Continuous Flow Cell-Recycle Fermentation of Biomass Hydrolysates

Abstract

Fermentation studies were conducted in batch and continuous flow cell-recycle reactors with glucose, xylose and biomass hydrolysates as substrates. Glucose fermentation at pH 6 indicates that Propionibacterium acidipropionici rapidly consumes the substrate in an initial exponential growth stage producing large amounts of lactic acid A slow growth stage follows, during which the lactic acid is further metabolized to produce acetic and propionic acids. The bacterial metabolism was altered when xylose was used as the substrate, and lactic acid was not produced during the fermentation. The product and cell yields obtained from the glucose fermentation were 0.76 g acid (g glucose)⁻¹ and 0.21 g dry cell (g glucose)⁻¹, respectively. Xylose fermentation gave about the same cell yield, but the total product yield was only 63% of that with glucose fermentation. Continuous fermentation with cell recycle using microfiltration resulted in a dramatic increase in cell concentration (X) and volumetric productivity (P). P ranged from 0.56 to 2.83 g dm⁻³h⁻¹ for dilution ratios (D) ranging from 0.036 to 0.23 h⁻¹. X and P at a D of 0.23 h⁻¹ were 11 and 22 times the values for batch fermentation at pH 6. Cell-recycle fermentation with hydrolysates from bakery waste and wood chips gave P equivalent to 93% and 71%, respectively, compared to that with glucose.