How do you do the food tests required practical?

What does the food tests practical investigate?

It is a set of qualitative tests. Each one tells you whether a nutrient is there, from a colour change, without measuring exactly how much. You test foods for sugars, starch, protein and lipids (fats and oils), each with its own reagent.

It is one of AQA's required practicals in both GCSE Biology and Combined Science: Trilogy, in the organisation topic alongside digestion and enzymes. The Benedict's test is a partial exception to "yes or no": the colour it ends on gives a rough idea of how much sugar there is.

What equipment do you need?

Besides the food, you need a reagent for each test and a hot water bath for the Benedict's test. The rest is ordinary glassware.

• The foods to test, and a pestle and mortar to grind solid ones
• Distilled water, a beaker, and a filter funnel with filter paper
• Test tubes, a rack and a test tube holder
• A spotting tile, for the starch test
• Dropping pipettes, a fresh one for each reagent
• Benedict's solution, iodine solution and biuret reagent
• Sudan III stain, or ethanol, for the lipid test
• A water bath at about 75 °C, or a beaker of hot water from a kettle
• Eye protection

What is the method?

Make a solution from the food, then split it between the tests. Run each test beside a tube of distilled water, so you can see what "no change" looks like.

• Grind a small amount of food with a little distilled water using a pestle and mortar. Stir in more water, then filter it or let the bits settle, to get a food solution.
• Sugars: put some food solution in a test tube, add about 10 drops of Benedict's solution, and stand the tube in the water bath for about 5 minutes. A change from blue to green, yellow or brick red means sugar is there.
• Starch: put a few drops of food solution on a spotting tile and add a drop or two of iodine solution. A change from orange-brown to blue-black means starch is there.
• Protein: add biuret reagent to some food solution in a test tube and shake gently. A change from blue to purple means protein is there.
• Lipids with Sudan III: add a few drops of Sudan III stain to some food solution and shake gently. A red-stained oily layer floating on top means lipids are there.
• Lipids with ethanol: shake a little of the food with ethanol, then pour the liquid into a tube of water. A cloudy white layer, called an emulsion, means lipids are there.
• Record the colour you see in every test, including the distilled water tubes.

What are the variables?

Food tests identify nutrients rather than measure an effect, so the variables are simple: the food is what you change, and the colour is what you record. Keeping amounts the same matters most when you compare Benedict's colours between foods.

How do you work out and present the results?

There is nothing to calculate and no graph to draw. Put your results in a table, with the foods down the side and the tests across the top. In each box, write the colour you saw and whether the nutrient is present.

For Benedict's, the final colour gives a rough amount of sugar: green for a little, then yellow and orange, up to brick red for a lot. That makes it semi-quantitative. It can put drinks in order of sugar content, but it can't give you a number.

Worked example: a sample turns Benedict's orange, leaves iodine orange-brown and turns biuret purple. It contains protein and quite a lot of sugar, but no starch.

How do you make it accurate and safe?

Use a clean pipette for each reagent and each food, because one drop carried between tubes can give a false result. Judge every colour against a white background and next to your distilled water tube. For Benedict's, give each tube the same time in the water bath, or the colours can't be compared. And if a sugary food stays blue, check which sugar it has: table sugar (sucrose) does not react with Benedict's solution.

Wear eye protection throughout. Biuret reagent contains sodium hydroxide, which can harm eyes and skin, and the other reagents can irritate your eyes. Heat the Benedict's test in a water bath rather than over a flame, and hold hot tubes with a test tube holder. Ethanol catches fire easily, and Sudan III stain is usually made up in ethanol, so keep both well away from flames. Never taste the food.

What do exam questions ask about it?

Most food test questions are about recall and reading results. You might be asked for the reagent that tests for a nutrient and the colour change for a positive result, or be given a table of colours and asked what each food contains. Other questions ask why the Benedict's test is heated in a water bath, or how you could compare the sugar in two drinks. An odd result can come from a pipette carrying one reagent into another tube. A 6-mark question may ask you to describe how to test one food for several nutrients.

An exam-style question we wrote: "A student adds biuret reagent to a sample of egg white. Give the colour change and what it shows." The answer is blue to purple, which shows protein is present.

Common mistakes

• Mixing up the reagents. Iodine tests for starch, Benedict's for sugars and biuret for protein.
• Forgetting to heat the Benedict's test. Without heat, it stays blue even when sugar is there.
• Giving only the final colour. Write the whole change, such as "blue to purple".
• Using one pipette for everything, which carries reagents from tube to tube.
• Doing the ethanol test near a lit Bunsen burner. Keep ethanol well away from flames.