The Heat is On - The Energy Stored in Food

INTRODUCTION:

Plants have evolved processes that convert light energy into the chemical bonds of complex molecules. The chemical bonds in carbohydrates, fats, and proteins store energy until needed by the plant. The plant can then release the energy by breaking the appropriate chemical bonds.

Every animal maintains its life processes by consuming complex molecules that store energy. The processed plants and animals we eat as foods contain varying amounts of proteins, carbohydrates, and fats. Because each of these types of foods contains varying amounts of energy, these foods will release varying amounts of energy when they are used by cells. Within our bodies, the energy is released slowly by a series of chemical reactions.

 PRE-LAB PREPARATION:

By burning pieces of food, the chemical energy stored in molecular bonds is released as heat and light. The heat can be measured in units called calories. A calorie is the amount of heat (energy) required to increase the temperature of one gram of water by one degree C. This process is the basis of the technique of calorimetry.

The more calories a food contains, the more heat is given off when burned. Foods high in calories will release large amounts of energy. One gram of a protein will release far fewer calories than one gram of fat. You will study foods with different proportions of protein, fats, and carbohydrates to see how much energy (calories) they release.

MATERIALS:
Test Foods		Test tube (18 x 150 mm)
Balance		25-mL Graduated cylinder
Utility clamp		Large cork with pin
Ring stand		Matches
Thermometer		Distilled water

 

HAZARDS:

The obvious concern is for burns. Also some students may be allergic to the nuts and/or their burning. Check with the students before proceeding. Some nuts may be substituted or omitted. Black soot will form on the bottom of the can which may stain clothing.

PROCEDURE:

 1. Assemble the ring stand and clamp so that a test tube placed in the clamp will be one cm above the food sample (see drawing).

 2. Place 15.0 mL of water in the test tube and put the test tube in the clamp. Place the thermometer in the test tube.

 3. Obtain a 1 to 3 g sample of test food number 1. Find the mass of the test food sample to the nearest 0.01 g (two decimal places), and record its name and mass in the DATA TABLE.

 4. Measure the temperature of the water in the test tube to the nearest 0.5 degrees C and record in the DATA TABLE as initial water temperature.

 5. Use the pin to affix the sample to the cork. Place the cork on the table away from the test tube. Then strike a match and set the food on fire. Immediately move the sample under the test tube. Gently stir the water with the thermometer, using an up and down motion.

 6. After the food sample is completely burned, measure the temperature of the water again to the nearest 0.5 degrees C, and record in the DATA TABLE as final water temperature. Be sure to watch the thermometer carefully, to catch the highest temperature reached.

7. Find the mass of the sample remaining to the nearest 0.01 g and record in the DATA TABLE as mass of sample after burning (ash weight).

DATA TABLE:
Test food # ____
Food name: __________________________
Mass of sample before burning. _______ g		Initial water temperature. _______ oC
Mass of sample after burning (ash weight). _______ g		Final water temperature. _______ oC

CALCULATIONS:

 8. Subtract the mass of the sample after burning (ash weight) from the mass of the sample before burning. This is the change in mass.

To estimate the calories in the food sample you will need the mass of the water you heated. By definition the density of water is 1g/mL, so 1 mL of water has a mass of 1 g. The 15.0 mL of water you used would be 15.0 g.

 The specific heat of water is 1 kilocalorie/Kg deg.C. So the formula would look like this.

 You will see that all units of measurement except kilocalorie cancil each other out of the equation. Everything is already in the equation except your change in temperature for the water. Put in your change in temperature and work the calculation. You now have the total kilocalories of energy given off by the food sample.

 Energy given off by sample = ________ Kcal 

Kilocalories per gram of sample = ________ ^Kcal / _g

Now repeat the procedure with the next food sample. You may collect the data for all the samples, and then do the calculations. Use a clean test tube each time.

 Compare the answer to step 11 for all the food samples.

Click here for the caloric value of various nuts.