Biomedical Engineering Reference
In-Depth Information
11.1 3. HEAT GENERATION BY MICROBIAL GROW TH
About 40% to 50% of the energy stored in a carbon and energy source is converted to bio-
logical energy (ATP) during aerobic metabolism, and the rest of the energy is released as heat.
For actively growing cells, the maintenance requirement is low, and heat evolution is directly
related to growth.
The heat generated during microbial growth can be calculated using the heat of combus-
tion of the substrate and of cellular material. A schematic of an enthalpy balance for microbial
utilization of substrate is presented in Fig. 11.9 . The heat of combustion of the substrate is
equal to the sum of the metabolic heat and the heat of combustion of the cellular material.
DH c; S
D H c ; P
YF X=S
YF X=S ¼
YF P=S þ DH c;X þ
YF H =X
(11.51)
where D H c,S is the heat of combustion of the substrate (kJ/g-substrate), YF X/S is the biomass
yield factor (g-cell/g-substrate), D H c,X is the heat of combustion of cells (kJ/g-cells), YF P/S is
the extracellular product yield factor (g-P/g-substrate), D H c,P is the heat of combustion of
extracellular products (kJ/g-P), and YF H/X is the heat yield factor on biomass or the meta-
bolic heat evolved per gram of cell mass produced (kJ/g-cells).
Equation (11.51) can be rearranged to yield
DH c;S
YF X=S DH c;X
YF P=S DH c;P
YF H =X ¼
(11.52)
YF X=S
D H c,S , D H c,P , and D H c,X can be determined from the combustion of substrate, extracellular
products, and cells. When these data are not available, one can estimate them via energy
regulations (Chapter 3). Typical D H c values for bacterial cells are 20 e 25 kJ/g-cells. Typical
values of YF H/X are glucose, 10 kJ/g; malate, 14 kJ/g; acetate, 20 kJ/g; ethanol, 23 kJ/g;
methanol, 35 kJ/g; and methane, 69 kJ/g. Clearly, the degree of oxidation of the substrate
has a strong effect on the amount of heat released.
The total rate of heat evolution in a batch fermentation is
V L m net X
YF X= H
DH G ¼
(11.53)
where V L is the liquid volume (L) and X is the cell concentration (g/L).
CO 2 + H 2 O + Microbial Cells + Extracellular Products (P)
Complete
combustion of
Extracellular
Products
Microbial cells
growth and
respiration
Complete
combustion of
microbial cells
YF H/X , kJ/g-cell
H c,X , kJ/g-cell
H c,P , kJ/g-P
Complete combustion
H c,S , kJ/g-substrate
Substrate + O 2
CO 2 + H 2 O
FIGURE 11.9 Enthalpy balance on substrate combustion and with microbial and extracellular product utilizations.
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