Propionic acid production in glycerol/glucose co-fermentation by Propionibacterium freudenreichii subsp. shermanii

•Glucose gave considerable cell biomass and acetate, with a low propionate yield.•Glycerol gave higher propionate yield and selectivity, but low productivity.•Co-fermentation of glycerol and glucose improved propionate productivity and yield.•Co-fermentation showed robust flux redistribution for redox balance and cell growth.•Propionate can be produced from low-cost crude glycerol and cassava bagasse. Propionibacterium freudenreichii subsp. shermanii can ferment glucose and glycerol to propionic acid with acetic and succinic acids as two by-products. Propionic acid production from glucose was relatively fast (0.19g/Lh) but gave low product yield (∼0.39g/g) and selectivity (P/A: ∼2.6; P/S: ∼4.8). In contrast, glycerol with a more reduced state gave a high propionic acid yield (∼0.65g/g) and selectivity (P/A: ∼31; P/S: ∼11) but low productivity (0.11g/Lh). On the other hand, co-fermentation of glycerol and glucose at an appropriate mass ratio gave both a high yield (0.54–0.65g/g) and productivity (0.18–0.23g/Lh) with high product selectivity (P/A: ∼14; P/S: ∼10). The carbon flux distributions in the co-fermentation as affected by the ratio of glycerol/glucose were investigated. Finally, co-fermentation with cassava bagasse hydrolysate and crude glycerol in a fibrous-bed bioreactor was demonstrated, providing an efficient way for economic production of bio-based propionic acid.