Food Tests: Starch, Sugar and Protein

Why food tests matter

Carbohydrate, protein, fat, vitamin, mineral, roughage and water are the seven components of a balanced diet. Starch and sugar supply energy, protein builds and repairs the body, and fat stores energy and protects organs.

You cannot tell which food contains what by looking at it, so the body's building blocks are detected by simple colour tests. Each test is specific: iodine reacts only with starch, Benedict's solution only with reducing sugar, and Biuret reagent only with protein.

The three tests

Iodine test for starch: add two or three drops of iodine solution. A blue-black colour means starch is present; a yellow-brown colour means it is absent. It works at room temperature — rice, potato and bread all turn blue-black.

Benedict's test for reducing sugar: add Benedict's solution and heat the tube gently in a water bath. If reducing sugar is present the blue solution turns green, then yellow, then brick red. Banana, milk and sugar solution test positive. Heating is essential — no heating, no colour change.

Biuret test for protein: add sodium hydroxide, then a few drops of copper sulphate solution. A violet colour means protein. Milk, egg white, soybean and groundnut test positive.

Reading the result honestly

The test tells you presence or absence, not how much. A faint brick red means a little sugar; a deep brick red means a lot. Record the colour you actually saw rather than what the book says you should see.

Two mistakes cost marks in the practical: heating the tube in the open flame instead of a water bath (the solution boils over and spits), and adding iodine to a solid without a drop of water, so the colour is patchy. Also, always write the observation and the inference separately: "Observation — the solution turned blue-black. Inference — starch is present."