Identification of Food Components
Senior 2 · Biology
Identification of Food Components
Lesson 2 of 3
20 min
Let's begin
A ripe banana tastes sweeter than a green one. Is that because starch has turned into sugar? You do not need to guess: two simple laboratory tests can tell you which carbohydrate a food contains, and roughly how much.
Food scientists, nutritionists and health workers who test urine for sugar all use these same colour reactions.
Most tests work on a liquid. Cut solid food into small pieces, grind it with a little distilled water using a mortar and pestle, and then filter it or let it settle. Always run a control with distilled water alone, so you can see the "negative" colour for comparison.
Safety: wear eye protection. Heat test tubes in a water bath, never directly in a flame, and point them away from people.
| Colour change | Meaning |
|---|---|
| stays yellow-brown (orange-brown) | no starch |
| turns blue-black | starch present |
The test needs no heating. Cassava, potato, maize flour and green banana all turn blue-black.
A reducing sugar changes the blue copper(II) ions into copper(I) oxide, which forms a coloured precipitate (a solid that separates out of the liquid). The more sugar there is, the further the colour moves along this scale:
| Final colour | Amount of reducing sugar |
|---|---|
| stays blue | none |
| green | a trace |
| yellow | a little |
| orange | a moderate amount |
| brick-red | a lot |
Because the colour indicates how much sugar is present, Benedict's test is semi-quantitative: it gives an estimate, not an exact amount.
Sucrose gives a negative (blue) Benedict's test. To detect it, first boil the sample with dilute hydrochloric acid for a few minutes, which splits sucrose into glucose and fructose. Then neutralise the acid by adding sodium hydrogencarbonate until fizzing stops, and do Benedict's test again. If the second test is positive when the first was negative, a non-reducing sugar was present.
Question: A student tested green and ripe banana. Results: green banana: iodine blue-black, Benedict's green; ripe banana: iodine yellow-brown, Benedict's brick-red. Explain the results.
Solution: The green banana contains a lot of starch (blue-black) and only a trace of reducing sugar (green). The ripe banana contains almost no starch (the iodine stays yellow-brown) but a lot of reducing sugar (brick-red). During ripening, enzymes break starch down into sugars, which is why ripe bananas taste sweet.
Tip
Memory hook: iodine is cold and blue-black; Benedict's is hot and brick-red. Only Benedict's needs heating.
Exam technique: always give BOTH colours, "from yellow-brown to blue-black" or "from blue to brick-red". Giving only the final colour usually loses a mark.
Describe how to test a sample of maize flour for starch, and give the positive result.
Solution: Put a little flour on a white tile and add a few drops of iodine solution. A change from yellow-brown to blue-black shows that starch is present.
Why is a boiling water bath used instead of heating the tube directly in a flame?
Solution: It heats the tube evenly and safely, so the mixture cannot boil over or spurt out, and all tubes are heated to the same temperature, which makes the test fair.
Three juices gave these Benedict's colours: A green, B brick-red, C yellow. Rank them from most to least reducing sugar.
Solution: B (brick-red) > C (yellow) > A (green).
A sugar solution stays blue in Benedict's test, but turns orange after being boiled with dilute acid, neutralised and tested again. What sugar could it be?
Solution: A non-reducing sugar, most likely sucrose. The acid split it into glucose and fructose, which are reducing sugars.
Summary
Sources and credits
The food-test procedures, the colour tables, the banana worked example and the practice questions are written by Ganzaa.
