Mutation of Aspergillus niger for hyperproduction of citric acid from black strap molasses

Abstract

Spore suspensions of Aspergillus niger GCB 75, which produced 31.1 g/l citric acid from 15% sugars in molasses, were subjected to u.v.-induced mutagenesis. Among three variants, GCM 45 was found to be the best citric acid producer and was further improved by chemical mutagenesis using NTG. Out of 3 deoxy-D-glucose-resistant variants, GCM 7 was selected as the best mutant which produced 86.1 ± 1.5 g/l citric acid after 168 h of fermentation of potassium ferricyanide + H2SO4-pretreated black strap molasses (containing 150 g sugars/l) in Vogel's medium. On the basis of comparison of kinetic parameters, namely the volumetric substrate uptake rate (Qs), and specific substrate uptake rate (qs), the volumetric productivity, theoretical yield and specific product formation rate, it was observed that the mutants were faster growing organisms and had the ability to overproduce citric acid.