The Food Test Lab

The Food Test Lab

What is really in your lunch? With four simple chemical tests you can reveal whether a food contains starch, sugar, protein or fat: each shown by a distinct colour change. By the end you will have run the tests on an unknown sample and written the method up the way the exam asks for it.

The four tests

Each nutrient has its own reagent and its own colour change. The reagent colour BEFORE the test matters too, because a positive result means the colour has changed from it:

Match each food to the result it would give

Before any reagent

A reagent can only reach nutrients that are in solution, so a solid food is never tested as it comes: - Grind the food with a pestle and mortar, add distilled water, stir, and filter to get a clear solution to test. - Use a fresh portion for each test. Reagents from one test would ruin the colour of the next. - Distilled water, not tap water, so nothing dissolved in the water affects the result. Safety: the Benedict's test needs a hot water bath, and ethanol is highly flammable, so the emulsion test is done well away from any flame, using an electric water bath if heat is needed.

The emulsion test method

Put the steps of the ethanol emulsion test for fats in the right order.

• Grind the food and put it in a dry test tube
• Add ethanol and shake, to dissolve any fat out of the food
• Let the solid settle, then pour the ethanol off into a second tube
• Add distilled water to the ethanol
• A cloudy white emulsion means fat is present

Why ethanol first?

A student adds water to the food FIRST, then adds ethanol, and sees no cloudiness even though the food definitely contains fat. Why did the test fail?

• Fat does not dissolve in water, so it was never in solution for the water to make it come back out
• The water diluted the food too much for the test to work
• The mixture needed heating in a water bath first
• Ethanol only tests for protein, not fat

How much sugar?

Benedict's is the one test that tells you how much, not just whether. The more reducing sugar there is, the further along the scale the colour travels: blue (none) → green (traces) → yellow → orange → brick-red (a lot) A grade-9 point: a tube that stays blue proves there is no reducing sugar. It does NOT prove there is no sugar at all, because some sugars, such as the sucrose in table sugar, are not reducing sugars and give a negative Benedict's result.

Read the rack

Four food samples were tested with Benedict's solution. Tap the tube from the food with the MOST reducing sugar.

A tube that stays blue

A student heats a food sample with Benedict's solution and it stays blue. Which conclusion can she safely write down?

• The food contains no reducing sugar
• The food contains no sugar of any kind
• The food contains no carbohydrate
• The test failed and must be repeated

Identify the sample

You are handed a soft white solid and asked to find out what nutrients it contains. Work through the practical.

• What is the first thing you do with it?
• Iodine turns blue-black, and Biuret turns purple. What can you say so far?
• You now want to test the same sample for fat. What do you use?

Describe the method

Describe how you would test a sample of milk to find out whether it contains protein and whether it contains fat.

• Say how you prepare the sample before any reagent is added
• Give the reagent and the positive result for protein
• Give the method for the fat test IN ORDER, and say what a positive result looks like
• Include one safety precaution, and say why it is needed

Food test summary

A solid food is ground up and mixed with _____ water before testing. To test for starch, add iodine solution: it turns blue-black. To test for reducing sugars, add Benedict's solution and _____ it, turning brick-red. _____ reagent turns purple with protein. For fat, dissolve the sample in ethanol and then add water, giving a cloudy white _____.