Biological Molecules » Food Testing Practical

What you'll learn this session

Study time: 30 minutes

The chemical tests used to identify carbohydrates, proteins and lipids in food samples
How to perform Benedict's test for reducing sugars
How to use the iodine test to identify starch
How to conduct the Biuret test for proteins
How to use the emulsion test to detect lipids
How to interpret test results and avoid common errors

Introduction to Food Testing

Food contains various biological molecules that our bodies need to function properly. Being able to test for these molecules helps us understand what's in our food and is an important skill in biology. In this practical session, we'll learn how to identify the main biological molecules in food using simple chemical tests.

Key Definitions:

Biological molecules: Large molecules that are essential for life, including carbohydrates, proteins and lipids.
Reagent: A substance used in a chemical reaction to detect or measure another substance.
Positive result: A visible change that confirms the presence of the molecule being tested for.
Negative result: No change, indicating the molecule being tested for is absent.

🍛 Why Test Food?

Food testing is used in many real-world situations:

Food manufacturers check nutritional content
Scientists analyse diets for health research
Doctors test for food allergies
Quality control in food production

🔬 Lab Safety First!

Always remember these safety rules:

Wear safety goggles and lab coat
Some reagents are corrosive or toxic
Never taste any chemicals
Report spills immediately
Wash hands after the practical

Testing for Carbohydrates

Carbohydrates are energy-providing molecules made of carbon, hydrogen and oxygen. There are two main types we can test for: reducing sugars (like glucose) and non-reducing sugars (like starch).

Benedict's Test for Reducing Sugars

This test identifies simple sugars like glucose and fructose, which have the ability to reduce copper(II) ions in Benedict's solution.

🔮 Method

Place 2ml of food sample in a test tube
Add 2ml of Benedict's solution (blue liquid)
Place the test tube in a water bath at 80°C
Heat for 5 minutes and observe any colour change

✅ Positive Result

The blue solution changes colour:

Green → small amount of sugar
Yellow → moderate amount
Orange → large amount
Brick-red → very large amount

❌ Negative Result

Solution remains blue, indicating no reducing sugars are present.

Common errors:

Not heating long enough
Temperature too low
Sample too dilute

Iodine Test for Starch

Starch is a complex carbohydrate made up of many glucose molecules joined together. It's found in foods like potatoes, rice and bread.

🔮 Method

Place a small amount of food sample on a white tile
Add a few drops of iodine solution (yellow-brown colour)
Observe any colour change immediately

✅ Results

Positive: Iodine turns blue-black when starch is present

Negative: Iodine remains yellow-brown when no starch is present

This test is quick and works at room temperature - no heating required!

Case Study: The Importance of Starch Testing

People with certain digestive disorders need to limit starch in their diet. Food manufacturers use the iodine test during product development to measure starch content. This helps them create suitable foods for people with conditions like diabetes, where controlling carbohydrate intake is essential. The test is also used in industry to check if starch has been completely broken down in products like glucose syrup.

Testing for Proteins

Proteins are large molecules made up of amino acids. They're essential for growth and repair of tissues in our bodies.

Biuret Test

The Biuret test identifies peptide bonds, which connect amino acids in proteins. This makes it specific for proteins and not other biological molecules.

🔮 Method

Place 2ml of food sample in a test tube
Add 2ml of sodium hydroxide solution
Add a few drops of copper(II) sulfate solution
Shake gently and observe colour change

✅ Positive Result

Solution turns purple or lilac, indicating proteins are present.

The intensity of the colour relates to the amount of protein:

Pale purple → small amount
Deep purple → large amount

❌ Negative Result

Solution remains blue or turns light blue, showing no proteins are present.

Warning: Sodium hydroxide is caustic - handle with care!

Testing for Lipids (Fats and Oils)

Lipids are fatty substances that don't dissolve in water. They include fats (solid at room temperature) and oils (liquid at room temperature).

Emulsion Test

This test works because lipids don't mix with water but can form an emulsion when shaken with ethanol.

🔮 Method

Place a small amount of food sample in a test tube
Add 2ml of ethanol and shake well
Pour the mixture into another test tube containing 2ml of water
Shake gently and observe what happens

✅ Results

Positive: A cloudy white emulsion forms when lipids are present

Negative: Solution remains clear when no lipids are present

The ethanol dissolves the lipids, but when added to water, the lipids can't stay dissolved and form tiny droplets that scatter light, creating the cloudy appearance.

Real-World Application: Food Labelling

Food manufacturers must accurately label the nutritional content of their products. They use more sophisticated versions of these tests to determine the exact amounts of carbohydrates, proteins and lipids. This information helps consumers make informed choices about their diet. For example, athletes might choose high-protein foods, while someone watching their weight might look for low-fat options.

Practical Tips and Common Errors

💡 Top Tips for Accurate Results

Always prepare a control sample (known positive and negative)
Make sure food samples are properly prepared (crushed, dissolved, etc.)
Follow the method precisely - timing and quantities matter!
Record results immediately after the test
Clean equipment thoroughly between tests to avoid contamination

⚠ Common Errors to Avoid

Using too much or too little reagent
Not heating for long enough in Benedict's test
Confusing colour changes (e.g., green vs. blue)
Cross-contamination between samples
Forgetting to record observations promptly
Not using a water bath for Benedict's test (direct heating can cause false results)

Summary of Food Tests

Here's a quick reference guide to help you remember the key points about each test:

🍨 Carbohydrates

Benedict's test (reducing sugars):

Blue → green/yellow/orange/red

Iodine test (starch):

Yellow-brown → blue-black

🥩 Proteins

Biuret test:

Blue → purple/lilac

Detects peptide bonds between amino acids

🥛 Lipids

Emulsion test:

Clear → cloudy white emulsion

Works because lipids are insoluble in water but soluble in ethanol

Remember, these tests are qualitative (they tell you if something is present) rather than quantitative (they don't tell you exactly how much is present). For more precise measurements, scientists use more advanced techniques in laboratories.

🧠 Test Your Knowledge!