 → Cu2+ + SO42-
Cu2+ + Reducing sugar → Cu+
Cu+ → Cu2O
4. Fehling’s Test
Take the given food sample and prepare its extract. Add Fehling’s A and Fehling’s B reagent to the prepared food extract. A red precipitate formed in the test tube confirms the presence of carbohydrates in the foodstuff.
5. Iodine Test
This test is used to identify starch. Starch is a type of carbohydrate. Take the given food sample, prepare the extract of the given food sample. Then add an iodine solution to the prepared food extract. A solution will turn blue in colour to confirm the presence of starch (carbohydrate) in the foodstuff.
Food Test for Proteins
Proteins can be tested by using the following methods.
1. Biuret Test
Take the given food sample and prepare its extract. Add the aqueous copper sulphate to the prepared food extract. A solution that turns violet in colour confirms the presence of protein in the foodstuff.
2. Ninhydrin Test
Take the given food sample and prepare its extract. Add the pyridine ninhydrin solution to the prepared food extract. A solution that turns violet in colour confirms the presence of protein in the foodstuff.
3. Xanthoproteic Test
Take the given food sample and prepare its extract. Add the nitric acid solution to the prepared food extract. A solution that turns yellow in colour confirms the presence of protein in the foodstuff.
4. Millon’s Test
Take the given food sample and prepare its extract. Add sodium nitrite and sulfuric acid to the above extract followed by Mercuric sulfate. A solution will turn brick-red in colour. This confirms the presence of protein in the food.
Food Test For Fats
1. Acrolein Test
Take the given food sample and prepare its extract. Add potassium bisulphite to the above extract. A pungent and irritating odour coming from the solution confirms the presence of fats in the food.
2. Translucent Spot Test
Take the food sample place it between the folds of filter paper and rub it lightly. The presence of translucent spots on the filter paper confirms the presence of fats in the foodstuff.
3. Solubility Test
Take the given food sample and prepare its extract. Add the above extract to the chloroform solution or alcohol solution. If the extract gets dissolved into the solution, it confirms the presence of fats in the food.
Observation Inference
The following is an example of an observation table for the samples of foodstuff containing carbohydrates, fats and proteins.
Result and the discussion:
The given sample of the foodstuff contains _______ carbohydrates.
The given sample of the foodstuff contains ______ proteins.
The given sample of the foodstuff contains ______ oils and fats.
Precautions:
Always use freshly prepared reagents in the minimum quantity to test the sample food.
Always make use of the droppers to take out the reagents from the bottle.
Always use personal protection gear like aprons and hand gloves while conducting the experiment.
Did You Know?
Zero food contains zero carbohydrates.
Carbohydrate is also known as brain food.
Occasional carbohydrate cutting can beat cancer.
A high protein diet aids fat loss.
All the tests of carbohydrates, fats and proteins in food are explained in the above article. The article provides useful information on food testing and its procedures.
FAQs on Tests of Carbohydrates, Fats and Proteins in Given Food Stuffs
1. What are the common tests to confirm the presence of carbohydrates in a food sample?
To confirm the presence of carbohydrates in a food sample, several tests can be performed. The most common ones as per the CBSE curriculum are:
- Molisch’s Test: A general test for all carbohydrates. A purple or violet ring at the junction of two liquids indicates a positive result.
- Fehling's Test: Used to detect reducing sugars. The formation of a reddish-brown precipitate indicates the presence of reducing sugars.
- Benedict's Test: Similar to Fehling's test, it also detects reducing sugars, indicated by a red, yellow, or green precipitate.
- Iodine Test: Specifically used to test for the presence of starch. A blue-black colour appears when iodine solution is added to the sample containing starch.
2. How can a student test for proteins in a given foodstuff like pulses or egg white?
To test for proteins in a food sample, the Biuret test is commonly used. First, an aqueous extract of the food sample is made. Then, a few drops of copper sulphate solution are added, followed by sodium hydroxide solution. The appearance of a violet or purple colour confirms the presence of proteins in the foodstuff.
3. What are the simplest tests to identify the presence of fats or oils in food?
Two simple tests can identify fats in food:
- Translucent Spot Test: The food sample is rubbed on a piece of paper. If the paper turns translucent and greasy, it indicates the presence of fats. The spot does not dry even after some time.
- Solubility Test: Fats are insoluble in water but soluble in organic solvents. If the food extract dissolves in a solvent like chloroform or alcohol but not in water, it confirms the presence of fats.
4. What is the scientific principle behind using the Iodine test for starch?
The Iodine test works because of the unique structure of starch. Starch contains a component called amylose, which has a helical or coiled shape. When iodine solution is added, the iodine molecules (specifically, triiodide ions, I₃⁻) get trapped inside these coils. This forms a starch-iodine complex that absorbs light differently than iodine alone, resulting in the characteristic deep blue-black colour. This reaction is highly specific to starch.
5. Why does the Biuret test give a positive result for proteins but not for individual amino acids?
The Biuret test does not detect amino acids because it specifically identifies the presence of peptide bonds—the chemical links between amino acids in a protein chain. For a positive test, at least two peptide bonds must be present. The copper ions (Cu²⁺) in the Biuret reagent form a coordination complex with these bonds, producing the violet colour. Since individual amino acids are not joined by peptide bonds, they do not react and will not produce a colour change.
6. What is the key difference between Fehling's test and Tollen's test for identifying carbohydrates?
Both Fehling's test and Tollen's test are used to identify reducing sugars. The primary difference lies in their reagents and the observed result:
- Fehling's Test uses a copper(II) solution. In the presence of a reducing sugar, the Cu²⁺ ions are reduced to Cu⁺, forming a reddish-brown precipitate of copper(I) oxide (Cu₂O).
- Tollen's Test uses an ammoniacal silver nitrate solution (Tollen's reagent). A positive test results in the reduction of silver ions (Ag⁺) to metallic silver (Ag), which forms a distinct silver mirror on the inner surface of the test tube.
7. Why is it necessary to prepare a food extract using water before performing these tests?
Preparing a food extract, usually with water, is a crucial first step because most chemical tests for nutrients are carried out in an aqueous medium. The reagents (like Fehling's solution or Biuret reagent) are dissolved in water. To ensure a chemical reaction can occur, the nutrients (carbohydrates, proteins) from the solid food sample must be brought into the solution. Grinding and mixing the food with water helps to dissolve or suspend these nutrients, making them accessible to react with the test reagents.
