Modern Engineering Thermodynamics

706 CHAPTER 17: Thermodynamics of Biological Systems
H
H
H
H
H
H
H
H
H
C
C
C
C
COOH
H
C
C
C
C
COOH
H
H
H
H
H
FIGURE 17.8
A saturated fatty acid hydrocarbon chain.
FIGURE 17.9
An unsaturated fatty acid hydrocarbon chain.
Mixer
motor
110 V
Thermometer
Fuse wire
Test sample
Bomb (closed
rigid vessel)
Water
Insulation
FIGURE 17.10
A schematic of a typical bomb calorimeter.
The energy value of different foods is normally determined by direct calorimetry in a device called a bomb calorimeter
(see Figure 17.10). In this device, a sample of known mass is ignited in a pressurized atmosphere of excess
pure oxygen. The liberated heat of combustion is transferred to water surrounding the combustion chamber, and
it can easily be calculated from an energy balance on the calorimeter. The end product of this type of combustion
is always CO 2 and H 2 O (and nitrogen products when the sample contains bound nitrogen). Since this is
exactly the same end state that occurs in the body as a result of enzyme decomposition of food molecules, the
same amount of energy must be released in each case. Thus, bomb calorimeter energy measurements represent
the total energy available in the sample that can be converted into heat or another form of energy.
Bomb calorimeter studies on dry (water-free) foods give the following averaged results for the specific energies
of the basic food components:
)
Carbohydrate: 18:0 MJ/kg
Protein: 22:2 MJ/kg Total energy content ðwater freeÞ
Fat:
39:8 MJ/kg
When these same substances are metabolized in the human body they produce the following specific energy
releases:
Carbohydrate:
)
17:2 MJ/kg
Protein: 17:2 MJ/kg Metabolizable energy content ðwater freeÞ
Fat:
38:9 MJ/kg
Using these two sets of values we can compute the food energy conversion efficiency of the human body as
η carbohydrate = 17:2 × 100 = 95:5%
18:0
η protein = 17:2 × 100 = 77:5%
22:2
η fat = 38:9 × 100 = 97:7%
39:8
Thus, 22.5% of the energy in the protein we eat passes through the body unused. The low protein energy conversion
efficiency supports the theory that humans were not always meat-eating animals.