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To Detect the Presence of Carbohydrates, Fats, and Proteins in Food Stuffs

Q: 2. A student adds a few drops of iodine solution to a boiled potato extract. What is the expected observation if starch is present, and what is the chemical reason for this change?

A positive test for starch will show a blue-black colouration. This happens because the iodine molecules (I₂) get trapped inside the helical structure of the amylose component of starch, forming a deep blue or black starch-iodine complex. This is a highly important and expected question for board practicals.

Q: 5. How would you perform a confirmatory test for fats in a sample of groundnut?

The simplest method is the Spot Test. A small amount of the crushed groundnut sample is pressed between the folds of a filter paper. If fat is present, it will leave a translucent, greasy spot on the paper. This spot does not disappear upon heating, unlike a water spot, which would evaporate.

Q: 6. Why does sucrose (table sugar) give a negative result with Fehling's solution, and what important step is required to test for it?

Sucrose is a non-reducing sugar because its reactive anomeric carbons are involved in the glycosidic bond, leaving no free aldehyde group to react with Fehling's solution. To test for sucrose, you must first perform hydrolysis by boiling it with a few drops of dilute HCl. This breaks sucrose into its reducing monosaccharide units, glucose and fructose, which will then give a positive result with Fehling's test after neutralisation.

Q: 9. What are three critical precautions a student must take to ensure accurate results and safety while performing these food tests for the Class 12 Chemistry practical exam?

To ensure accuracy and safety, a student should:Use clean and dry apparatus: Any contamination in the test tubes can interfere with the results and lead to incorrect observations.Use fresh reagents: Reagents, especially Fehling's solution, can degrade over time. Always use freshly prepared reagents for reliable results.Handle chemicals with care: Use droppers to dispense reagents and wear appropriate safety gear. Be cautious while heating solutions to avoid bumping or splashing.

Chemistry Experiment: To Determine the Presence of Carbohydrates, Fats and Proteins in Grapes, Potatoes, Rice, Butter, Biscuits, Milk, Groundnut, Boiled Egg

Carbohydrates are necessary food elements that affect taste and nutrition. Determining the types and concentrations of carbohydrates in foods is integral for energy analysis, nutritionary labelling, quality control, and distinguishing possible adulteration. Proteins are biomolecules created from amino acids. They're essential for building muscle mass.

Fats are a class of lipids containing 2 forms of monomers, fatty acids and glycerol. It's a subgroup of lipids. This article will explain how to perform characteristic tests of carbohydrates and proteins in the given foodstuffs.

Table of Content

Aim
Material Required
Theory
Procedure
Result
Observation
Precautions
Viva Questions
Summary

Aim

Characteristic tests of carbohydrates, proteins and fat in the given samples.

Materials Required

Test tubes
Beakers
Glass Rod
Pestle Mortar
Burner

Theory

The presence of carbohydrates, fats and proteins in any foodstuff is confirmed by performing the tests with the extract of the food samples. The advantage of these tests is that they do not interfere with one another.

Procedure

First, prepare the extract of the given foodstuff by either dry grinding within the mortar with a pestle or by boiling with a minimum amount of water and extracting with a little amount of an organic solvent.

The extracts of a number of the foodstuffs may be prepared as given under

For grapes — extract the juice.

For potatoes — cut the potato into two slices and boil it in water.

For the rice — boil with water

For butter — test directly

For biscuits — boil with water

For milk — test directly

For groundnut — grind in the mortar

For a boiled egg — take the white part of the egg, grind it and shake it with water.

Food Test for Carbohydrates

Carbohydrate tests can be performed by using the following methods:

Experiment	Procedure	Observation	Inference
Tollen’s Test	The given food sample is taken in a beaker and the extract is prepared. Add the Tollen’s chemical agent to the prepared food extract.	A silver mirror is formed on the wall of the test tube.	Confirms the presence of carbohydrates in the sample.
Iodine Test	The given food sample is taken, and prepared the extract of the given food sample. Then add an iodine solution to the prepared food extract.	The solution will turn blue in colour.	Confirms the presence of starch (carbohydrate) in the sample.
Fehling’s Test	The given food sample and its extract is prepared. Add Fehling’s A and Fehling’s B chemical agent to the prepared food extract.	A red precipitate formed in the test tube.	Confirms the presence of carbohydrates in the foodstuff.
Molisch’s Test	The given food sample and its extract is prepared.Add the Molisch’s chemical agent to the prepared food extract.	A purple-violet ring formed in the test tube.	confirms the presence of carbohydrates.
Benedict’s Test	The given food sample and its extract is prepared. Add Benedict’s chemical agent to the prepared food extract.	red precipitate formed in the test tube.	confirms the presence of carbohydrates.

Food Test for Proteins

Proteins may be tested by using the subsequent methods:

Experiment	Procedure	Observation	Inference
Ninhydrin Test	The given food sample and its extract is prepared. The pyridine ninhydrin solution is added to the prepared food extract.	A solution that turns violet in colour.	Confirms the presence of protein.
Biuret Test	The given food sample and its extract are prepared. Add the aqueous copper sulfate to the prepared food extract.	The solution turns violet in colour.	Confirms the presence of protein.
Millon’s Test	The given food sample and its extract is prepared. Add NaNO3 and sulphuric acid to the above extract followed by mercuric sulfate.	The solution turns brick-red in colour.	Confirms the presence of protein in the food.

Food Test For Fats

Fats may be tested using the following methods:

Experiment	Procedure	Observation	Inference
Translucent Spot Test	Take the food sample, place it between the folds of filter paper and rub it gently.	Translucent spots on the paper.	Confirms the presence of fats in the foodstuff.
Solubility Test	The given food sample and its extract is prepared. The chloroform solution is added to the above extract.	The extract gets dissolved into the solution.	Confirms the presence of fats.
Acrolein Test	The given food sample and its extract is prepared. Add potassium bisulphite to the above extract.	Pungent and aggravating odor coming from the solution.	Confirms the presence of fats .

Observation

Food Stuff	Carbohydrates(g)	Proteins(g)	Fats(g)
Grapes	13.67	0.54	0.12
Boiled Egg	0.6	6	5
Groundnuts	16.1	25.8	49.2
Rice	14.84	1.42	0.15
Biscuit	19.3	3.2	8.5
Butter	0	0	12
Milk	12	8	8
Potatoes	20.1	1.9	0.1

Result

The given sample of the foodstuff contains 13.67 g carbohydrates.

The given sample of the foodstuff contains 0.54 g of proteins.

The given sample of the foodstuff contains 0.12 g of oils and fats.

Precautions

Following precautions should be taken while completing the experiment:

Always use freshly prepared reagents within the minimum amount to test the sample food.
Always use the droppers for taking out the reagents from the bottle.
Always use personal protection gear like a lab coat and hand gloves while experimenting.

Lab Manual Questions

1. What are the best factors to consider when evaluating the quality of a carbohydrate?

Ans. The elements of carbohydrate quality that should be considered are the number of dietary fibres, the proportion of whole-grain carbohydrates compared with total-grain carbohydrates, the low glycemic index and the proportion of solid carbohydrates.

2. Which reagent is used to detect the presence of proteins?

Ans. Biuret reagent.

3. Which colour modification represents a positive reaction to the presence of starch?

Ans. Using an iodine solution, you'll be able to test for the presence of starch. Once the starch is present, the iodine changes from brown to dark-blue or purple.

4. What are the 4 main functions of carbohydrates?

Ans. They are energy production, storage, building macromolecules, sparing protein, and assisting in lipid metabolism.

5. What enzymes break down carbohydrates?

Ans. Amylase.

Viva Questions

Q1.Give two examples of monosaccharides.

Ans. Fructose and glucose.

Q2. Give two examples of disaccharides.

Ans. Maltose and Sucrose.

Q3.What are the functions of carbohydrates?

Ans. To supply energy to the body as biofuel and to act as storage of chemical energy in the form of glycogen in the liver and as a constituent of cell membranes.

Q4. Why do we get a red ppt. in Fehling’s test?

Ans. Because of the formation of cuprous oxide (Cu20).

Summary

the food test aims to analyze the use of chemical reagents to detect starch, reducing sugars, proteins, and fats. Carbohydrates give energy to cells within the body. Protein is the major useful and structural part of all body cells. Fats and oils are the highest energy sources that contain fatty acids that are essential for health and aren't made by the human body. This experiment confirms the presence of carbohydrates, proteins, and fats in the given food sample.

FAQs on To Detect the Presence of Carbohydrates, Fats, and Proteins in Food Stuffs

1. What are the essential qualitative tests required in the CBSE Class 12 practicals to detect the presence of carbohydrates, proteins, and fats in food items?

For the CBSE 2025-26 syllabus, the key tests to identify these biomolecules in given foodstuffs are:

For Carbohydrates: The Iodine test is used for starch, while Fehling's test or Benedict's test is used for reducing sugars like glucose.
For Proteins: The Biuret test is the standard confirmatory test.
For Fats and Oils: The Spot test or Solubility test is commonly performed.

2. A student adds a few drops of iodine solution to a boiled potato extract. What is the expected observation if starch is present, and what is the chemical reason for this change?

A positive test for starch will show a blue-black colouration. This happens because the iodine molecules (I₂) get trapped inside the helical structure of the amylose component of starch, forming a deep blue or black starch-iodine complex. This is a highly important and expected question for board practicals.

3. Explain the principle behind Fehling's test for detecting reducing sugars. Why is a red precipitate an important observation?

Fehling's test is used to identify reducing sugars which have a free aldehyde or ketone group. The principle is based on a redox reaction. The reducing sugar is oxidised, while the Cupric ions (Cu²⁺) in the Fehling's solution are reduced. The formation of a red precipitate of cuprous oxide (Cu₂O) confirms the presence of a reducing sugar.

4. Describe the procedure and the chemical basis of the Biuret test for proteins in a given food sample like milk or egg albumin.

To perform the Biuret test, you first add a few drops of an alkaline solution like sodium hydroxide (NaOH) to the food extract. Then, you add a 1% copper sulphate solution drop-wise. The appearance of a purple or violet colour confirms the presence of protein. This colour results from the formation of a coordination complex between the Cu²⁺ ions and the nitrogen atoms in the peptide bonds of the protein molecule in an alkaline medium.

5. How would you perform a confirmatory test for fats in a sample of groundnut?

The simplest method is the Spot Test. A small amount of the crushed groundnut sample is pressed between the folds of a filter paper. If fat is present, it will leave a translucent, greasy spot on the paper. This spot does not disappear upon heating, unlike a water spot, which would evaporate.

6. Why does sucrose (table sugar) give a negative result with Fehling's solution, and what important step is required to test for it?

Sucrose is a non-reducing sugar because its reactive anomeric carbons are involved in the glycosidic bond, leaving no free aldehyde group to react with Fehling's solution. To test for sucrose, you must first perform hydrolysis by boiling it with a few drops of dilute HCl. This breaks sucrose into its reducing monosaccharide units, glucose and fructose, which will then give a positive result with Fehling's test after neutralisation.

7. In the Biuret test, what is the specific role of adding sodium hydroxide (NaOH) before the copper sulphate solution? What happens if it's not added?

The addition of sodium hydroxide is crucial as it provides the alkaline medium necessary for the reaction. The complex between the Cu²⁺ ions and the peptide bonds only forms under alkaline conditions. If NaOH is not added, the solution will not be alkaline, and the characteristic purple colour will not appear, leading to a false negative result even if protein is present.

8. What are the key differences between fats and oils that can be identified in a lab setting?

The main difference is their physical state at room temperature, which is a direct result of their chemical structure.

Fats (e.g., butter) are typically solid at room temperature because they contain a higher proportion of saturated fatty acids.
Oils (e.g., groundnut oil) are liquid at room temperature as they have a higher proportion of unsaturated fatty acids.

9. What are three critical precautions a student must take to ensure accurate results and safety while performing these food tests for the Class 12 Chemistry practical exam?

To ensure accuracy and safety, a student should:

Use clean and dry apparatus: Any contamination in the test tubes can interfere with the results and lead to incorrect observations.
Use fresh reagents: Reagents, especially Fehling's solution, can degrade over time. Always use freshly prepared reagents for reliable results.
Handle chemicals with care: Use droppers to dispense reagents and wear appropriate safety gear. Be cautious while heating solutions to avoid bumping or splashing.