(6) Calorific values

Measuring the calorific value of foods e.g. the theoretical energy released on metabolising edible oil or fat using simple calorimeter experiments

Plant based carbohydrate products like starch and sugars, animal fats or vegetable oil products are all oxidised to carbon dioxide and water via many complex chemical reactions.

You can measure the output of energy from metabolising energy rich foods via some simple laboratory experiments.

The most energy rich foods are vegetable oils and animal fats, more so than carbohydrates.

You need to burn a known mass of material in a controlled way that involves trapping the heat energy and gaining a measurement of the chemical potential energy released as thermal energy ('heat').

 

Example 1. Very simple method using a boiling tube

You can ignite a peanut or small lump bread of known mass and hold it in a mounted needle under a boiling tube of cold water, knowing the mass and initial temperature of the water.

After burning so much of it, the flame is extinguished, and, when cool, reweigh the peanut, not forgetting to measure the final temperature.

Suppose, after burning a 2.50 g lump of bread, 1.75 g remains and the temperature of the water rose from 22.0^o to 39.5^oC.

Assume there is 30.0 g (~30 cm³) of water in the boiling tube and the heat capacity of water is 4.20 J per g per degree rise in temperature.

Calculate the energy in kJ released per gram of bread.

Mass of bread burned = 2.50 - 1.75 = 0.75 g.  Temperature rise = 39.5 - 22.0 = 19.5^oC

Heat energy released = mass of water x heat capacity x temperature rise

Chemical potential energy released from the bread = 30 x  4.2 x 19.5 = 2457 J

Energy released from the bread = 2457 / 0.75 = 3276 J/g, 3.28 kJ/g (3 sf)

It is a crude method with heat loss by convection and glass is a poor thermal conductor.

 

Example 2. Using a more accurate calorimeter (see energy changes in chemistry)

You can use a more sophisticated calorimeter system (illustrated on the right).

You can use this system employing a copper can (good heat conductor) to hold the water and employ a draught shield to minimise heat losses by convection..

You fill the little wick burner with vegetable oil and weigh it.

Pour in 100 g (~100 cm³) of water into the copper calorimeter and measure its temperature.

Light the burner and place it carefully under the suspended calorimeter (clamp not shown).

Burn for 5 minutes, extinguish the flame and take the final increased temperature of the water after stirring it to get the average temperature of the bulk liquid.

Suppose the burner containing the vegetable oil weighed 20.55 g.

After burning it weighed 20.05 g and the temperature rose from 24.5^oC to 40.0 ^oC.

Mass of oil burned = 20.55 - 20.05 = 0.50 g

Temperature rise = 40.0 - 24.5 = 15.5^oC.

Calculate the heat released in kg/g of vegetable oil.

Heat energy released = mass of water x heat capacity x temperature rise

Chemical potential energy released from the oil = 100  x  4.2 x 15.5 = 6510 J

Energy released from the oil = 6510 / 0.50 = 13020 J/g, 13.0 kJ/g (3 sf)

Although it is a better method than the boiling tube, the flame may be smoky and therefore incomplete combustion, so the calorific value of J/kg will be less than the theoretical 100% conversion of chemical potential energy.

See also page on respiration.

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Key points Source of information is based on textbooks & syllabus-specifications for students taking the AQA GCSE, Edexcel GCSE and OCR GCSE level biology examinations (~US grades 9-10).

Key points about measuring calorific values of foods and examples of calorific values in kJ/g

Measuring the Calorific Value of Food: GCSE Biology Revision

Understanding how to measure the energy content of food is a key practical skill.

This topic links food, energy, and human biology, helping students explore how the body gets fuel from what we eat.

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1. What Is Calorific Value?

The calorific value of a food is the amount of energy it provides when consumed.

It’s usually measured in joules (J) or kilojoules (kJ) per gram.

This energy is released during respiration and used for all bodily functions.

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2. Simple Calorimetry Experiment

You can estimate the energy content of food using a basic calorimeter setup.

Apparatus:

• Boiling tube

• Clamp stand

• Thermometer

• Measuring cylinder

• Mounted needle

• Food sample (e.g. nut, crisp)

• Bunsen burner or lighter

• Balance (to measure mass)

Method:

1. Measure 25 cm³ of water and pour it into the boiling tube.

2. Record the initial temperature of the water.

3. Weigh the food sample.

4. Skewer the food on the mounted needle.

5. Ignite the food and hold it under the boiling tube until it burns completely.

6. Stir and record the final temperature of the water.

7. Use the formula to calculate energy released:

8. Energy (J) = mass of water (g) x temperature rise (°C) x 4.2

9. Divide by the mass of food burned to get energy per gram (J/g).

10. Divide by 1000 to get calorific value in kJ/g of food.

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3. Example Calorific Values of Foods

These values can vary depending on the brand and preparation method.

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4. Limitations of the Experiment

• Heat loss to surroundings and glassware

• Incomplete combustion of food

• Not all energy transferred to water

Despite these, the method is useful for comparing energy content between foods.

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5. Importance in Human Biology

Studying calorific values helps students:

• Understand how food fuels the body

• Link diet to energy balance and body mass

• Explore how different foods contribute to daily energy needs

• Develop skills in data analysis and experimental design

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Summary of learning objectives and key words or phrases about calorific values of food

Know what the 'calorific value' of a food means e.g. energy in kilojoules obtained per kilogram of fat, oil or carbohydrate metabolised in a diet.

Know the laboratory procedure for measuring the calorific value of foods using simple calorimeter experiments calorimeter and be able to calculate the calorific value from the experimental results or if you are provided with data.

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