Enhanced solid-state citric acid bio-production using apple pomace waste through surface response methodology

Aims: To evaluate the potential of apple pomace (AP) supplemented with rice husk for hyper citric acid production through solid-state fermentation by Aspergillus niger NRRL-567. Optimization of two key parameters, such as moisture content and inducer (ethanol and methanol) concentration was carried out by response surface methodology. Methods and Results: In this study, the effect of two crucial process parameters for solid-state citric acid fermentation by A. niger using AP waste supplemented with rice husk were thoroughly investigated in Erlenmeyer flasks through response surface methodology. Moisture and methanol had significant positive effect on citric acid production by A. niger grown on AP (P < 0 center dot 05). Higher values of citric acid on AP by A. niger (342 center dot 41 g kg-1 and 248 center dot 42 g kg-1 dry substrate) were obtained with 75% (v/w) moisture along with two inducers [3% (v/w) methanol and 3% (v/w) ethanol] with fermentation efficiency of 93 center dot 90% and 66 center dot 42%, respectively depending upon the total carbon utilized after 144 h of incubation period. With the same optimized parameters, conventional tray fermentation was conducted. The citric acid concentration of 187 center dot 96 g kg-1 dry substrate with 3% (v/w) ethanol and 303 center dot 34 g kg-1 dry substrate with 3% (v/w) methanol were achieved representing fermentation efficiency of 50 center dot 80% and 82 center dot 89% in tray fermentation depending upon carbon utilization after 120 h of incubation period. Conclusions: Apple pomace proved to be the promising substrate for the hyper production of citric acid through solid-state tray fermentation, which is an economical technique and does not require any sophisticated instrumentation. Significance and Impact of the Study: The study established that the utilization of agro-industrial wastes have positive repercussions on the economy and will help to meet the increasing demands of citric acid and moreover will help to alleviate the environmental problems resulting from the disposal of agro-industrial wastes.