Carbohydrate concentration and type drive product selectivity to a mixture of volatile fatty acids or lactic acid in thermophilic mixed-culture fermentation

Abstract Carboxylic acids are key platform chemicals that can be biologically produced in mixed-culture fermentations. In these systems, product formation is determined by the microbial community composition, which is shaped by operational conditions. A general trend has been observed linking substrate availability to the production of either lactic acid or volatile fatty acids, but it remains unclear at which substrate concentration this shift occurs. This study investigates the effect of carbohydrate concentrations (from 0 to 670 mg COD·L −1 ) and type (hexoses and pentoses) on product selectivity and microbial community composition in thermophilic mixed-culture fermentations. Fermentation experiments were conducted in thermophilic reactors (55 °C, pH 5.3, HRT 4 days), fed either with glucose or xylose. Higher substrate concentrations (650–670 mg COD·L −1 ) favored lactic acid production, accounting for 60–76% of the soluble products. Lower carbohydrate concentrations reduced lactic acid production and increased volatile fatty acid concentrations. Volatile fatty acids became the main product under low substrate concentrations (continuous operation; 0 mg COD·L −1 ), with a selectivity of 83–97%. Despite this general trend, the substrate concentration at which the shift from lactic acid to volatile fatty acids occurred depended on the carbohydrate type. Microbial community analyses revealed Thermoanaerobacterium as the dominant genus in all reactors, with genera within the Bacillaceae family (putatively involved in lactic acid production) increasing in relative abundance under high carbohydrate concentrations. This suggests that the product shift resulted from both a metabolic shift within the dominant species and a change in the microbial community composition. Key points Low carbohydrate concentrations favor VFA production. High carbohydrate concentrations promote lactic acid formation. Carbohydrate type influences the shift from lactic acid to VFAs.