10.3 - Food Tests
The tests
Aim
To find out which food molecules, starch, reducing sugars, protein, and lipid (fat), are present in a food, using a different reagent for each. Each reagent changes colour if its molecule is there.
Background
- Foods contain different types of molecule, such as starch, reducing sugars, protein, and lipids.
- Each food test uses a chemical, a reagent, that reacts with just one type of molecule to give a colour you can recognise.
- You can't see the molecules in a food, but you can see the colour a reagent turns.
- So in this practical you add each reagent to a separate portion of a food solution, and use the colours to tell which nutrients the food contains.
Method
- Prepare the food solution - Grind the food with distilled water, and filter it to get a clear food solution.
- Split it into portions - Put the same volume of food solution into a separate test tube for each of the starch, sugar, and protein tests.
- Carry out each test - Add the reagent as the table shows, using the same volume each time and fresh reagents, and record the colour.
- Test a positive control - Run each test on a food you know contains the nutrient, so a negative result means the nutrient is absent, not that the test has failed.
Each nutrient has its own test and colour change.
| Nutrient | What you do | Positive result (present) | Negative result (absent) |
|---|---|---|---|
| Starch | Add a few drops of iodine solution (no heating) | Orange-brown changes to blue-black | Stays orange-brown |
| Reducing sugar | Add Benedict's solution and heat in a water bath at about 75 °C for 5 minutes | Blue changes through green and yellow to brick-red (the more sugar, the further it goes) | Stays blue |
| Protein | Add Biuret reagent and mix (no heating) | Blue changes to purple/lilac | Stays blue |
| Lipid (fat) | Shake a small sample of the food itself (not the filtered solution) with ethanol, then pour the ethanol into water | A cloudy white layer forms | Stays clear |
Safety
- Eye protection - Wear it throughout.
- Ethanol - It's flammable, so heat Benedict's solution in a water bath rather than over a flame.
- Biuret reagent - It contains sodium hydroxide, which is corrosive, so handle it carefully and wash off any splashes.
Common exam questions
Question 1
Describe how you would test a food for reducing sugars, including the result for a positive test.
Add Benedict's solution to the food solution and heat it in a water bath at about 75 °C for 5 minutes. If a reducing sugar is present, the solution turns from blue to brick-red.
Question 2
A food turned blue-black with iodine, stayed blue with Benedict's, and turned purple with Biuret. Which nutrients does it contain?
Starch (blue-black with iodine) and protein (purple with Biuret). It contains no reducing sugar, because Benedict's stayed blue.
Question 3
Why is Benedict's solution heated in a water bath and not over a Bunsen flame?
A water bath gives even, gentle heat and stops the contents spitting. It also keeps a flame away from the flammable ethanol used nearby.
Question 4
Why might a strongly coloured food give an unclear result?
Its own colour can hide the colour change of the test, making it hard to judge. Using a colourless food solution, or comparing against a known control, makes the change easier to see.