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Calorific Value

Calorific Value: Types, Measurement using Dulong’s Formula, Bomb Calorimeter and Oxy-Calorimeter, Significance of Calorific Value, Practice Problems and FAQs

Off late the world we live in has become more conscious about health, fitness and the type of food that we put inside our bodies. ‘Healthy eating’, ‘clean eating’ and ‘counting the calories’ are some phrases that have spread like wildfire in the past few years. You must have noticed that there are many people who would actively research the calorie content of their food and prepare meal strategies to intake a fixed amount of calories throughout the day. In fact, many of us have the habit of checking the calorie and nutritional value of the food we buy from the back of the packet. So what is this calorie and why has it become so important? Calorie is nothing but the unit to express the amount of energy a specific mass of a food substance will offer you when its metabolised completely in your body. If we intake more calories than what we are burning, then it leads to weight gain and that is what has kept the world on its edge.

We often say that food is the fuel for the body, right? So just like food, the fuels that we use for our automobiles, engines and other machines also have a calorific value that is a measure of how much energy the fuel can generate when it’s burnt. Let us dig deeper into this concept of calorific value.

Table of Contents:

  • Calorific Value
  • Types of Calorific Value
  • How can the Calorific Value be Measured?
  • Significance of Calorific Value
  • Practice Problems
  • FAQs

Calorific Value

The total amount of heat produced when a specific mass of fuel or food undergoes complete combustion is known as its calorific value. Calorific value of food or fuel is the measure of their efficiency. Higher the calorific value of food or fuel, more is its efficiency. The scientific measurements of food energy are made in calorie (cal) or joule (J). One calorie is the amount of heat energy required to raise the temperature of 1g of water by 1oC.

Since calorie is too small a unit to measure energy released from food or fuel, higher units such as kilocalories or Kcal and kilojoules or kJ are used. Kilojoule is the unit officially recommended by the World Health Organisation and hence the unit for calorific value is kJ/kg or Kcal/kg.

Relationship between the units of calorific value

1 calorie = 4.184 joules or 4.2 joules

1 Kcal = 1000 calories

1 Kcal = 4.184 kJ or 4.2 kJ

Types of Calorific Value

When it comes to hydrogen-based fuels, there are two calorific values to consider:

  • Higher Calorific Value (Gross Calorific Value)
  • Lower Calorific Value (Net Calorific Value)

Another type of calorific value, known as physiological calorific value is relevant with respect to calorific value of food.

Gross Calorific Value

Gross Calorific Value or Higher Calorific Value (HCV) refers to the maximum amount of heat that is released when a specific mass of food or fuel is completely burned and the products of combustion are brought down to room temperature.

When the combustion products are allowed to cool down to ambient room temperature, the water vapour released due to combustion of the fuel or food releases its latent heat of vaporisation and condenses to liquid water. This latent heat of vaporisation is also considered while calculating the gross calorific value.

Fuel

HHV

kJ/mol

Hydrogen

286

Methane

890

Ethane

1,560

Propane

2,220

Butane

2,877

Pentane

3,509

Net Calorific Value

Net Calorific Value or Lower Calorific Value (LCV) is the total amount of heat generated when a food or a fuel is completely burned but the products of combustion are allowed to be released.

As the condensation of water vapour produced due to combustion does not occur, the latent heat of vaporisation is not factored in while calculating net calorific value. This results in a lower quantity of thermal energy accessed and hence the name lower or net calorific value.

Difference between HCV and LCV

The combustion products are permitted to cool to room temperature while considering HCV, whereas products are allowed to escape while considering LCV.

As a result, in HCV, the latent heat of vaporisation released during condensation of water vapour, that would otherwise be lost due to escape of combustion products, can be recovered. On the other hand, in LCV, part of the latent heat is taken away as the water vapour escapes. Thus, we can say.

LCV = HCV – heat lost through vapour

OR

HCV = LCV + heat lost through vapour

Physiological Calorific Value

The gross calorific value of a foodstuff is the amount of heat that is release due to complete combustion of 1 g of the foodstuff in a bomb calorimeter. But the actual amount of heat released when food is completely metabolised in the body is different from what is seen in a bomb calorimeter. The actual amount of energy that is liberated from the combustion of 1 g of food in the body is known as the physiological calorific value. The given table compares the gross and physiological calorific values of different macronutrients in the human body.

Food group

Gross calorific value

Physiological calorific value

Carbohydrates

4.1 Kcal/g

4 Kcal/g

Proteins

5.65 Kcal/g

4 Kcal/g

Fats

9.45 Kcal/g

9 Kcal/g

How can the Calorific Value be Measured?

Calorific value can be calculated theoretically using the Dulong’s formula and practically using a Bomb Calorimeter (direct method) or an oxy-calorimeter (indirect method). Let’s have a look at all the different approaches.

Dulong's Formula

The LHV of a fuel is calculated as the aggregate of the calorific values of all the fuel components such as carbon, hydrogen, sulphur, etc. From fuel analysis, the percentage of these components is obtained and their calorific values are already known. Thus the Dulong’s formula is used to calculate the calorific value of the fuel. The formula is written as -

LHV [kJ/g]= 33.87mC + 122.3(mH - mO ÷ 8) + 9.4mS

where, the initials mC, mH, mO, and mS in the formula represent the amount of carbon, hydrogen, oxygen, and sulphur present in the fuel.

Direct method using Bomb Calorimeter

Bomb calorimeter gives the most effective measurement of the calorific value. The quantity of the sample burnt is calculated based on the difference in its mass before and after the combustion. It is a calorimeter with a fixed volume that evaluates the heat of combustion of the fuels or food samples.

Parts

1. A tank with weighed amount water

2. A heavy steel bomb with a copper lining that is plated with gold or platinum

3. The bomb has a cover that is held tightly by a screw collar

4. An oxygen valve to charge the bomb

5. Electrodes which are connected to an electric circuit to generate sparks which can ignite the sample.

6. Differential thermometer dipped in the water tank.

Working

To begin, a predetermined amount of sample is inserted in the bomb and it is charged with oxygen through the valve. The valve is then sealed and the bomb is immersed in the container with water. The electric circuit connected to the electrodes is closed and electric sparks are generated to ignite the sample.

The heat released by burning of the sample is measured by observing the rise in the temperature of the water in the container. Heat arising from accessory combustions is deducted to obtain accurate measurement of heat liberated in calories from the burning of the actual sample.

Indirect method using Oxy-calorimeter

The indirect determination of caloric value is done by using an oxy-calorimeter. A fixed amount of food sample is burnt in oxygen and the volume of oxygen needed to completely burn the food sample is measured. Heat energy production due to combustion is related to oxygen utilisation and thus the volume of oxygen used up is used to calculate the calorific value.

The underlying principle is that 4.8 Kcal of heat is released when 1litre of oxygen is used to oxidise organic nutrients.

Significance of Calorific Value

Calorific value helps us determine the efficiency of a fuel. Higher the calorific value, higher is the efficiency of the fuel. This helps in generating bills for fuel consumption.

It also gives us an idea of the amount of energy that is generated in the body when we eat a particular food item. This helps us in creating meal plans and workout plans to balance the calories that we take in and those that we burn.

Practice Problems

1. Which of the following correctly describes the relationship between Kcal and joule?

  1. 1Kcal = 4.184 J
  2. 1Kcal = 1000 J
  3. 1Kcal = 4184 J
  4. 1Kcal = 4.2 J

Solution: We know that,

1 calorie = 4.184 joules or 4.2 joules

1 Kcal = 1000 calories

1 Kcal = 1000 X 4.184 joules = 4184 joules or 4184 J

Thus, the correct option is c.

2. Which of the following options is correct?

  1. HCV + LCV = heat lost through vapour
  2. LCV = HCV + heat lost through vapour
  3. HCV = LCV - heat lost through vapour
  4. HCV = LCV + heat lost through vapour

Solution: In HCV, the latent heat of vaporisation released during condensation of water vapour, that would otherwise be lost due to escape of combustion products, can be recovered. On the other hand, in LCV, part of the latent heat is taken away as the water vapour escapes. Thus, we can say.

LCV = HCV – heat lost through vapour

OR

HCV = LCV + heat lost through vapour

Thus, the correct option is d.

3. The physiological calorific value of proteins is much less compared to its gross calorific value. Why?

Answer: The gross calorific value a foodstuff is the amount of energy obtained due to complete combustion of a weighed amount of the food sample in the bomb calorimeter. It is nothing but the potential energy of the sample being converted to heat energy upon being burnt. But the energy liberated by the same amount of foodstuff in the body is not the same, and is termed as physiological energy. Carbohydrates and fats are made up of carbon, hydrogen and oxygen, which are completely burnt to carbon dioxide and water vapour within the body and hence their gross and physiological calorific values do not differ greatly. However, for proteins, deamination of the constituent amino acids causes the nitrogen to be removed as urea from the body. Hence, the physiological calorific value is much less than the gross calorific value.

4. In indirect calorimetry, the heat generated due to complete combustion of a sample is measured in times of

  1. Difference in temperature of surrounding water
  2. Difference in mass of sample
  3. Volume of oxygen used for complete combustion of sample
  4. Difference in weight of sample

Solution: The indirect determination of caloric value is done by using an oxy-calorimeter. A fixed amount of food sample is burnt in oxygen and the volume of oxygen needed to completely burn the food sample is measured. Heat energy production due to combustion is related to oxygen utilisation and thus the volume of oxygen used up is used to calculate the calorific value.

The underlying principle is that 4.8 Kcal of heat is released when 1litre of oxygen is used to oxidise organic nutrients. Thus, the correct option is c.

FAQs

1. What are empty calorie foods?
Answer:
Food as added sugars or solid fats which supply high calories of food energy but little to no nutrition are known as empty calorie foods. Excessive intake of empty calorie foods leads to Hidden Hunger. This can be one of the reasons for weight gain.

2. Do negative calorie foods exist?
Answer:
Foods that would require more energy to be digested than the amount of energy they would produce upon being metabolised, are said to be negative calorie foods. Such foods are a myth and there is no scientific evidence of any food item having such an effect.

3. Why is the bomb calorimeter so named?
Answer:
The bomb calorimeter uses oxygen placed under huge pressure and hence requires a strong vessel to hold the sample and oxygen. Thus, it gets the name ‘bomb’ calorimeter.

4. Who was the first person to develop a bomb calorimeter?
Answer:
The first bomb calorimeter was developed in 1878 by Paul Vieille for measuring heat emitted due to explosions at the French service of explosives.

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