Tests of Carbohydrates, Fats and Proteins in Given Food Stuffs

Dhristi JEE 2022-24

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:


(Image will be Uploaded Soon)

 

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. 

Foodstuff

For Carbohydrates

For Proteins

For Oils and Fats

Butter












Potato












Butter












Boiled eggs












Milk












 

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:

  1. Always use freshly prepared reagents in the minimum quantity to test the sample food.

  2. Always make use of the droppers to take out the reagents from the bottle.

  3. 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.

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FAQs (Frequently Asked Questions)

1. Name Three Tests that Can be Used for the Identification of Carbohydrates.

Three tests that can be used for the identification of carbohydrate are: 

  1. Fehling’s Test

  2. Molisch’s Test

  3. Tollen’s Test

2. Explain the Biuret Test.

Biuret Test is used for the identification of protein in the food. In this test, we take the given food sample, prepare the extract of the given food sample. Then add the aqueous copper sulphate to the prepared food extract. A solution turns violet in colour confirms the presence of protein in the foodstuff.

3. What are carbohydrates? Name five carbohydrates.

Carbohydrates or carbs are known as sugar molecules. Along with proteins and fats, carbohydrates are also a very essential part of the three main nutrients that are found in most drinks and food. Carbohydrates are broken down into glucose by the body. Thus, glucose and blood sugar become the main source of energy for the body cells, tissues and various organs. The five main carbohydrates are as follows:-

  • Monosaccharides.

  • Disaccharides.

  • Oligosaccharides.

  • Polysaccharides.

  • Nucleotides.

4. Which one is considered the healthiest carbs?

Unprocessed or minimally processed whole grains, vegetables, fruits and beans are the healthiest source of carbohydrates. It promotes good health by delivering vitamins, minerals, fibre, and a host of important phytonutrients. 

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