Energy in Food and Nutrition

Thermochemistry

Energy in Food and Nutrition

  • Determine the energy value in food (in kilojoules (kJ) or kilocalories (kcal or Cal) .


16-Crunchy

How much energy does a snack food contain?

Food that we eat gives us energy to go through our day.  It gives us energy by providing energy to the cells inside our body.  Carbohydrates in food are used first.  When they are all used up, the body then uses fats, and then proteins as energy sources.  So carbohydrates, fats and proteins provide energy to our bodies through the foods that we eat.

The energy in the food that we it is measured in units of kilocalories or Calories.  The Calorie (Cal, with an uppercase C) used to measure the nutrition in food is actually 1000 calories (cal) (with a lowercase c) or 1 kilocalorie (kcal).  While the Calorie unit is used widely in the U.S., the kilojoule (kJ) is in widespread use internationally.  The conversion factors for calories, kilocalories, joules, kilojoules, and Calories are as follows:

Energy Values used in Nutrition

1000cal = 1 kcal = 1 Cal

4184 J = 4.184 kJ = 1 Cal

Nutrition labels are currently a requirement on most packaged foods sold in the United States and internationally.  The nutrition facts (as they are also called) usually include how many calories are in the food, how much carbohydrates, fat and proteins are present as well as the Calories per gram of each type of energy source.  In order to determine the energy in the food, the food is burned in a calorimeter (described in more detail in the following section) in the laboratory to obtain the energy value in units of kJ/g or kcal/g, typically.  A food sample is put into a steel container containing oxygen and in the surrounding chamber, a measured amount of water.  The energy value is determined based on the mass of the food and water and the increase in temperature.

Sample Problem: Calculating Specific Heat

Use the information in this nutrition facts label to determine the amount of Calories (Cal = kcal) and kilojoules (kJ) from fat, carbohydrates and protein in the snack mix.

Step 1: List the known quantities and plan the problem .

Known

Energy per gram

  • Fat = 9 kcal/g (Cal/g)
  • Carbohydrate = 4 kcal/g (Cal/g)
  • Protein = 4 kcal/g (Cal/g)


Grams

  • Fat = 11 g
  • Carbohydrate = 12 g
  • Protein = 5 g


The amount of grams of each energy source (fat, carbohydrate, and protein) and the amount of energy per gram of each energy source is given on the label.  Multiply the grams by the Energy per gram to obtain the Energy.  Note: The Energy per gram is the conversion factor.  It should be multiplied in such a way that the energy is on top and the grams are on bottom because you want your final answer to be energy in Cal or kcal or kJ.

Step 2: Solve .

 Energy from Fat: 

11 g x 9 kcal/g = 99 kcal = 99 Cal

Convert to kJ

99 Cal x 4.184kJ/Cal = 414.216 kJ= 414kJ

Energy from Carbohydrate:

12 g x 4 kcal/g = 48 kcal = 48 Cal

Convert to kJ

48 Cal x 4.184kJ/Cal = 200.832 kJ = 201 kJ

Energy from Protein:

5 g x 4 kcal/g = 20 kcal = 20 Cal

Convert to kJ

20 Cal x 4.184kJ/Cal = 83.68 kJ = 84 kJ

Step 3: Think about your result .

For the snack food in this example, most of the energy is coming from fat first, then from carbohydrates and finally from protein when these three sources of energy are considered.

Summary

  • An example of how to calculated the energy in the food that we consume, based on food nutrition labels has been illustrated..


Practice

For more practice, find nutrition labels on the foods you consume at home or online and determine the amount of energy from fat, protein and carbohydrates in each.

Review

Questions

  1. Is there energy in the food that we eat?
  2. Which of the 3 energy sources mentioned is the primary fuel for the body?
  3. How is the calorie different from the Calorie (also known as the kilocalorie)?


  • Calorie: The a unit of energy used to measure the energy in nutrition/food and 1 Calorie is equal to 1000 kilocalories.






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