Utilization of agro-food by-products for gluconic acid production by Aspergillus niger ORS-4 under surface culture cultivation

Among the seven isolated microbial strains from dumping sites of the sugarcane industry waste, a potent fungal strain Aspergillus niger ORS-4 was selected, that gave 48 g/L of gluconic acid with 74 per cent yield when glucose was used as the carbon source. Starch hydrolysate, molasses and the banana must were evaluated as the cheaper carbohydrate sources for gluconic acid production by A niger ORS-4 in surface culture fermentation process. The banana must was found to be a better source with significant gluconic acid production (39.6 g/L, 40 per cent yield) after 12 d incubation. The untreated sugarcane molasses gave marginal production of gluconic acid (2.4 g/L), however, the production increased significantly (34.6 g/L, yield 38.5 per cent) after the molasses were subjected to the hexacynoferrate (HCF) treatment. Starch hydrolysate on the other hand, resulted into comparative production (30.2 g/L, yield 35.9 per cent) but lower than that obtained with HCF treated molasses, whereas the acid production was low (10 g/L) with unhydrolyzed starch. Gluconic acid production from these substrates was comparable to that obtained with glucose.