Mixed inoculum origin and lignocellulosic substrate type both influence the production of volatile fatty acids during acidogenic fermentation

Volatile fatty acids (VFAs) are increasingly viewed as platform substances for the production of sustainable biofuels and green chemicals. Optimising the acidogenic fermentation requires determining which environmental parameters (temperature, pH, substrate, inoculum, etc) have the largest impact on the metabolite profile. The present study investigates the influence of substrate and mixed inoculum origin. Nine combinations of three complex lignocellulosic substrates and three different mixed inocula were monitored under batch conditions (35 °C; pH 6.5; triplicates, monitoring of pH, soluble COD and VFAs) during 30 days. All nine combinations led to a significantly different fermentation profile, and each combination represented a unique situation. The metabolite profiles were highly reproducible within each substrate-inoculum combination. The substrate COD conversion yield to VFAs was up to 64% (average 31%). VFAs represented most (between 61 and 98%) of the soluble COD. Acetic, propionic and butyric acids were always the major VFAs.