Edexcel iGCSE Biology-2.9 practical: food samples- Study Notes- New Syllabus

Practical: Investigate Food Samples for Glucose, Starch, Protein and Fat

📝 Introduction

• Different foods contain different nutrients.
• Scientists use food tests to find out which nutrient is present in a food sample.
• Each nutrient has its own chemical test.
• A positive result shows that the nutrient is present.
• A negative result shows that the nutrient is absent.
• These tests are qualitative tests, which means they tell us whether a nutrient is present or not, but not the exact amount.

🎯 Aim

To test food samples for the presence of:

• Glucose (reducing sugar)
• Starch
• Protein
• Fat (lipid)

🧪 Food Samples

Some common food samples used in this practical are:

• Glucose solution
• Bread
• Potato
• Milk
• Egg white
• Cheese
• Vegetable oil

🧰 Apparatus and Chemicals

Apparatus

• Test tubes
• Test tube rack
• Test tube holder
• Pipettes or droppers
• Measuring cylinder
• Beaker
• Water bath
• Spotting tile or white tile
• Safety goggles

Chemicals

• Benedict’s solution
• Iodine solution
• Biuret reagent
• Sudan III stain (or Ethanol for the emulsion test)
• Distilled water

⚖️ Controls

• Using controls helps us know whether the test has worked correctly.

Positive Control

• A sample that is already known to contain the nutrient.

Examples:

Negative Control

• Distilled water
• It should give a negative result in every food test.

🍬 Test for Glucose (Reducing Sugar)

Reagent Used

• Benedict’s Solution

Procedure

1. Place about 2 cm³ of the food sample into a test tube.
2. Add 2 cm³ of Benedict’s solution.
3. Mix the contents gently.
4. Place the test tube in a hot water bath for about 2–5 minutes.
5. Observe the final colour.

Positive Result

If glucose is present, the solution changes from:

Blue → Green → Yellow → Orange → Brick Red

The colour depends on the amount of glucose present.

• Green = Small amount
• Yellow = Moderate amount
• Orange = High amount
• Brick red = Very high amount

Negative Result

• Solution remains blue.

Important Points

• Benedict’s test only works after heating.
• A water bath is safer than heating directly over a flame.
• Always use clean test tubes to avoid contamination.

🌾 Test for Starch

Reagent Used

• Iodine Solution

Procedure

1. Place a small amount of food sample on a spotting tile or in a test tube.
2. Add 2–3 drops of iodine solution.
3. Mix gently if necessary.
4. Observe the colour.

Positive Result

Colour changes:

Yellow-brown → Blue-black

This shows starch is present.

Negative Result

Colour stays:

Yellow-brown

Important Points

• Heating is not required.
• Iodine reacts only with starch.
• Even a small amount of starch gives a clear colour change.

🥚 Test for Protein

Reagent Used

• Biuret Reagent

Procedure

1. Place 2 cm³ of food sample into a test tube.
2. Add 2 cm³ of Biuret reagent.
3. Shake gently.
4. Observe the colour.

Positive Result

Colour changes:

Blue → Lilac or Purple

This shows protein is present.

Negative Result

Solution remains:

Blue

Important Points

• Heating is not needed.
• The purple colour appears because the reagent reacts with peptide bonds in proteins.

🥜 Test for Fat (Lipids)

There are two commonly used methods.

Method 1 – Sudan III Test

Reagent Used

• Sudan III Solution

Procedure

1. Place 2 cm³ of food sample into a test tube.
2. Add a few drops of Sudan III solution.
3. Shake gently.
4. Allow the mixture to stand.

Positive Result

• A red-stained oil layer forms on the surface.

Negative Result

• No red oil layer appears.

Method 2 – Ethanol Emulsion Test

Reagents Used

• Ethanol
• Water

Procedure

1. Add the food sample to a test tube.
2. Add ethanol and shake well.
3. Pour the mixture into water.
4. Observe the solution.

Positive Result

• A white cloudy emulsion appears.

Negative Result

• Solution remains clear.

📊 Expected Results

⚠️ Safety Precautions

• Wear safety goggles throughout the practical.
• Tie back long hair.
• Handle hot test tubes carefully using a holder.
• Never point heated test tubes towards yourself or others.
• Benedict’s and Biuret solutions should not touch the skin or eyes.
• Iodine can stain clothes and skin.
• Ethanol is highly flammable, so keep it away from naked flames.
• Wash hands after completing the experiment.

❗ Common Errors and How to Avoid Them

💡 Important Points

• Benedict’s test must be heated.
• Iodine test does not require heating.
• Biuret test does not require heating.
• Sudan III and Ethanol tests are both used to detect fats.
• A colour change indicates that the nutrient is present.
• No colour change means the nutrient is absent.
• These tests only show the presence or absence of nutrients, not the exact quantity.

📋 Summary Table

⚡ Quick Recap
✔ Glucose → Benedict’s solution + heat → Brick red (positive).
✔ Starch → Iodine solution → Blue-black (positive).
✔ Protein → Biuret reagent → Lilac/Purple (positive).
✔ Fat → Sudan III → Red oil layer (positive).
✔ Fat → Ethanol test → White cloudy emulsion (positive).
✔ Benedict’s test is the only test that requires heating.
✔ Always use clean apparatus, correct reagents, and follow safety precautions for accurate results.