Food Tests

Cambridge requires five tests for this chapter: iodine solution for starch, Benedict’s solution for reducing sugars, the biuret test for protein, the ethanol emulsion test for fats and oils, and DCPIP for vitamin C. Food tests are also named explicitly as an experimental context for the practical papers, so expect them to turn up as questions about method and interpretation, not only as recall.

Iodine test for starch

Add iodine solution to the sample. No heating is required, and none should be added — a heating step here is a detail borrowed from Benedict’s test, and it does not belong in this answer.

If starch is absent, the iodine simply stays its starting colour — yellow-brown, orange-brown or brown are all acceptable descriptions of that colour, and any of them is fine to use. If starch is present, that colour changes to blue-black. The positive result has to be given as the full change, not just the end colour: “turns black” on its own is not the same statement as “changes from orange-brown to blue-black”, and only the second earns full credit. It is also worth being precise about what the test actually detects — iodine is specific to starch, not to carbohydrates generally, so it will not go blue-black for a sample that contains only glucose.

Benedict’s test for reducing sugars

Add Benedict’s solution to the sample, then heat the mixture in a hot or boiling water bath — a water bath rather than a direct flame, partly for even heating and partly because a naked flame is a genuine hazard in a lab where a flammable reagent like ethanol may also be in use nearby.

Benedict’s solution starts blue. With no reducing sugar present, it stays blue. With reducing sugar present, heating drives a colour progression: blue → green → yellow → orange → brick-red, and the exact point the colour reaches depends on how much reducing sugar is there — a green result signals a small amount, while an orange or brick-red result signals a large amount. That means Benedict’s is semi-quantitative: it lets you say one sample contains more reducing sugar than another, but it cannot give you an exact concentration on its own, so an answer that reads a colour straight into a percentage has gone further than the test actually supports. Glucose is the reducing sugar students meet most often, which is why this test sometimes gets described casually as “the test for glucose” — in an exam, use the syllabus term, reducing sugars, since that is the category the test actually detects.

Biuret test for protein

Add biuret reagent to the sample. No heating is required — the most common way to lose a mark here is importing the heating step from Benedict’s test, when biuret works at room temperature. Biuret reagent can also be set up as two separate additions, sodium hydroxide followed by copper(II) sulfate; that is the same test presented a different way, not a different or lesser-credit method.

With no protein present, the mixture stays blue. With protein present, it changes to lilac or purple. Keep that colour separate in your memory from Benedict’s orange or brick-red — the two positive results are sometimes confused precisely because both tests start from a blue reagent.

Ethanol emulsion test for fats and oils

This test works in a fixed order that matters: add ethanol to the sample and shake or mix it, so that any fat or oil dissolves into the ethanol, then add water. Fats and oils do not dissolve in water, but they do dissolve in ethanol — the ethanol step is what gets the fat into solution in the first place, so that when water is then added, the fat comes back out as tiny droplets spread through the liquid, which is what scatters light and produces the cloudy appearance you are looking for. Reverse that order and add water first, and the fat never dissolves, so no emulsion forms at all.

With no fat or oil present, the mixture stays clear. With fat or oil present, a cloudy or milky-white emulsionforms. This is the one food test in the set where the positive result is a change in appearance rather than a colour change, so describing it as “turns white” misses the point slightly — the correct language is that an emulsion forms. No heating is used in this test, and that matters for a second reason beyond method accuracy: ethanol is flammable, so it must be kept away from naked flames throughout.

DCPIP test for vitamin C

DCPIP solution starts blue. Add the sample — often a fruit juice — drop by drop, mixing as you go, and count how many drops it takes for the blue colour to disappear completely.

A positive result is DCPIP being decolourised, going from blue to colourless; it does not turn blue as a positive, since it starts that way. With no vitamin C present, the DCPIP simply stays blue, or needs an unusually large number of drops before any change appears. The comparison between samples depends on which volume is held fixed: with a fixed volume of DCPIP and the sample added dropwise, fewer drops needed to decolourise it means more vitamin C in that sample, and more drops needed means less. If the arrangement is reversed — a fixed volume of sample with DCPIP added to it — the volume relationship reverses too, so a stronger sample decolourises a larger volume of DCPIP rather than a smaller one. Never state “fewer drops means more vitamin C” as a fixed rule without first checking which liquid the question has actually held constant.

Food tests at a glance

Interpreting and comparing results

Each test answers exactly one question about a sample, so an unfamiliar-sample question is really asking you to combine several single-nutrient answers into one conclusion. Read every result independently first — iodine positive means starch is present, biuret positive means protein is present, a Benedict’s negative means no reducing sugar was detected — and only then combine them: a sample giving a positive iodine test and a positive biuret test, with the other tests negative, contains starch and protein but no detected reducing sugar, fat or vitamin C.

Word that last part carefully. A negative result means the test did not detect that nutrient, not that the sample definitely contains none of it at all — a small enough amount can sit below what a particular test can pick up. When a table compares two or more samples, work through each sample’s full row of results first, then compare samples against each other; matching a food to an unknown result set means matching the whole pattern of positives and negatives, not just reacting to a single line that looks familiar.

Recording results and running a fair test

Cambridge also expects you to describe food tests as a practical procedure, not just recall their colours, which means keeping two things separate that students often collapse into one. An observation is what you actually saw — “the solution changed from blue to brick-red” — while a conclusionis what that observation means — “reducing sugar is present”. If a question asks you to record what happened, give the observation; swapping in the conclusion instead usually loses the mark even though the underlying chemistry is understood correctly.

When two samples are being compared, the conditions that are not being investigated should be kept identical between them: the volume of sample used, the volume or concentration of reagent added, the heating temperature and time where a test involves heating, and the method used to judge the final colour or appearance. Only the one factor actually under investigation should be allowed to differ. A negative control — the same test carried out on a substance known not to contain the nutrient, such as distilled water — gives you a reliable picture of what “no change” genuinely looks like, so a faint or ambiguous result is not misread as positive.

Two safety points come up specifically in this topic: take care with hot water when heating a Benedict’s test, and keep ethanol away from naked flames at every stage of the emulsion test, since it is flammable. Cambridge’s practical assessment explicitly covers safe use of materials, the use of suitable controls, accurate observation and food tests together, so treat these as part of the answer, not as background detail around it.