Efficient production of lactic acid from anaerobic co-fermentation of starch and nitrogen-rich agro-industrial waste using a batch system

Anaerobic co-fermentation is an interesting process that can efficiently valorize complex organic substrates by converting them into lactic acid. One of the main issues, however, is the high substrate cost, and its reduction is of great significance to the practical application. Therefore, this study investigated the feasibility of lactic acid production from carbohydrate-enrich corn starch, tapioca starch, potato starch, and rice starch with inexpensive nitrogen-rich agro-industrial waste of corn steep liquor, palm kernel cake, and soybean meal. Fermentation results indicated that the co-fermentation of starch with corn steep liquor significantly enhanced the lactic acid concentration compared with the co-fermentation of starch with palm kernel cake or soybean meal (p < 0.05). The highest lactic acid concentrations of 68.89 & PLUSMN; 6.30, 71.78 & PLUSMN; 13.74, 67.90 & PLUSMN; 2.94, and 68.83 & PLUSMN; 2.68 g/L were achieved under the co-fermentation of corn starch, tapioca starch, potato starch, and rice starch with corn steep liquor, respectively. The microbial community structure showed the dominance of Lactobacillus (60.78%95.97%) in the co-fermentation of different substrates, which is believed to be beneficial to the growth of such fermentative bacteria with the main products of lactic acid. Furthermore, the economic analysis demonstrated that the co-fermentation of corn starch and corn steep liquor had better economic benefits than other treatments. This study provides a promising way to reduce lactic acid production costs, which will change the economic feasibility of lactic acid fermentation.