3.5. Effect of incubation time
Fermentation was carried out at different incubation periods to standardize the optimum incubation time. High levels of citric acid formed at 72 h of incubation. Further incubation results in decreased citric acid production (Fig. 5). Maximum biomass production was observed at 72 h after that the increase was insignificant. It has been reported that optimum incubation time for A. niger was 120 h for citric acid production when cassava bagasse was used as a substrate (Vandenberghe et al., 2000); 4 days in jack fruit carpel fiber (Angumeenal and Venkappayya, 2005) and 5 days in Kumara (Lu et al., 1997). Citric acid production and biomass were highly correlated (R2 = 0.83) at different incubation periods (Fig. 5a).