Microbial granulation for lactic acid production

Abstract

This work investigated the formation of microbial granules to boost the productivity of lactic acid (LA). The flocculated form of LA-producing microbial consortium, dominated by Lactobacillus sp. (91.5% of total sequence), was initially obtained in a continuous stirred-tank reactor (CSTR), which was fed with 2% glucose and operated at a hydraulic retention time (HRT) of 12h and pH 5.0 +/- 0.1 under a thermophilic condition (50 degrees C). The mixed liquor in the CSTR was then transferred to an up-flow anaerobic sludge blanket reactor (UASB). The fermentation performance and granulation process were monitored with a gradual decrease of HRT from 8.0 to 0.17h, corresponding to an increase in the substrate loading from 60 to 2,880g glucose L(-1)d(-1). As the operation continued, the accumulation of biomass in the UASB was clearly observed, which changed from flocculent to granular form with decrease in HRT. Up to the HRT decrease to 0.5h, the LA concentration was maintained at 19-20gL(-1) with over 90% of substrate removal efficiency. However, further decrease of HRT resulted in a decrease of LA concentration with increase in residual glucose. Nevertheless, the volumetric LA productivity continuously increased, reaching 67g L-fermenter(-1)h(-1) at HRT 0.17h. The size of LA-producing granules and hydrophobicity gradually increased with decrease in HRT, reaching 6.0mm and 60%, respectively. These biogranules were also found to have high settling velocities and low porosities, ranging 2.69-4.73cms(-1) and 0.39-0.92, respectively. Biotechnol. Bioeng. 2016;113: 101-111.