Energy Content of Foods

All human activity requires “burning” food for energy. In this experiment, you will determine the energy released (in kJ/g) as various foods, such as cashews, marshmallows, peanuts, and popcorn, burn. You will look for patterns in the amounts of energy released during burning of the different foods.

Figure 1

MATERIALS

CBL System	utility clamp and slit stopper
TI Graphing Calculator	2 stirring rods
Vernier Temperature Probe	ring stand and 4-inch ring
Vernier adapter cable	100-mL graduated cylinder
2 food samples	small can
food holder	cold water
wooden splint	matches

PROCEDURE

1. Obtain and wear goggles.

2. Plug the temperature probe into the adapter cable in Channel 1 of the CBL System. Use the link cable to connect the CBL System to the TI Graphing Calculator. Firmly press in the cable ends.

3. Turn on the CBL unit and the calculator. Start the CHEMBIO program and proceed to the MAIN MENU.

4. Set up the calculator and CBL for one temperature probe and a temperature calibration.

• Select SET UP PROBES from the MAIN MENU.

• Enter “1” as the number of probes.

• Select TEMPERATURE from the SELECT PROBE menu.

• Enter “1” as the channel number.

• Select USE STORED from the CALIBRATION menu.

5. Set up the calculator and CBL for data collection.

• Select COLLECT DATA from the MAIN MENU.

• Select TIME GRAPH from the DATA COLLECTION menu.

• Enter “6” as the time between samples, in seconds.

• Enter “99” as the number of samples (the CBL will collect data for a total of 9.9 minutes).

• Press . Select USE TIME SETUP to continue. If you want to change the sample time or sample number, select MODIFY SETUP.

• Enter “0” as the minimum temperature (Ymin).

• Enter “100” as the maximum temperature (Ymax).

• Enter “10” as the temperature increment (Yscl).

6. Obtain a piece of one of the two foods assigned to you and a food holder like the one shown in Figure 1. Find and record the initial mass of the food sample and food holder. CAUTION: Do not eat or drink in the laboratory.

7. Determine and record the mass of an empty can. Add 50 mL of cold water to the can. Obtain the cold water from your teacher. Determine and record the mass of the can and water.

8. Set up the apparatus as shown in Figure 1. Use a ring and stirring rod to suspend the can about 2.5 cm (1 inch) above the food sample. Use a utility clamp and slit stopper to suspend the temperature probe in the water. The probe should not touch the bottom of the can. Remember: The temperature probe must be in the water for at least 45 seconds before you do Step 9.

9. Press on the calculator to begin data collection. Monitor temperature (in °C) on the CBL screen and record the initial temperature of the water, t1, in your data table. Note: A real-time graph of temperature vs. time will also be displayed on the TI calculator screen. Remove the food sample from under the can and use a wooden splint to light it. Quickly place the burning food sample directly under the center of the can. Allow the water to be heated until the food sample stops burning. CAUTION: Keep hair and clothing away from open flames.

10. Continue stirring the water until the temperature stops rising. Record this maximum temperature, t2. After 9.9 minutes, data collection is completed (“DONE” appears on the CBL screen).

11. Determine and record the final mass of the food sample and food holder.

12. Press to display a graph of temperature vs. time. To confirm the initial (t1) and maximum (t2) temperature values you recorded earlier, examine the data points along the curve. As you move the cursor right or left, the time (X) and temperature (Y) values of each data point are displayed below the graph.

13. Press , then choose YES to repeat the data collection for the second food sample. Use the same Y-axis settings as in Part I. Use a new 50-mL portion of cold water. Repeat Steps 6‑12.

14. When you are done, place burned food, used matches, and partially-burned wooden splints in the container provided by the teacher.

PROCESSING THE DATA

1. Find the mass of water heated for each sample.

2. Find the change in temperature of the water, Dt, for each sample.

3. Calculate the heat absorbed by the water, q, using the equation

q = Cp•m•Dt

where q is heat, Cp is the specific heat capacity, m is the mass of water, and Dt is the change in temperature. For water, Cp is 4.18 J/g°C. Change your final answer to kJ.

4. Find the mass (in g) of each food sample burned.

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

6. Record your results and the results of other groups in the Class Results Table. Which food had the highest energy content? The lowest energy content?

7. Food energy is often expressed in a unit called a Calorie. There are 4.18 kJ in one Calorie. Based on the class average for peanuts, calculate the number of Calories in a 50-g package of peanuts.

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

DATA AND CALCULATIONS

Food type	___________________	___________________
Initial mass of food and holder	________ g	________ g
Final mass of food and holder	________ g	________ g
Mass of food burned	________ g	________ g
Mass of can and water	________ g	________ g
Mass of empty can	________ g	________ g
Mass of water heated	________ g	________ g
Final temperature, t2	________°C	________°C
Initial temperature, t1	________°C	________°C
Temperature change, Dt	________°C	________°C
Heat, q	________ kJ	________ kJ
Energy content in kJ/g	________ kJ/g	________ kJ/g

CLASS RESULTS TABLE

Marshmallows	Peanuts	Cashews	Popcorn
________ kJ/g	________ kJ/g	________ kJ/g	________ kJ/g
________ kJ/g	________ kJ/g	________ kJ/g	________ kJ/g
________ kJ/g	________ kJ/g	________ kJ/g	________ kJ/g
________ kJ/g	________ kJ/g	________ kJ/g	________ kJ/g
________ kJ/g	________ kJ/g	________ kJ/g	________ kJ/g

Average for each food type:

________ kJ/g