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

), 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.