Experiment 9: caloric content of food

 

EXPERIMENT 9: Caloric Content of Food

 

Read the entire experiment and organize time, materials, and work space before beginning.

Remember to review the safety sections and wear goggles when appropriate.

Objectives: To measure the energy content of various food items, and

To become familiar with energy units like calories and joules.

Materials: Student Provides: Lighter or candle

Aluminum foil, tray, or pie tin

Metal fork

Food items including marshmallows, peanuts or

walnuts, and others such as potato chips or popcorn

From LabPaq: Goggles Thermometer

Burner stand Digital scale

100-mL Beaker Test tube holder

Discussion & Review: A “calorie” is a unit of energy, just like a joule or a kilowatt-hour.

It simply represents the amount of heat energy it would take to raise the temperature of

one gram of water by one degree Celsius. The Systeme International (SI) unit of energy

is the joule (J), which can be calculated from calories by multiplying by 4.184 (the

specific heat of water). When dealing with food calories the term “Calorie” (note the

capital C) represents a kilocalorie or 1000 calories.

We will 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 the

final temperatures, the energy released can be calculated using the equation

Q = Δt•m•cp

where Q = heat energy absorbed (in J), Δt = change in temperature (in °C), m = mass

(in grams), and cp= specific heat capacity (which is 4.184 J/g°C for water). Dividing the

resulting energy value by grams of food burned gives the energy content (in J/g).

Since some of the heat given off by the burning food will be absorbed by the beaker and

some more energy will escape into the air, only the remaining heat goes into heating the

water. This means that the results we will get from this exercise will be lower than the

accepted values. However, we will get quite reasonable approximations.

PROCEDURES: Before you begin, set up a data table as shown before the Analysis

section to record your observations.

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1. With your digital scale determine the empty weight of your 100-mL beaker.

2. Half-fill the beaker with water (approximately 50 ml) and weigh it again.

3. The beaker and water weight minus the empty beaker weight is the net weight of

water used for this experiment. Record it.

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

upward.

5. Set up the burner stand over the aluminum foil and place the beaker of water on top

of the burner stand.

6. Measure and record the initial temperature of the water.

7. Marshmallow: With your digital scale determine the mass of a marshmallow and the

mass of your empty fork, and record the figures.

A. Place the marshmallow on the fork and light it with a lighter or a candle.

B. As soon as the marshmallow is well lit and burning, hold it under the beaker of

water while you occasionally stir the water with the thermometer.

C. When the marshmallow is completely burnt some of it may stick to the fork.

Weigh the fork with the marshmallow remnants and record. Subtract the empty

fork weight to obtain the weight of the unburned marshmallow residue.

D. Record the temperature of the water – this is the final temperature.

E. To analyze another food item place a fresh beaker of water on the burner stand.

Don’t forget to determine the mass of the water and the initial temperature.

8. Peanut or walnut: Record the weight of the peanut or walnut.

A. The peanut or walnut is best held in place by the wire test tube holder, but a long

needle will also work. Food items like peanuts may take a while to stay lit and

you may have to move them around in a flame to get them burning. When well lit

and burning, immediately hold the peanut or walnut under the beaker of water

and occasionally stir the water with the thermometer.

B. When the peanut or walnut is almost completely burned and the flame has been

extinguished, record the final temperature of the water and determine the weight

of the nut residue and record.

9. Repeat this procedure on some other food in your home, such as a potato chip or

popped popcorn.

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DATA TABLE:

Marshmallow Peanut or Walnut Other Sample

Food Item Description

Mass of food & holder –

initial

Mass of food & holder – final

Mass of food burnt

Mass of beaker

Mass of beaker & water

Mass of water

Water temp. – initial

Water temp. – final

Delta T (oC change)

Analysis:

1. For each of the samples tested, calculate the change in water temperature, Δt, by

subtracting the initial temperature from the final temperature (Δt = tfinal – tinitial). Record

all information in the data table.

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

the empty beaker from the mass of the beaker water. Record your findings in the

data table.

3. Use the results of Steps 1 and 2 above to determine the heat energy gained by the

water (in J). Use the equation:

Q = Δt•m•cp

where Q = heat absorbed (in J), Δt = change in temperature (in °C), m = mass of the

water heated (in g), and cp = specific heat capacity (4.184 J/g°C for water).

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

the initial mass. Record your findings in the data table. The heat energy determined

in step 3 above is for this mass of food burned.

5. To calculate the heat energy per unit mass – such as per one gram of food – divide

the heat energy obtained in item 3 above by the mass of the food burned.

For example, if you burned a peanut with a net mass of .8 g and you heated 50 mL

of water from an initial temperature of 21o C to a final temperature of 48o C, you

would get the following results:

Q = Δt•m•cp = (48 – 21 o C)(50 g H20)(4.184 J/g o C) = 5,648 J (Joules) (for .5 g of

peanuts)

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To determine the heat energy per gram: 5 648 J/ .5g = 11,296 J/gram of peanuts or

11,296 J/4.184 J/cal = 2,700 calories = 2.7 kilocalories or 2.7 Cal (food calories)

Typical experimental 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)

Cashews and peanuts have the highest energy content. A generalization is that

carbohydrates such as unbuttered popcorn and marshmallows provide about 4 Cal/g

while fat-rich peanuts and cashews provide about 9 Ca/g. (1 Cal = 1 dietary calorie).

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