Required practical - Food testing (AQA GCSE Biology Combined Science): Revision Notes
Required practical - Food testing
What are food tests?
Food tests are qualitative tests that help us identify what nutrients are present in different foods. These tests tell us if a substance is there or not, but they don't tell us exactly how much is present.
We can test for four main types of nutrients:
- Reducing sugars (like glucose and fructose)
- Starch
- Lipids (fats and oils)
- Proteins
Qualitative tests are different from quantitative tests. While qualitative tests simply tell us whether a nutrient is present or absent, quantitative tests would measure the exact amount of each nutrient present in the sample.
Aim of this practical
The goal is to investigate different food samples and work out which nutrients they contain using chemical tests.
This practical helps you develop important laboratory skills including careful observation, accurate recording of results, and understanding how chemical reactions can be used as diagnostic tools in biology.
Equipment you need
- Safety equipment: Eye protection
- Basic lab equipment: Test tubes, test tube rack, measuring equipment
- Heating equipment: Hot water bath, distilled water
- Chemical reagents:
- Benedict's solution (for sugars)
- Iodine solution (for starch)
- Sudan III stain in ethanol (for lipids)
- Biuret reagent (for proteins)
- Food samples to test
Important safety notes
Critical Safety Information
- Always use a hot water bath instead of a Bunsen burner - this is much safer
- Make your hot water bath by half-filling a beaker with hot water from a kettle (not boiling)
- Ethanol is highly flammable - keep away from flames
- If you spill chemicals on skin or clothes, wash immediately with water
- If you have large food pieces, cut them up small or grind them with a pestle and mortar first
Method for each test
Setting up samples
Before starting any test, mix a small amount of your food sample with about 2 cm³ of distilled water in a test tube.
Test for reducing sugars (Benedict's test)
The Benedict's test is used to detect reducing sugars such as glucose and fructose in food samples.
- Add a few drops of Benedict's solution to your food sample
- Place the test tube in a hot water bath for 5 minutes
- Look for any colour changes
- Positive result: Blue → green → orange → red (depending on how much sugar is present)
- Negative result: Stays blue
Glucose and fructose are reducing sugars, but table sugar (sucrose) is not. This is because reducing sugars have free anomeric carbons that can react with Benedict's solution, while sucrose's anomeric carbons are locked in a glycosidic bond.
Test for starch (Iodine test)
The iodine test is a rapid test for detecting starch that works at room temperature.
- Add a few drops of iodine solution to your food sample
- Mix gently and observe any colour changes
- Positive result: Orange-brown → blue-black
- Negative result: Stays orange-brown
Test for lipids (Sudan III test)
This test detects the presence of fats and oils by using a lipid-soluble dye.
- Add a few drops of Sudan III stain to your food sample
- Shake the test tube to mix well
- Look to see if a separate layer forms on top
- Positive result: Red layer appears on top
- Negative result: No separate layer forms (stays colourless on top)
Alternative method: You can use ethanol instead and look for a cloudy white layer when water is added. This is called the emulsion test and works because lipids are insoluble in water but soluble in ethanol.
Test for proteins (Biuret test)
The Biuret test detects peptide bonds present in proteins and polypeptides.
- Add an equal volume of Biuret reagent to your food sample
- Shake the test tube to mix
- Look for colour changes
- Positive result: Blue → pink or purple
- Negative result: Stays blue
Worked Example: Testing Unknown Food Sample
Sample: Unknown white powder
Step 1: Benedict's test
- Added Benedict's solution and heated for 5 minutes
- Result: Remained blue (negative for reducing sugars)
Step 2: Iodine test
- Added iodine solution at room temperature
- Result: Changed from orange-brown to blue-black (positive for starch)
Step 3: Sudan III test
- Added Sudan III stain and shook
- Result: No red layer formed (negative for lipids)
Step 4: Biuret test
- Added equal volume of Biuret reagent
- Result: Remained blue (negative for proteins)
Conclusion: The unknown sample contains starch but no reducing sugars, lipids, or proteins. This sample could be corn starch or wheat flour.
Recording your results
Make a table to record your results clearly. Here's what you should expect to see:
| Test reagent | What it detects | Negative result colour | Positive result colour |
|---|---|---|---|
| Benedict's solution | Reducing sugars | Blue | Green → orange → red |
| Iodine solution | Starch | Orange-brown | Blue-black |
| Sudan III stain | Lipids | Colourless upper layer | Red upper layer |
| Biuret reagent | Proteins | Blue | Pink or purple |
When recording results, be specific about colour changes. For Benedict's test, note that the intensity of the colour (green vs orange vs red) indicates the concentration of reducing sugars present.
Why these tests work
Understanding the scientific principles behind each test helps explain why specific colour changes occur:
- Benedict's test: Reducing sugars react with Benedict's solution when heated, causing the copper sulphate to be reduced and creating the colour change from blue through green, orange, to red
- Iodine test: Iodine forms a complex with starch molecules, specifically with the amylose component, creating the characteristic blue-black colour
- Sudan III test: This dye dissolves in fats but not in water, so it separates out and forms a distinct coloured layer when lipids are present
- Biuret test: Proteins contain peptide bonds that react with the copper ions in Biuret reagent, forming a coordination complex that appears pink or purple
Key Points to Remember:
- Qualitative tests show if something is present, not how much
- Always use a hot water bath for safety, never direct flame
- Benedict's test needs heating for 5 minutes to work properly
- Iodine test works immediately at room temperature
- Sudan III creates a separate coloured layer if lipids are present
- Biuret reagent changes from blue to pink/purple with proteins
- Record all colour changes clearly in a results table
- Each test detects a specific type of biological molecule through characteristic chemical reactions