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Diet and exercise : 6. Measuring the calorific value of food experiments

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(6) Calorific values

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

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 22.0o to 39.5oC.

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 = 19.5oC

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)

(c) doc bYou 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 24.5oC to 40.0 oC.

Mass of oil burned = 20.55 - 20.05 = 0.50 g

Temperature rise = 40.0 - 24.5 = 15.5oC.

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.

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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