Estimating the Calorie Content of Food

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Estimating the Calorie Content of FoodGeneral Chemistry II Lab

Introduction:When a typical fuel is burnt in an oxygenated atmosphere three things are usually

generated.

Fuel + O2(g)CO2(g) + H2O(g) + heat

The amount of carbon dioxide and water vapor that is evolved from the reaction dependson the stoichiometry of the reaction, but the heat that evolved depends on the chemical

structure of the fuel. When we talk about fuels we often think about gasoline or coal,

but our bodies use the food we ingest as fuel to operate our body. The goal of this lab is

to examine the amount of heat that different foods will evolve. At this point we shouldrealize that a reaction that evolves heat is called an exothermic reaction, and that means

energy is being released to the surroundings. In the body does not burn food, but breaks

it down by metabolic processes involved in digestion. The key to this lab is that theamount of energy evolved from digestion is the same as that evolved from burning the

food outside the body.

The International System of Units (S.I.) uses the joule (J) as an energy unit, but food

science uses the often-heard calorie (Cal) as the unit of energy. This calorie (with capital

C) is what we call a food calorie, and is equal to 1 kcal or 1000 normal calories. To keepthe two straight, we use Calfor the food calorie, and calfor the calorie. So it may be

handy to know the following conversions:

1 Cal = 1 kcal = 1000 cal

1 cal = 4.148 J

You may recall that one calorie (cal) is equal to the amount of energy needed to raise 1 g

of water 1C, this will be useful as well to remember. From this knowledge we know the

specific heat of water is 1.00 cal/C or 4.148 J/C.

Foods that are high in fat are also high in calories. Lipids that make up fat are high inenergy. For example, ~50% by mass of the common peanut is from the lipids that make

up the peanut oil. This peanut oil accounts for ~75% of the food energy of the peanut.

In this lab we will use a homemade calorimeter to measure the amount of energy evolved

from a variety of foods to find the calories content of the food. We will then compare our

experimental values to those found on the packages for the food.

Using a Calorimeter to Determine Energy Released From a Food Item.

The details of how to build your calorimeter will be discussed later. The design will be

used to optimize the capture of the energy released from the burning of your food item.This energy will be transferred to the water reservoir in the calorimeter. Optimally,


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100% of the energy released from burning will be transferred to the water. This

absorption of energy by the water results in an increase in the internal energy of thewater, or more commonly called a raise in temperature. To find the amount of energy

released we can therefore use the following equation:

E= msT

Where E is the energy absorbed by the water, mis the mass of the water used, sis the

specific heat of water

Cg

calo

1 , and Tis the change in temperature of the water. This

will give you the energy in calories (cal), so you then need to convert to food calories

(Cal). After that is done you will have the total energy evolved from the food. To

convert this to a useful value you will take this total energy and divide it by the mass offood burned, to get the food calories in 1 gram of food.

Procedure

Construction of the Calorimeter

You will construct a calorimeter out of a pop can, 2 skewers, a paper clip, and a rubber

cork as shown in the following figure:

Figure 1. A simple calorimeter: (a) food holder; (b) pop can assembly; (c) setup for burning food.

Your lab instructor may demonstrate this construction. After you have the calorimeter

built you, you need to prepare your ring stand to hold the can. You will need to make

sure the height is correct. You will need to allow enough room under the can to place the

food holder to the concave of the can captures most of the energy from the food.


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If you are using a previously prepared calorimeter, make sure to clean off any remaining

soot from the can prior to use.

Preparing to Burn the Peanut/Food Item

1. Take the mass of your can with skewers/wires and record this value on line 1.2. Use your graduated cylinder to measure out ~100 mL of water and add to the can.

Make sure there is no water around the lip of the can, if so wipe dry with a papertowel. Take the mass of the can plus the water and record on line 2.

3. Place the can on the ring stand and place your thermometer in the can, you mayuse a string to suspend it or have someone hold it when you start the experiment.

You should not let the thermometer rest on the bottom of the can during the

burning of the food item as this may cause inaccurate measurements.

4. Find the mass of a food item and record on line 4. Remember to never placechemicals or food directly on the balance, use weigh paper or a weigh boat.

5. Rest the food item on the holder. Use your thermometer to gently stir the water inthe can and record the initial temperature on line 7.

6. Light your food item using a match and quickly slide the holder under the can.Do not light the food item while it is under the can, this will transfer energy from

the match to the water. Allow the food to burn until done, or until the food falls

off the holder. If the food falls off the holder, extinguish the flame immediately.This will not ruin the experiment as long as it burned long enough to transfer a

measurable amount of energy to the calorimeter.

7. Monitor the temperature of the water while gently stirring with the thermometer.After the temperature has leveled off, record the final temperature on line 8.

8. Recover the unburned food and take the mass. Record on line 5.9. Clean the can as needed and repeat steps for other food items. You do not need to

empty out the water and refill, the mass of the water should not change, just the

initial temperature.

10.Record your results on the chalkboard as directed.Cautions:

- If you have any food allergies (especially for peanuts) please notify theinstructor or TA prior to class.

- Some of the food items will emit heavy soot when burning, for this reasonuse only the size of each food item indicated by your instructor.

- Be careful to not transfer soot to your clothes or others.


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Name:_________________________________________Section:_______ Date:_______

Data Sheet

II. Burning the Food Item

Name of food item ____________ ____________ ____________

1. Mass of emptycalorimeter can ____________ ____________ ____________

2. Mass of can plus water ____________ ____________ ____________3. Mass of water in can ____________ ____________ ____________

4. Mass of food item ____________ ____________ ____________5. Mass of unburned

food item ____________ ____________ ____________

6. Mass of fooditem consumed ____________ ____________ ____________

7. Initial water temperature ____________ ____________ ____________8. Final water temperature ____________ ____________ ____________9. Change in temperature ____________ ____________ ____________

10.Energy evolvedfrom burning ____________ ____________ ____________

11.Energy evolvedper gram ____________ ____________ ____________

12.Average energyevolved per gram ____________ ____________ ____________

13.Standard deviation ____________ ____________ ____________14.Energy evolved per

gram from package ____________ ____________ ____________

15.Percent difference ____________ ____________ ____________


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Name:_________________________________________Section:_______ Date:_______

Post-Laboratory Questions

1. When using a calorimeter like the one we in this laboratory, we make the assumptionthat all the energy is absorbed by the water. In reality the can itself absorbed someenergy as well. Given that the specific heat of aluminum is 0.900 J/gC, if the

calorimeter has 100 g of water and the temperature raises 1C:

a. What is the amount of energy absorbed by the water?

b.

What is the amount of energy absorbed by the can?

c. What is the total energy absorbed by the calorimeter?

d. What is the percent of energy is absorbed by the can?

2. List some other sources of experimental error. Comment on how the soot on the cancould affect your results.

3. From the packages, which food had the highest amount of fat per gram?

4. From the classs results, which food releases the largest amount of energy per gram?

Estimating the Calorie Content of Food

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