Welcome to Nutrition and Food Tests!
Have you ever wondered why we need to eat a variety of foods, or how scientists can tell what chemicals are hidden inside a slice of bread or a drop of milk? In this chapter, we explore what makes up a balanced diet, how your body uses different nutrients, how to perform classic food tests in the lab, and how to measure the energy content of food.
Don't worry if this seems like a lot to remember at first! We will break each concept down into small, simple steps with easy memory tricks to help you ace your GCSE exams.
1. The Components of a Balanced Diet
A balanced diet provides all the essential nutrients in the correct proportions to keep your body healthy and functioning properly. Think of your body like a house: you need bricks to build it, fuel to keep it warm, and maintenance workers to keep everything running smoothly.
The Seven Essential Food Groups
1. Carbohydrates
• Function: Provide our main source of usable energy.
• Simple carbohydrates (Sugars): Give fast-acting, short-term energy (e.g., glucose, found in fruits and sweets).
• Complex carbohydrates (Starch): Broken down slowly to give sustained, long-term energy (e.g., bread, pasta, potatoes, rice).
2. Proteins
• Function: Essential for growth and the repair of damaged tissues. They are also used to build enzymes and hormones.
• Building blocks: Proteins are large molecules made up of smaller units called amino acids.
• Good sources: Meat, fish, eggs, beans, pulses, and nuts.
3. Lipids (Fats and Oils)
• Function: Long-term energy storage, insulation (keeping us warm), and protecting vital organs.
• Building blocks: One lipid molecule is made of one glycerol attached to three fatty acids.
• Good sources: Butter, vegetable oils, cheese, and oily fish.
4. Vitamins
• Function: Needed in tiny amounts for healthy cell function and preventing deficiency diseases.
• Vitamin C: Maintains healthy skin, gums, and connective tissue. A severe lack of Vitamin C causes scurvy (bleeding gums and poor wound healing). Found in citrus fruits.
• Vitamin D: Helps the body absorb calcium for strong bones and teeth. A severe lack causes rickets (soft, bendy bones). Found in dairy, eggs, and made by skin in sunlight.
5. Minerals
• Function: Inorganic elements needed in small quantities for specific bodily structures and chemical processes.
• Calcium: Needed for strong bones and healthy teeth. Found in milk and dairy products.
• Iron: An essential component of haemoglobin, the red pigment in red blood cells that carries oxygen. A severe lack leads to anaemia (tiredness and lack of energy). Found in red meat and spinach.
6. Dietary Fibre (Roughage)
• Function: Fibre cannot be digested by human enzymes, but it provides bulk that the muscles of the gut can push against. This keeps food moving through the digestive system by peristalsis and prevents constipation.
• Good sources: Wholemeal bread, oats, bran cereals, vegetables, and fruit skins.
7. Water
• Function: Acts as a biological solvent, transports substances around the body in blood plasma, and helps cool the body down through sweating.
• Good sources: Drinking water, juice, and watery foods like cucumbers and watermelon.
Did you know? Water makes up around \(60\%\) to \(70\%\) of your total body mass!
Key Takeaway: A Balanced Diet
A healthy diet requires seven parts: carbohydrates (energy), lipids (stored energy/insulation), proteins (growth/repair), vitamins and minerals (maintenance and health), fibre (digestive movement), and water (solvent and transport).
2. Diagnostic Food Tests
In the laboratory, you need to know how to identify the presence of starch, reducing sugars (glucose), proteins, and fats using specific chemical reagents.
A. Test for Starch (Iodine Solution)
• Method: Place a small food sample on a spotting tile. Add a few drops of iodine solution.
• Initial colour: Yellow-brown.
• Positive result: Turns blue-black.
• Negative result: Remains yellow-brown.
B. Test for Reducing Sugar (Benedict's Reagent)
• Method: Place the food sample into a test tube and add Benedict's reagent. Place the tube into a hot water bath (at least \(80^\circ\text{C}\)) for \(5\) minutes.
• Initial colour: Light blue.
• Positive result: Changes from blue to green \(\rightarrow\) yellow \(\rightarrow\) orange \(\rightarrow\) brick-red precipitate (the colour depends on the concentration of sugar present; brick-red indicates a high concentration).
• Negative result: Remains blue.
• Common Exam Trap: Forgetting to state that Benedict's test must be heated in a water bath!
C. Test for Protein (Biuret Reagent)
• Method: Add an equal volume of Biuret reagent (or sodium hydroxide followed by copper sulfate) to the liquid food sample in a test tube. Shake gently.
• Initial colour: Blue.
• Positive result: Turns lilac / purple.
• Negative result: Remains blue.
• Memory Trick: Biuret turns Purple for Protein (or think of the letter P in Purple).
D. Test for Fats / Lipids (Ethanol Emulsion Test)
• Method: Dissolve the food sample in a small volume of ethanol in a test tube. Pour the clear liquid into a tube containing cold water.
• Initial colour: Colourless.
• Positive result: A cloudy-white emulsion forms (like tiny droplets of fat suspended in water).
• Negative result: Liquid remains colourless and clear.
• Safety Note: Ethanol is highly flammable, so keep it away from open Bunsen burner flames.
Summary Table of Food Tests
• Starch: Reagent = Iodine | Initial = Yellow-brown | Positive = Blue-black | Special condition = None.
• Sugar (Glucose): Reagent = Benedict's | Initial = Blue | Positive = Brick-red precipitate | Special condition = Heat in a water bath.
• Protein: Reagent = Biuret | Initial = Blue | Positive = Lilac / Purple | Special condition = None.
• Fats / Lipids: Reagent = Ethanol + water | Initial = Colourless | Positive = Cloudy-white emulsion | Special condition = No naked flames.
Key Takeaway: Food Tests
Make sure you can state the reagent, the starting colour, and the positive colour change for each of the four main tests, remembering that Benedict's requires a hot water bath!
3. Measuring Energy in Food (Calorimetry)
We can measure the amount of chemical energy stored in dry food samples (such as a piece of pasta, bread, or a peanut) by burning them and using the heat released to warm up a measured volume of water.
The Experimental Setup
1. Measure a known volume of water (e.g., \(20\text{ cm}^3\)) into a boiling tube and record its starting temperature (\(T_1\)) using a thermometer.
2. Weigh the mass of the dry food sample using an electronic balance.
3. Mount the food sample onto a mounted needle and ignite it using a Bunsen burner flame.
4. Immediately hold the burning food underneath the boiling tube of water until the food has completely burned out (relight it if it goes out, until only ash remains).
5. Stir the water gently with the thermometer and record the highest final temperature reached (\(T_2\)).
Calculating the Energy Content
The energy released can be calculated using the formula:
\(\text{Energy released (J)} = \text{mass of water (g)} \times 4.2 \times \text{temperature rise } (^\circ\text{C})\)
Note: \(1\text{ cm}^3\) of water has a mass of \(1\text{ g}\). The value \(4.2\text{ J/g}^\circ\text{C}\) is the specific heat capacity of water (the energy required to heat \(1\text{ g}\) of water by \(1^\circ\text{C}\)).
To compare different foods fairly, we calculate the energy released per gram of food:
\(\text{Energy per gram (J/g)} = \frac{\text{Energy released (J)}}{\text{mass of food sample (g)}}\)
Sources of Experimental Error
Simple school calorimetry experiments usually give values that are much lower than the actual energy printed on food labels. Why does this happen?
• Heat loss to the surroundings: Much of the heat radiates into the air rather than heating the water.
• Incomplete combustion: Some of the food does not burn completely, leaving unburned carbon/soot.
• Heat absorbed by equipment: The glass boiling tube absorbs some of the heat energy.
• Evaporation: Some water may evaporate during heating.
How Can the Experiment Be Improved?
• Place a heat shield or draught excluder around the apparatus to reduce heat loss to the air.
• Use a bomb calorimeter in professional labs, which surrounds the burning chamber with water, uses pure oxygen to ensure complete combustion, and includes a stirrer to distribute heat evenly.
Key Takeaway: Food Energy
Energy from food is transferred into thermal energy in water. We use the formula \(\text{Energy} = \text{mass of water} \times 4.2 \times \text{temperature change}\), but simple lab tests underestimate energy due to heat loss and incomplete burning.
4. Quick Revision Check
Before moving on, test yourself on these essential facts:
• What reagent tests for protein, and what is the positive result? (Biuret reagent; changes from blue to lilac/purple).
• Why do we need dietary fibre? (To provide bulk for peristalsis and prevent constipation).
• Which nutrient is tested using Benedict's solution, and what extra step is needed? (Reducing sugars/glucose; must be heated in a hot water bath).
• What is the deficiency disease caused by a lack of Vitamin C? (Scurvy).
• Why does a peanut release more energy per gram than a piece of bread? (Peanuts are rich in lipids/fats, which store more energy per unit mass than carbohydrates).