Our objective is to test for the presence of starch in a given food sample.
Starch is the most common carbohydrate in the human diet and is contained in many staple foods. The major sources of starch intake worldwide are cereals (rice, wheat, and maize) and root vegetables (potatoes and cassava). We can use iodine solution to test for the presence of starch. If starch is present is a food item, it turns blue-black colour when iodine solution is added to it.
What are Carbohydrates?
Carbohydrates are one of the main components of our daily diet. This category of foods include sugars, starches, and fibre. Carbohydrates are built up of sugar molecules that contain hydrogen, carbon and oxygen. Chemically, most of these carbon atoms have a hydrogen group and hydroxyl group attached to it. Thus, the word Carbohydrates Cn(H2O)n literally mean carbon (carbo) + water (hydrate).
Classification of Carbohydrates
Carbohydrates are classified as simple or complex. This classification depends on the chemical structure of the food, and how quickly the sugar is digested and absorbed.
Simple carbohydrates are simple sugars with a chemical structure that is composed of one or two sugars. There are two types of simple carbohydrates - monosaccharides and disaccharides.
Monosaccharides consist of only one sugar, and examples are glucose, fructose and galactose.
Disaccharides consist of two chemically-linked monosaccharides, and examples are sucrose, lactose and matose.
Complex carbohydrates have three or more sugars and are classified as oligosaccharides and polysaccharides.
Oligosaccharides consist of a small number of monosaccharides, which does not exceed 10. They are important in the absorption of certain minerals and the formation of fatty acids. Examples are raffinose and stachyose.
Polysaccharides are often made up of a large number of monosaccharides and disaccharides. Examples are starch, glycogen and cellulose.
What is Starch?
Starch is the major carbohydrate reserved in plant tubers and seed endosperm. It is stored in plant cells. Starch molecules arrange themselves in the plant in semi-crystalline granules. A starch molecule contains a large number of glucose molecules. These glucose molecules are arranged either as occasionally branched chains (amylopectin) or as unbranched chains (amylose). Amylose is a much smaller molecule than amylopectin.
The colour of natural starch is milky white and the pure form of starch is insoluble in water and alcohol. Depending on the plant, starch generally contains 20-25% amylase and 75-80% amylopectin. Each plant species have a unique starch granular size. Rice starch is relatively small (about 2μm), while potato starches have larger granules (up to 100μm). Starch becomes soluble in water when heated.
Students understand the terms carbohydrates, monosaccharides, disaccharides, oligosaccharides, polysaccharides and starch.
Students understand the method to test for the presence of starch.
Students acquire the skills to perform the experiment using different food samples having observed the animation and simulation.