Screening of agro-industrial wastes for citric acid bioproduction by Aspergillus niger NRRL 2001 through solid state fermentation

Background The citric acid (CA) industry is currently struggling to develop a sustainable and economical process owing to high substrate and energy costs. Increasing interest in the replacement of costly synthetic substrates by renewable waste biomass has fostered research on agro-industrial wastes and screening of raw materials for economical CA production. The food-processing industry generates substantial quantities of waste biomass that could be used as a valuable low-cost fermentation substrate. The present study evaluated the potential of different agro-industrial wastes, namely apple pomace (AP), brewer's spent grain, citrus waste and sphagnum peat moss, as substrates for solid state CA production using Aspergillus niger NRRL 2001. Results Among the four substrates, AP resulted in highest CA production of 61.06 +/- 1.9 g kg1 dry substrate (DS) after a 72 h incubation period. Based on the screening studies, AP was selected for optimisation studies through response surface methodology (RSM). Maximum CA production of 312.32 g kg1 DS was achieved at 75% (v/w) moisture and 3% (v/w) methanol after a 144 h incubation period. The validation of RSM-optimised parameters in plastic trays resulted in maximum CA production of 364.4 +/- 4.50 g kg1 DS after a 120 h incubation period. Conclusion The study demonstrated the potential of AP as a cheap substrate for higher CA production. This study contributes to knowledge about the future application of carbon rich agro-industrial wastes for their value addition to CA. It also offers economic and environmental benefits over traditional ways used to dispose off agro-industrial wastes. (c) 2012 Society of Chemical Industry