Polysaccharides are macromolecules, without sweetness, insoluble or slightly soluble in water, non crystalline, made up of many monosaccharides, general formula is Cx(H2O)y, non reducing type carbohydrates. Polysaccharides are polymers of monosaccharides and they function chiefly as food and energy storage in the form of starch and glycogen and structural materials e.g., cellulose. They are convenient storage molecules because:

They exert no osmotic or chemical influence on the cell 

These occur in highly folded structures which on hydrolysis obtains monosaccharide.

Starch is a group of carbohydrates made from various cereals and root vegetables. The alternative word for starch is “carbohydrate” (although “carbohydrate food” would have been a better choice). 

Starch 

It is homopolysaccharide, a polymer of a D-glucose, gets accumulated as food storage, only found in plant cells, absent in animal cells. The reserve food is either in simple (maize, pea, potato) or compound (rice, oat) form. Each grain has a central point known as hilum around which starch forms ring and present in concentric (wheat, rice, maize) or accentric (e.g., potato) form.

Starch has different properties; some require heating (e.g., rice, tapioca) while others are soluble in hot water – these include maize and potato that dissolve in hot water or arrowroot, which is soluble in cold water. Finally, a few starches are insoluble even in boiling water – for example, wheat starch and pinene. 

Structure of Starch Polysaccharide

Starch is made up of two components; amylose and amylopectin.

Amylose: It is a long chain of D-glucose bounded by α-linkage or a 1-4 linkage. Insoluble in water but forms hydrated micelles in aqueous solution, broken down by α-amylase enzymes secreted by pancreas and saliva. It leads to formation of a mixture of glucose, maltose and polysaccharides of intermediate β-amylose found in malt that leads to formation of maltose. Amylose constitutes around 20-30% of starch molecule.Amylose gives blue-black colour with iodine solution.

Amylopectins: These are highly branched structures and each branch is about 12 glucose units long, attached with α-linkage or a(1-4 linkage). Branching points are α (1-6) linkage i.e., glycosidic bond between C atoms C1 and C6 of neighbouring glucose units formed by condensation. They form colloidal or micellar solution, molecular weight is as high as 1 million, they are also attached by α and β-amyloses. Since α (1-6) linkage cannot break by these enzymes, the end product is a large and highly branched core called limit dextrin. They constitute around 70-80% of starch. It develops red-violet colour with iodine solution.

Tests for Starch

1. If you dissolve the starch in water, you have a milky-looking solution. You can tell if the starch is amylose or amylopectin because amylose leaves behind thicker white sediment after settling.
2. If flour and water are mixed, they bind together and form a paste. The starch inside the paste is made up of long chains in parallel. The longer the chains, the more dense the solution will be. Amylose gives a slightly viscous solution, whereas amylopectin gives a much more viscous solution.
3. If you cook cornflour for too long, it will taste bitter, indicating that it has broken down into glucose and other sugars. This indicates hydrolysis, and longer heating leads to more breakdown.
4. Boiling with acids can also break down starch into simpler chains, e.g., the enzyme lactase.
5. Fatty acids may react with starch and form esters, indicating that the starch has been degraded.

Uses of starch

• As a thickener: starch can be used as a thickener in food by mixing them with water. Starch molecules can absorb large amounts of water and swell. As the starch swells, it forms a thick paste which gives the food a more solid texture.
• Food preparation: starch is used as binders, fillers, and for making various other kinds of additives to food products. They are used extensively in the food industry, bakery products, confectionery products, etc. For example, starch can be used to make a glue-like substance, which is useful in preparing candies and some kinds of food wraps.
• Generally, food made from starch has low nutritional value because they are only composed of glucose (one sugar molecule). However, starch can be more nutritious when combined with other substances. 

Starch Examples

Rice, potatoes, whole grain bread, cereals, wholewheat pasta, chips, noodles, chapatis, maize and fast food, are all very nutritious.

Conclusion

Starch is a very important group of carbohydrates, and carbohydrates are an important energy source for the human body. Carbohydrates are stored in the form of starch in seeds, roots and tubers. Starch is found in various foods, and some are used as substitutes for fats. 

Starch molecules are found in the form of amylose and amylopectin. Starch hydrolysis is how fat is converted into glucose by acids. In a nutshell, starch is vital for functioning and living healthily.