Metabolic measurement of biomass
- Respiratory metabolism
Oxygen consumption and carbon dioxide release result from respiration, the metabolic process by which aerobic micro-organisms derive most of their energy for growth. These metabolic activities are therefore growth associated and can be used for the estimation of biomass synthesis.
As carbon compounds within the substrate are metabolised, they are converted into biomass and carbon dioxide. Production of carbon dioxide causes the weight of fermenting substrate to decrease during growth, and the amount of weight lost can be correlated to the amount of growth that has occurred.
Growth estimation based on carbon dioxide release or oxygen consumption assumes that the metabolism of these compounds is completely growth associated, which means that the amount of biomass produced per unit of gas metabolised must be constant. Sugama and Okazaki (1979) reported that the ratio of mg CO2 evolved to mg dry mycelia formed by Aspergillus oryzae on rice ranged from 0.91 to 1.26 mg CO2 per mg dry mycelium. A gradual increase in this ratio was observed late in growth due to endogenous respiration. Drastic changes can be observed for the respiratory quotient which commonly changes with the growth phase, i.e: germination, rapid and vegetative growth, secondary metabolism, conidiation and degeneration of the mycelium. Evolution of CO2 and O2 during SSF of Rhizopus on cassava is presented in Figure 1.