Testing for nutrients

How can we find out which nutrients a food has? Starch (a kind of carbohydrate), fat and protein can be found with simple tests. Other nutrients need a well-equipped laboratory.

Test for starch. Put 2–3 drops of diluted iodine solution on a small piece of food, such as a slice of potato or some boiled rice. If the food turns blue-black, it has starch.

Test for fat. Wrap a small piece of food in paper and press it. If the food has fat, it leaves an oily patch on the paper. Hold the paper up to the light: the light shines faintly through the oily patch. That shows the food has fat.

Test for protein. Your teacher does this one, because the chemicals are harmful. Never touch them unless asked, never put them in your mouth, never smell them.

• Make the food into a paste or powder.
• Put about half a teaspoon in a clean test tube with 2–3 teaspoons of water.
• Add two drops of copper sulfate solution.
• Then add ten drops of caustic soda solution, and shake.
• If the contents turn violet, the food has protein.

The book reports that peanuts show both protein and fat. So one food can contain more than one nutrient.