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Key points

  • Starch is a type of . Its molecules contain carbon, hydrogen and oxygen atoms.

  • Iodine solution is used to test for starch.

  • The digestion of starch makes .

Video - Starch in cooking

A case study video explaining how starch molecules react in cooking.

How is starch useful?

Starch is a type of carbohydrate. Its molecules are made up of large numbers of carbon, hydrogen and oxygen atoms. Starch is a white solid at room temperature, and does not dissolve in cold water.

Most plants, including rice, potatoes and wheat, store their energy as starch. This explains why these foods – and anything made from wheat flour – are high in starch.

You can use iodine to test foods for starch. If starch is present, the orange-yellow iodine solution becomes blue-black.

Starch has many uses. Your body digests starch to make glucose, which is a vital energy source for every cell. Food companies use starch to thicken processed foods, and to make sweeteners. Scientists are investigating the effects of these sweeteners on health.

A rotating model of a starch molecule.

Polymerisation of starch

Starch is an example of a natural polymer. A polymer is a long and repeating chain of the same molecule stuck together. Starch is a long-chain polymer of glucose molecules joined together.

As the plant adds one glucose molecule to the starch polymer, one molecule of water is released. You can see this mechanism in the video opposite. Plants create starch polymers, for example in grains of wheat, to store the glucose made by photosynthesis.

A model of the polymerisation of starch.