Energy Values in Food Samples

Aim

To determine and compare the energy value (calorific value) of different food samples, such as peanut, bread, potato chip and coconut, by burning each sample and measuring the rise in temperature it produces in a fixed volume of water.

Variables

• Manipulated variable: the type of food sample burnt (e.g. peanut, bread, potato chip, coconut).
• Responding variable: the rise in temperature of the water, used to calculate the energy released per gram of food.
• Controlled variables: the volume of water in the boiling tube, the initial temperature of the water, the distance between the flame and the boiling tube, shielding from draughts, and the type of container used.

Materials and apparatus

• Dried food samples mounted on a mounted needle (e.g. peanut, bread, potato chip, coconut)
• Boiling tube
• Water
• Thermometer
• Retort stand and clamp
• Electronic balance
• Matches or a lighter
• Draught shield or screen
• Heatproof mat

Procedure

1. Measure a fixed volume of water (e.g. 20 cm3) into a boiling tube using a measuring cylinder, and record its initial temperature with a thermometer.
2. Weigh a food sample on the electronic balance and record its mass.
3. Mount the food sample on a mounted needle and position it directly under the boiling tube, clamped at a fixed height on the retort stand.
4. Ignite the food sample with a match or lighter and let it burn completely under the boiling tube, using a draught shield to protect the flame from air currents.
5. Stir the water gently with the thermometer and record the maximum temperature reached.
6. Calculate the temperature rise (final temperature minus initial temperature), then calculate the energy value per gram of food using E = mcΔT ÷ mass of food burnt, where m is the mass of water and c is the specific heat capacity of water.
7. Repeat the procedure for each of the other food samples, keeping the volume of water and all other conditions the same.

Expected results

Burning each food sample raises the temperature of the water by a different amount, depending on how much energy the food releases per gram when it burns. Typical results for a class experiment are shown below.

Conclusion

Different food samples release different amounts of energy per gram when burnt. In this experiment, foods that are rich in fat, such as peanut and coconut, produced a greater temperature rise per gram burnt than foods that are mainly carbohydrate, such as bread and potato chip, showing that fats generally have a higher energy value than carbohydrates.

The calculated energy values are usually much lower than the true energy values printed on food labels, because most of the heat produced escapes into the surrounding air rather than heating the water; the method is therefore more reliable for comparing food samples against each other than for finding an absolute energy value.

Paper 3-style questions

These original questions are written in the style of Paper 3 to practise the science process skills this experiment assesses.

Question 1 (hypothesis). A student burns equal masses of peanut and biscuit under identical boiling tubes of water. Write a suitable hypothesis.

Model answer. The peanut, which contains more fat, produces a greater temperature rise in the water per gram burnt than the biscuit, so it has a higher energy value.

Question 2 (variables). State the manipulated and responding variables, and describe how one controlled variable is kept constant. Model answer. Manipulated variable: the type of food sample burnt.

Responding variable: the temperature rise of the water. Controlled variable: the volume of water, kept constant by measuring the same volume, such as 20 cm3, with a measuring cylinder for every sample.

Question 3 (tabulation and calculation). A peanut of mass 0.40 g raises the temperature of 20 cm3 of water by 25 °C. Calculate its energy value in joules per gram, taking the specific heat capacity of water as 4.2 J g⁻¹ °C⁻¹.

Model answer. Heat gained by the water = mcΔT = 20 × 4.2 × 25 = 2100 J; energy value = 2100 ÷ 0.40 = 5250 J g⁻¹.

Question 4 (inference). In one trial a coconut sample gives a smaller temperature rise than a peanut sample of the same mass. State what this tells you, and give one reason the calculated value is lower than the value printed on a food label.

Model answer. The coconut releases less energy per gram than the peanut, so its energy value is lower; the calculated value is lower than the label value because much of the heat is lost to the surroundings rather than heating the water, and the sample may not burn completely.

Safety

• Tie back long hair and loose sleeves before lighting the food sample, and keep the flame away from other flammable materials.
• Hold the burning sample only on the mounted needle, never with bare fingers, because it stays hot after the flame goes out.
• Stand the apparatus on a heatproof mat and let the boiling tube and mounted needle cool before handling them.
• Point the mouth of the boiling tube away from yourself and others, and work in a well-ventilated area.

Common mistakes

Source:SRC-DSKP-EN

Frequently asked questions