Detection of Carbohydrates, Proteins and Fats


Our objective is to detect the presence of carbohydrates, protein and fats and to show their presence in suitable plant and animal materials.


The food we eat is one of the necessary factors in our daily life that provides nutritional support for the human body. Food consists of both organic and inorganic substances. Carbohydrates, fats and proteins are the main organic substances present in the food, which provide energy.

Let’s start with carbohydrates.

What are Carbohydrates?

One of the main components of our daily diet is carbohydrates. This type of foods includes sugars, starch and fibres. They are composed of sugar molecules that contain carbon, hydrogen and oxygen.

Carbohydrates are classified into simple carbohydrates and complex carbohydrates.

Simple Carbohydrates

Simple carbohydrates are composed of one or two sugar units. Because of their smaller size, simple carbohydrates can be broken down and can be absorbed by the body easily. So they act as the quickest source of energy. They are found in fruits in the form of fructose, milk in the form of lactose and table sugar in the form of sucrose.

There are two types of simple carbohydrates: Monosaccharides and Disaccharides.

Monosaccharides are the simplest carbohydrates, consisting of only one sugar unit.  Glucose, fructose and galactose are examples of monosaccharides. They have the capability of reducing cupric (Cu2+) ions into cuprous (Cu+) ions due to the presence of free aldehydic and ketonic groups  and are called reducing sugars. These reducing sugars give positive results in Benedict’s test and Fehling’s test because they reduce the cupric ion present in the Benedict’s and Fehling’s solution and form a precipitate of cuprous oxide. Depending upon the concentration of sugar, green, orange or brick red precipitates are obtained.

Disaccharides are composed of two chemically-linked monosaccharide units. Sucrose, lactose and maltose are examples of disaccharides. Sucrose is a non-reducing disaccharide. When it is boiled with HCl, sucrose undergoes hydrolysis to form glucose and fructose, which gives positive result with Benedict’s and Fehling’s solutions.

Complex Carbohydrates

Complex carbohydrates are composed of long chains of simple carbohydrate units. Because of their larger size, they can be broken down into simple carbohydrates.

Complex carbohydrates can be classified as Oligosaccharides and Polysaccharides.

Oligosaccharides consist of less than 10 monosaccharide units. Disaccharides are also a class of oligosaccharides. Raffinose and stachyose are examples of oligosaccharides.

Carbohydrates, made up of large number of monosaccharide units, are called polysaccharides. Starch, glycogen and cellulose are example of polysaccharides. Starch gives a blue-black complex with iodine.

What are Proteins?

Proteins are large biological molecules made up of large number of amino acid units. Amino acids are molecules consisting of both the amino (-NH2) group and carboxylic group (-COOH). In proteins, the amino acid units are linked together by specific linkages called peptide linkages. Because of the complex nature of protein, our body takes a longer time to break down protein molecules. Compared to carbohydrates, proteins are a much slower and longer-lasting source of energy. Most proteins are soluble in acidic or alkaline solutions, but insoluble in water.

Proteins give colour reactions due to one or more radicals or groups present in the complex protein molecules. All proteins do not give all colour reactions because all of them do not contain the same amino acid units. This property of protein can be used for the detection of protein in a given sample.

Biuret test is a method used for the detection of peptide bond in a protein molecule. In the Biuret test, the nitrogen atoms in the peptide chain react with copper ions in the reagent to form a violet coloured complex.

Xanthoproteic test is used for the identification of protein containing aromatic amino acid units.  By heating with nitric acid, the benzene ring in the amino acid unit is nitrated and forms a yellow coloured nitro-compound which turns to orange colour with alkali.

What are Fats?

Fats are complex molecules made up of fatty acids and glycerol. Our body needs fats for growth and energy. Fats contain carbon, hydrogen and sometimes oxygen. Phosphorous, nitrogen and sulphur are also present in some fats. They are insoluble in water, but soluble in non polar s like chloroform and benzene. They are found stored in many oil seeds and some animal tissues. They produce translucent spots on paper due to the diffraction of light. They also give a pink colour with azo dye, sudan III.

Learning Outcomes:

  • Students understand the characteristics of carbohydrates, proteins and fats.
  • Students learn the tests to detect the presence of carbohydrates, proteins and fats.
  • Students will be able to do the experiment more accurately in the real lab once they understand the steps through the animation and simulation.