Skill:
• Determination of the energy content of food by combustion
Determining Energy Content
The energy content of food can be estimated by burning a sample of known mass and measuring the energy released via calorimetry
- Combustion of the food source causes the stored energy to be released as heat, which raises the temperature of water
- The amount of energy required to raise 1 g of water by 1ºC is 4.18 J – this is the specific heat capacity of water
The equation for calculating the energy content of a food source via calorimetry is as follows:
- Energy (joules) = Mass of water (g) × 4.2 (J/gºC) × Temperature increase (ºC)
The biggest source of error in calorimetry is usually caused by the unwanted loss of heat to the surrounding environment
- The food sources should be burnt at a constant distance from the water to ensure reliability of results
- The initial temperature and volume of water should also be kept constant (1 g of water = 1 cm3 or 1 ml)
Measuring Energy Content of Food via Calorimetry
Comparing Energy Content
The three types of nutrients that are commonly used as energy sources are carbohydrates, lipids (fats) and proteins
- Carbohydrates are preferentially used as an energy source because they are easier to digest and transport
- Lipids can store more energy per gram but are harder to digest and transport (hence are used for long-term storage)
- Protein metabolism produces nitrogenous waste products which must be removed from cells
The relative energy content of carbohydrates, proteins and fats are as follows:
- Carbohydrates – 1,760 kJ per 100 grams
- Proteins – 1,720 kJ per 100 grams
- Fats – 4,000 kJ per 100 grams
Role of Organic Molecules in Energy Production