Energy Content of Food

Energy Content of Foods

Energy content is an important property of food. The energy your body needs for running, talking, and thinking comes from the food you eat. Energy content is the amount of heat produced by the burning of 1 gram of a substance, and is measured in joules per gram (J/g).

You can determine energy content by burning a portion of food and capturing the heat released to a known mass of water in a calorimeter. If you measure the initial and final temperatures, the energy released can be calculated using the equation

Q = Dt•m•c_p

where Q = heat energy absorbed (in J), Dt = change in temperature (in °C), m = mass (in g), and c_p= specific heat capacity (4.185 J/g°C for water). Dividing the resulting energy value by grams of food burned gives the energy content (in J/g).

PROCEDURE

1. Obtain and wear goggles/aprons.

2. Get a sample of food and a food holder. Find and record the initial mass of the food sample and food holder. CAUTION: Do not eat or drink in the laboratory.

4. Set up the apparatus. Place aluminum foil on the table top to catch any spills and also to reflect heat upward.

• Determine and record the mass of an empty can.

• Place about 50.0 mL of cold water into the can.

• Determine and record the mass of the can plus water.

• Use a rod to suspend the can about 2.5 cm (1") above the food sample.

• Use a utility clamp and stopper to suspend the thermometer in the water. The thermometer should not touch the bottom of the can.

5. Record the initial temperature of the water.

6. 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 an open flame.

7. Stir the water until the temperature stops rising. Record this final temperature.

8. Determine the final mass of the food sample and food holder.

9. Repeat the procedure for a second food sample. Use a new 50.0 mL portion of cold water.

10. When you are done, place burned food, used matches, and partly-burned wooden splints in the container supplied by the teacher.

DATA

Sample 1 Sample 2

Food used _______ _______

Mass of food and holder (initial) ______ g ______ g

Mass of food and holder (final) ______ g ______ g

Mass of empty can ______ g ______ g

Mass of can plus water ______ g ______ g

Initial water temperature ______ °C ______ °C

Final water temperature ______ °C ______ °C

PROCESSING THE DATA

1. Calculate change in water temperature, Dt, for each sample, by subtracting the initial temperature from the final temperature (Dt = t_final – t_initial).

2. Calculate the mass (in g) of the water heated for each sample. Subtract the mass of the empty can from the mass of the can plus water.

3. Use the results of Steps 1 and 2 to determine the heat energy gained by the water (in J). Use the equation

Q = Dt•m•c_p

where Q = heat absorbed (in J), Dt = change in temperature (in °C), m = mass of the water heated (in g), and c_p = specific heat capacity (4.185 J/g°C for water).

4. Calculate the mass (in g) of each food sample burned. Subtract the final mass from the initial mass.

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

6. Record your results and the results of other groups below. Share data with other groups.

Class Results

Food Type Food Type Food Type Food Type

____________ ____________ ____________ ____________

_________ J/g _________ J/g _________ J/g _________ J/g

Avg. _________ J/g _________ J/g _________ J/g _________ J/g

7. Which of the foods has the greatest energy content? Why do these foods have the greatest energy content?

Energy Content Information

The lab can be performed with food or different fuels such as ethyl alcohol, lamp oil, or candles (paraffin wax). See Heats of Combustion for an example of using the candle as a source.

Suggestions and possible results:

1. Alcohol burners may be used but be aware of the danger in their use. Due to the rapid evaporation of the alcohol the mass of the alcohol and burner should be determined immediately before and after use.

2. Lamp oil can be used in alcohol burners.

3. Aluminum drink cans work well.

4. Have ice water available and make sure all the ice is removed before use. Water initially at 4-5° C gives best results, because starting 17-19° below and finishing 17-19° C above room temperature tends to equalize heat exchange with the room.

5. Make sure candles are mounted on cardboard bases. The base will catch the drippings. Use 100 mL of water for candles and 200 mL of water for alcohol and oil.

6. Typical class averages:

candle 23 000 – 25 000 J/g

lamp oil 16 000 – 18 000 J/g

ethyl alcohol 12 000 – 14 000 J/g

7. Actual values are about 42.0 kJ/g for paraffin wax and 26.8 kJ/g for ethyl alcohol. You may want to discuss factors causing the results to be lower than actual values.

8. You may want to look at other considerations such as convenience, cost, availability, burning efficiency, and renewability.

9. If you are doing the energy content of food, use a paper clip and small jar lid (baby food jar). Partly straighten the paper clip, bending a small loop in one end. This will cradle the food sample. Glue the other end into the lid which will help catch burned food that might fall.

10. Good choices are cashews, marshmallows, peanuts, and popped popcorn. Because peanuts and cashews release very large amounts of heat you may want to use 100 mL of water in the can and only 50 mL for the others.

11. You can substitute calories for the heat energy unit. One Calorie = 1000 calories.

12. Typical class averages:

Cashews 11 000 – 12 000 J/g (2.5 – 2.9 Cal/g)

Marshmallows 4 200 - 5 800 J/g (1.0 – 1.4 Cal/g)

Peanuts 11 000 – 12 500 J/g (2.6 – 3.0 Cal/g)

Popcorn 5 000 - 8 400 J/g (1.2 – 2.0 Cal/g)

13. Cashews and peanuts have the highest energy content. A generalization is that carbohydrates provide about 4 Cal/g (unbuttered popcorn and marshmallows) while fat rich peanuts and cashews provide about 9 Ca/g. (1 Cal = 1 dietary calorie).

The Energy Content of Foods lab and data from this page from “Physical Science with Computers” of Vernier Software, Portland Oregon.