A critical review of the advances in valorizing agro-industrial wastes through mixed culture fermentation

Unsustainable management practices of agro-industrial wastes - generated across supply-chain, on-farm operations, and post-harvest stages - result in myriad environmental problems, with greenhouse gas emissions being the most severe. Recently, however, several biotechnological approaches - anaerobically valorizing agro-industrial wastes - have been developed; these approaches rely on novel microbial strains, which serve as cell factories and engineered using indigenous microbial species and modern genetic techniques (e.g., genetic encoding, recombinant DNA technology); additionally, these approaches take half the conventional digestion periods and generate a variety of liquid and gaseous bioproducts - which have a host of uses in different industries. The novel microbial (or 'ethanologenic') strains, such as Saccharomyces cerevisiae EJ2 and SR8, result in high lactic acid and ethanol yields (> 90% of the theoretical maximum) and production rates (> 1 g L-1 h(-1)). In this review, the important aspects of mixed culture fermentation involving recent works on adaptive laboratory evolution, directed evolution, separate hydrolysis fermentation, simultaneous saccharification and fermentation, and dark fermentation have been critically reviewed; these aspects include (i) development of new microbial strains (e.g., Ralstonia eutropha 5119, Yarrowia lipolytica PSA02004PP, S. cerevisiae strain WXY74), (ii) the role of these engineered strains to utilize the different lignocellulosic components (e.g., xylose) towards deriving high yields of value-added downstream compounds and platform chemicals, and (iii) the role of H-2 producing anaerobes in the DF pathways to generate fatty acids and solvents via chain elongation and reverse beta-oxidation. The present review, therefore, attempts to deliver critical insights on the recent advances vis-& agrave;-vis MCF of agro-industrial wastes.