Determination of yield and maintenance coefficients in citric acid production by Aspergillus niger

One method for economic planning and management of production processes is a balance of substrate mass and energy it contains. Theoretical assumptions and practical applications of mass and energy balances in the process of citric acid production by Aspergillus niger are discussed in the study.

In the calculation of biomass and product yields as well as the maintenance coefficients mechanistic values such as true biomass and citric acid yields Y^max_ATP,X(P) on ATP and specific ATP consumption m_ATP due to maintenance processes were used. A set of limitations imposed on theoretical and true yield coefficients in the linear growth equation is also presented.

The experimental measurements were performed in an air-lift bioreactor of 220 dm³ operating capacity, with external circulation loop. The sole carbon source for the cultivation of A. niger was sucrose at an initial concentration of 100 g l⁻¹.

The experimentally obtained yield coefficients relative to cell growth and citric acid production have reached 96% and 83% respectively of real maximum theoretical values. Calculated true biomass and citric acid yield coefficients were closely correlated with the values from the balance analysis of stoichiometric equations. Calculated value of specific maintenance requirements m_ATP was 0.015 mol ATP [mol C(dry mass)]⁻¹ h⁻¹.