Energy in Food Practical

Explained

Measuring the energy in food

This practical answers a simple question with a simple method. Burn a piece of food underneath a tube of water, measure how much the water warms up, and use that temperature rise to work out how much energy the food released. Everything examinable about it follows from that one chain.

The method

• Measure a known mass of water into a boiling tube and record its starting temperature.
• Weigh the food sample.
• Set the food alight and hold it under the tube until it stops burning.
• Record the highest temperature the water reaches.
• Reweigh the food to find the mass that actually burned.

Reweighing matters. The mass burned is the starting mass minus whatever is left, and using the starting mass instead makes every later figure wrong.

The calculation

Energy in joules equals the mass of water multiplied by 4.2 multiplied by the temperature rise. The 4.2 is the energy in joules needed to raise one gram of water by one degree Celsius.

Two traps live in this equation. The mass is the mass of the water, not of the food. And the temperature rise is the final temperature minus the starting one, not the final temperature on its own.

To compare different foods fairly, divide the energy by the mass of food burned to get energy per gram. Without that step you are comparing sample sizes rather than foods.

Why the answer is always too low

Every result from this practical underestimates the true energy content, and knowing why is worth more marks than the calculation itself.

• Much of the energy heats the surrounding air rather than the water.
• Some is lost through the glass of the boiling tube.
• The food often does not burn completely, so some energy is never released.

Say that energy is transferred to the surroundings rather than that it disappears. Improvements follow directly from the causes: shield the apparatus from draughts, insulate the tube, and hold the burning food as close to the tube as is safe.

Controls that make it a fair test

Use the same volume of water each time, the same starting temperature, and the same distance between the food and the tube. Stir the water before taking the final reading so the temperature is even throughout, otherwise the thermometer reads only the part nearest the flame.