Food Tests GCSE Biology: Benedict's, Iodine, Biuret

There are four food tests you need for GCSE and IGCSE Biology. Benedict’s test detects reducing sugars (blue to green, yellow, orange or brick-red on heating), the iodine test detects starch (orange-brown to blue-black), the biuret test detects protein (blue to purple), and the ethanol emulsion test detects lipids (a cloudy white emulsion). Each one is a single colour change, and the mark is usually for naming the reagent, the conditions and the exact colour, so all three have to be right.

Food tests at a glance

• Reducing sugars: Benedict’s solution, heated in a water bath. Blue to green, yellow, orange or brick-red.
• Starch: iodine solution, no heating. Orange-brown to blue-black.
• Protein: biuret reagent, no heating. Blue to purple (lilac).
• Lipids: ethanol, then water. A cloudy white emulsion forms.

Food tests are practical content, so they turn up both as recall questions and as questions about a method you may never have run yourself. Learn the list above, then learn the detail under each test below.

Test for sugar: Benedict’s test

Benedict’s test is the answer to “test for sugar” in GCSE Biology, but note the precise wording: it detects reducing sugars such as glucose, not sugar in general. Table sugar (sucrose) is not a reducing sugar and gives no colour change unless it has first been broken down.

• Crush or grind the food and mix it with distilled water. Filter if the mixture is lumpy.
• Add a few drops of Benedict’s solution, which starts blue.
• Heat in a hot water bath (a beaker of heated water, not a flame under the tube) for a few minutes.
• Record the colour. Blue means no reducing sugar. Green, yellow, orange and brick-red mean increasing amounts.

That last point is why the test is described as semi-quantitative: the colour you reach gives a rough idea of how much reducing sugar there is, not an exact amount. If a question asks you to compare two foods, say which one reached a colour further along the scale (brick-red has more than green). A negative result is a solution that stays blue.

Iodine test for starch

Add a few drops of iodine solution to the sample, either as a liquid or directly onto the food. Iodine solution is orange-brown (some boards say yellow-brown). If starch is present it turns blue-black. If not, it stays orange-brown. No heating is needed.

This is the test you will also see in practicals on enzymes, where iodine is dropped into samples at intervals to show when the starch has been broken down by amylase. The result you are looking for there is the iodine staying orange-brown, meaning the starch has gone.

Biuret test for protein

The biuret test uses biuret reagent, or sodium hydroxide followed by dilute copper sulfate solution, depending on your school’s method. Add it to the food sample and mix. The solution starts blue and turns purple (often described as lilac) if protein is present. If there is no protein, it stays blue. No heating is needed.

The trap here is the starting colour. Because the reagent is blue, writing “it turns blue” for a positive result scores nothing, and “it stays blue” is the correct negative result.

Ethanol emulsion test for lipids

To test for fats and oils, crush the food and shake it with ethanol so that any lipid dissolves. Pour the ethanol into a tube of water. If lipid is present, a cloudy white emulsion forms. If not, the liquid stays clear.

Some boards and school practicals use Sudan III instead. It stains lipid red, so the lipid shows as a red layer or red droplets. Check your own specification to see which version your board expects, and write the method that matches it.

Vitamin C is tested with DCPIP, a blue dye that is decolourised by vitamin C. Not every board includes this test, so confirm it is in your specification before you spend time on it.

The colour words mark schemes want

Mark schemes for these tests are strict about colour, in the same way that biology is strict about key terms. A few habits protect you.

• Name the start and the end. “Blue to brick-red” is a complete result. “It changes colour” earns nothing.
• Colourless is not the same as clear. Clear means you can see through it, and a blue solution can be clear. If the answer is that all colour has gone, write “colourless”. If you mean there is no cloudiness, write “clear”.
• Use the colour the scheme uses. Write “purple” for biuret and “blue-black” for iodine. Alternatives such as “lilac” are often accepted for biuret, but do not rely on vague words like “dark” being allowed.
• Do not skip the condition. Benedict’s earns its mark only with heating. Writing “add Benedict’s” with no heat is an incomplete method.
• Write the negative result when asked. “Stays blue” or “stays orange-brown” is a mark in its own right.
• Say “emulsion” for lipids. “Cloudy” alone is weak. “Cloudy white emulsion” is what the mark scheme is looking for.

Safety in food tests

Questions on safety are common, and the answer should name the hazard and the precaution together.

• Wear eye protection, because Benedict’s solution, sodium hydroxide and heated liquids can all splash.
• Use a test tube holder and point the tube away from people when heating in a water bath.
• Ethanol is flammable, so keep it away from flames and heat sources.
• Sodium hydroxide is an irritant and can burn. Rinse spills with plenty of water.
• Iodine stains skin and clothing.

Where the marks usually are

Most practical questions on food tests give you results and ask you to interpret them, or give you a food and ask you to plan a test. For a plan, the marks are normally for the right reagent, the right conditions, the right positive colour and a negative control, so write all four. Our post on six-mark questions covers how longer answers are structured.

Revise and practise food tests

Food tests are easy to recognise and easy to lose marks on, and the way to fix that is to write out answers and compare them with the mark scheme. Our revision notes cover this in the right place for your course, for example IGCSE Cambridge Biology, IGCSE Edexcel Biology and GCSE AQA Biology. Use the flashcards to drill the colours, then try the worksheets. Every written answer you practise on Exaim is marked against the board’s mark scheme, so you see exactly where the colour word or the condition was missing.