Selective short-chain carboxylates production: A review of control mechanisms to direct mixed culture fermentations

Anaerobic digestion of organic residual streams can be directed to produce carboxylates such as acetate, propionate, and n-butyrate, which can be either directly used in industry or converted into other valuable compounds. This paper reviews the studies working with mixed culture conversion of organic substrates toward carboxylates. It draws connections between standard fermentation parameters and the carboxylate product concentrations and composition. The use of more concentrated carbohydrate-rich substrates, at longer residence times and at neutral pH ranges, stimulates total acid production. When increasing pH to the neutral range, acetate and propionate fractions are elevated. High propionate concentrations and fractions are infrequently reported and mainly appear on high-protein-containing feedstock. High n-butyrate fraction >70% is usually found when pH < 6, at longer retention times or organic loading rates, under CO2 atmosphere or on substrates with high lactate concentrations. The review concludes with prospects for further developments related to the carboxylate platform.