Nutrition and digestion · Investigation
Food tests
Four tests, four colour changes. Each one answers exactly one question — and three questions you will want to ask, none of them can answer.
Start here
Two tubes of milk. One goes brick red, one stays blue.
Same bottle of milk, same Benedict's solution, same water bath. Tube 1 comes out brick red. Tube 2 comes out blue. Nobody made a mistake — the difference is that tube 1 was heated for five minutes and tube 2 for thirty seconds.
What can you conclude from tube 2 on its own?
Nothing. A blue tube means this test did not detect sugar here — which is not the same claim as there is no sugar in milk. Every negative result in this lesson is a statement about the test, not about the food.
At the bench · five foods, four tests
Predict, then run it
0 of 20 combinations run
Pick a food, pick a test, say what you expect, then run it. The tube shows what you would actually see.
Food
Test
Method
A few drops of iodine solution straight onto the food. No heating.
Detects: starch
Equal volumes of food solution and Benedict’s, then five minutes in a water bath at 80 °C.
Detects: reducing sugar (glucose, fructose)
Shake the food with ethanol, let it settle, then pour the ethanol into water.
Detects: lipid (fat and oil)
Equal volumes of food solution and Biuret solution, shaken. No heating.
Detects: protein
Before you run it — will Iodine detect starch in potato?
Before you run it — will Benedict's detect reducing sugar in potato?
Before you run it — will Ethanol emulsion detect lipid in potato?
Before you run it — will Biuret detect protein in potato?
Before you run it — will Iodine detect starch in milk?
Before you run it — will Benedict's detect reducing sugar in milk?
Before you run it — will Ethanol emulsion detect lipid in milk?
Before you run it — will Biuret detect protein in milk?
Before you run it — will Iodine detect starch in egg white?
Before you run it — will Benedict's detect reducing sugar in egg white?
Before you run it — will Ethanol emulsion detect lipid in egg white?
Before you run it — will Biuret detect protein in egg white?
Before you run it — will Iodine detect starch in apple juice?
Before you run it — will Benedict's detect reducing sugar in apple juice?
Before you run it — will Ethanol emulsion detect lipid in apple juice?
Before you run it — will Biuret detect protein in apple juice?
Before you run it — will Iodine detect starch in olive oil?
Before you run it — will Benedict's detect reducing sugar in olive oil?
Before you run it — will Ethanol emulsion detect lipid in olive oil?
Before you run it — will Biuret detect protein in olive oil?
You predicted this
Not what you predicted
Blue-black — starch detected.
Potato is mostly starch, and starch is what iodine is for. A strong blue-black comes up within seconds.
What you may write down: “Potato contains starch.” Nothing about how much.
You predicted this
Not what you predicted
Blue — unchanged — not detected.
Raw potato stores its carbohydrate as starch, not as free glucose, so there is very little reducing sugar to find. Leave a potato in a warm cupboard for a fortnight and this test starts to go positive — the starch is being broken down.
What you may write down: “No reducing sugar was detected in potato under these conditions.” Not “there is none”.
You predicted this
Not what you predicted
Clear — unchanged — not detected.
Potato has almost no lipid. Under a hundredth of its mass.
What you may write down: “No lipid was detected in potato under these conditions.” Not “there is none”.
You predicted this
Not what you predicted
Blue — unchanged — not detected.
A potato is about 2% protein — present, but too dilute for the school test to show. This is a false negative, and it is the most common one in the lesson.
What you may write down: “No protein was detected in potato under these conditions.” Not “there is none”.
You predicted this
Not what you predicted
Orange-brown — unchanged — not detected.
No starch in milk. This is a true negative — there really is none.
What you may write down: “No starch was detected in milk under these conditions.” Not “there is none”.
You predicted this
Not what you predicted
Brick red — reducing sugar detected.
Lactose, the sugar in milk, is a reducing sugar. Brick red if you heat it long enough; green or orange if you rush it.
What you may write down: “Milk contains reducing sugar.” Nothing about how much.
You predicted this
Not what you predicted
Cloudy white — lipid detected.
Whole milk is a suspension of fat droplets in water. Strong cloudiness.
What you may write down: “Milk contains lipid.” Nothing about how much.
You predicted this
Not what you predicted
Lilac purple — protein detected.
Milk is rich in casein and whey proteins. Clear lilac.
What you may write down: “Milk contains protein.” Nothing about how much.
You predicted this
Not what you predicted
Orange-brown — unchanged — not detected.
None. Egg white has essentially no carbohydrate.
What you may write down: “No starch was detected in egg white under these conditions.” Not “there is none”.
You predicted this
Not what you predicted
Blue — unchanged — not detected.
Negative, and truly so — egg white is about 90% water and 10% protein, with no sugar to speak of.
What you may write down: “No reducing sugar was detected in egg white under these conditions.” Not “there is none”.
You predicted this
Not what you predicted
Clear — unchanged — not detected.
The lipid in an egg is all in the yolk. Separate them and the white gives a clean negative.
What you may write down: “No lipid was detected in egg white under these conditions.” Not “there is none”.
You predicted this
Not what you predicted
Lilac purple — protein detected.
The clearest positive in the lesson. Egg white is almost pure protein in water.
What you may write down: “Egg white contains protein.” Nothing about how much.
You predicted this
Not what you predicted
Orange-brown — unchanged — not detected.
No starch — in a ripe apple it has all been converted to sugar. An unripe apple would test positive, which is a nice reminder that a food is not a fixed thing.
What you may write down: “No starch was detected in apple juice under these conditions.” Not “there is none”.
You predicted this
Not what you predicted
Brick red — reducing sugar detected.
Fructose and glucose, both reducing sugars. Rapid brick red.
What you may write down: “Apple juice contains reducing sugar.” Nothing about how much.
You predicted this
Not what you predicted
Clear — unchanged — not detected.
Nothing. Fruit juice is sugar and water.
What you may write down: “No lipid was detected in apple juice under these conditions.” Not “there is none”.
You predicted this
Not what you predicted
Blue — unchanged — not detected.
Under 0.3% protein. Negative on the school test, and not quite zero in the fruit.
What you may write down: “No protein was detected in apple juice under these conditions.” Not “there is none”.
You predicted this
Not what you predicted
Orange-brown — unchanged — not detected.
No starch.
What you may write down: “No starch was detected in olive oil under these conditions.” Not “there is none”.
You predicted this
Not what you predicted
Blue — unchanged — not detected.
No sugar. Note that a negative here tells you nothing about the energy in the tube — olive oil carries more energy per gram than anything else on the bench.
What you may write down: “No reducing sugar was detected in olive oil under these conditions.” Not “there is none”.
You predicted this
Not what you predicted
Cloudy white — lipid detected.
Textbook positive: a dense white cloud the moment the ethanol hits the water.
What you may write down: “Olive oil contains lipid.” Nothing about how much.
You predicted this
Not what you predicted
Blue — unchanged — not detected.
No protein.
What you may write down: “No protein was detected in olive oil under these conditions.” Not “there is none”.
What a colour change is worth
Three questions these tests cannot answer.
How much is there?
The test is qualitative. Colour intensity also changes with heating time, volume and concentration, none of which you controlled — so a deeper colour is not evidence of more nutrient.
Is there really none?
A negative means below what this test can detect, with this sample, prepared this way. Potato does contain protein; Biuret cannot see it at 2%.
Is this food healthy?
Nothing in a colour change is about health. Olive oil is one positive out of four and is a required nutrient; a glucose solution is one positive and is nothing but fuel.
Key fact
A food test is qualitative: it says present or not detected. It never says how much, and “not detected” is a fact about your test, not about the food.
Think again
“The redder the Benedict's went, the more sugar there was.”
Tempting, and almost useful, but not something you may claim from a school test. The colour does move through green, yellow and orange to brick red as more sugar is present — but it also moves that way if you heat it for longer, use more Benedict's, or start with a warmer bath. In your experiment those things were not controlled, so a redder tube is not evidence of more sugar; it is evidence that something was different, and you cannot say what. To make the colour mean an amount you would have to fix the volume, the concentration of Benedict's, the temperature and the time, and compare against tubes of known sugar concentration. That is a different experiment, and it is the one a laboratory actually runs.
“The iodine stayed orange, so there is no starch in it.”
Write the two sentences next to each other and the gap is obvious. The test did not detect starch is what you observed. There is no starch is a claim about the food. Between them sit at least four ways the first can be true while the second is false: too little starch to see, iodine too dilute, the sample not crushed so the iodine never reached the inside, or the starch already broken down to sugar — which is exactly what happens if you chew bread for two minutes before testing it. Negative results are the ones that need the most care, because a negative feels like nothing happened, and nothing happening is very easy to achieve by accident.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Read the result
A student shakes double cream with ethanol, pours the ethanol into water and gets a thick white cloud. What may they write down?
Rung 2 · The one that catches people
Bread gives a strong blue-black with iodine. A student chews a piece for two minutes, spits it out, tests it again, and now gets orange-brown and a positive Benedict’s. What happened?
Rung 3 · Write the honest conclusion
A group tests potato with Biuret and gets no colour change. They conclude “potato contains no protein”. Potato is in fact about 2% protein. Explain what is wrong with their conclusion, write a version you could defend, and say what you would change to detect the protein.
Rung 4 · Take it somewhere new
A company claims its cereal bar is “protein-packed and sugar-free”. You have iodine, Benedict’s, ethanol and Biuret. Say which tests you would run, what result would support each claim, and which of the two claims you could never fully check with this equipment.
Key note
Iodine turns blue-black with starch. Benedict's solution turns from blue to brick red with reducing sugar, on heating. Ethanol then water turns cloudy white with lipid. Biuret solution turns from blue to lilac with protein. Each result is positive or not detected — never an amount.
Going further
Benedict's test does not detect all sugars. It needs a sugar that can hand an electron to copper — a reducing sugar — which glucose and fructose can do and sucrose, ordinary table sugar, cannot. So a spoonful of caster sugar in water gives a blue tube and a perfectly negative result, and a student who concludes there is no sugar in sugar has followed the method correctly and reasoned wrongly. Boil the same solution with a little acid first, splitting the sucrose into glucose and fructose, and it goes brick red. The test was never asking “is there sugar here” — it was asking a narrower question that happens to be easy to confuse with that one.
Before this lesson
Connects to
- Enzymes in digestion
- Testing the model
Where the same question — what a result actually licenses you to claim — is asked of a model rather than of a tube.
At GCSE this becomes
- Required practical: qualitative food tests, plus quantitative comparison against known standards.
Where to next
Ask Mr Badmus AI
Unsure what a negative result lets you write down?
Benedict's and Biuret solutions are irritants and Biuret contains sodium hydroxide — wear eye protection, use a water bath rather than a naked flame, and never taste anything on the bench. Ethanol is highly flammable: no Bunsen alight while it is open.
Lesson content © MrBadmusAI.