Biochemistry 1 – Energy Content of Foods – 3

Introduction

All human activity – whether it’s running a marathon, playing football, or even simply breathing – requires “burning” food for energy. In this experiment, you will determine the amount of energy (in kJ/g) released as various foods burn. You’ll be looking for patterns in the amounts of energy released during the actual burning of foods such as cashews, marshmallows, peanuts, and popcorn (or whatever else your instructor comes up with).

Materials and Equipment

Utility clamp2 food samples

2 stirring rodsFood holder

Ring stand and 4-inch ringSmall can

100-mL graduated cylinderCold water

Bunsen burner and gas hoseStriker, matches, or long-barrel lighter

Triple-beam, top-loading, or analytical balance

Procedure

  1. Get a piece of one of the two foods you’ve been assigned and a food holder like the one shown underneath the can in Figure 1.

/ 6. Find and record the mass of your food sample and food holder. While
you’re still using the balance, determine and record the mass of the small
can provided by your instructor.
7. Get some cold water from your instructor and pour 50 mL of it into the
can. Determine and record the mass of the can and the water.
8. Set up the apparatus as shown in Figure 1. Your instructor may want you
to set the can on a wire mesh instead of using a stirring rod and clamp as
shown in the illustration; if he or she hasn’t said so one way or the other,
ask.
Figure 1. Setup for this experiment. / The can should be about 2.5 cm (1 inch) above the food sample. Use a
utility clamp to suspend the thermometer in the water; it should not
touch the bottom of the can.
  1. Give the thermometer at least 30 seconds to stabilize before starting to collect data.
  2. Record the initial temperature of the water, t1, in your data table (round it to the nearest 0.1 C).
  3. Remove the food sample from under the can and light it using a wood splint, Bunsen burner, matches, or long-barreled lighter (whichever one was provided for you). Quickly place the burning food sample under the center of the can. Heat the water in the can, stirring it constantly and carefully, until the food sample stops burning.
  4. Continue stirring the water until the temperature stops rising. Record this maximum temperature, t2.
  1. Determine and record the final mass of the food sample and food holder.

Data Table I

Food Type
Initial Mass of Food and Holder, g
Final Mass of Food and Holder, g
Mass of Food Burned, g
Mass of Can and Water, g
Mass of Empty Can, g
Mass of Water Heated, g
Final Temperature (t2), C
Initial Temperature (t1), C
Temperature Change (t), C

Calculations and Questions

1. In the blank space on the back of this data sheet, calculate the mass of water you heated in each trial. Record your answers

in the appropriate spaces.
2. Calculate the change in temperature of the water (t) for each trial and record your answers.

3. Calculate the heat absorbed by the water, q, using the equation
q = (m)(T)(Cp)

where m is the mass of the water, T is the change in temperature of the water, and Cp is the specific heat of water. Use the
value 4.18J / g C for the specific heat of water. Record your answers in Table II on the back of this sheet. Convert your

answers into kJ (kilojoules) and record these values as well.

4. Find the mass of food that burned in each trial. Record your answers in the appropriate spaces.

5. Use the results of Questions 3 and 4 to calculate the energy content (in kJ/g) of each food sample.

6. Record your results and the results of the other lab groups in Data Table III on the next page. Which food had the highest
energy content? Which had the lowest energy content?
7. Food energy is often expressed in a unit called the Calorie; there are 4.18 kJ in one Calorie. Based on the class average for
peanuts, calculate the number of Calories in a 50-gram bag of peanuts.

8. Two of the foods in this experiment have a high fat content (peanuts and cashews), and two have a high carbohydrate con-
tent (marshmallows and popcorn). From your results, what generalization can you make about the relative energy content of
fats and carbohydrates?

Data Table II

Food Sample 1 / Food Sample 2
Heat Gained by Water, J
Heat Gained by Water, kJ
Energy Content of Food, kJ / g

Data Table III

Your Results, kJ/g
Average, kJ/g

University of SouthAlabamaAdvanced Manual: Biochemistry

Science in Motion 2005Energy Content of Foods