Abstract
Background: Simultaneous and effective conversion of both pentose and hexoses in fermentation is a critical and challenging task toward the lignocellulosic economy. This study aims to investigate the feasibility of an innovative co-fermentation process featured with cell recycle unit (CF/CR) for mixed sugar utilization. A l-lactic acid producing strain Enterococcus mundtii QU 25 was applied in the continuous fermentation process to utilize the mixed sugar at different productivities over the changes of flowing conditions. Structured numerical platform were constructed with the experiments to optimize the biological process and clarify the cell metabolisms through kinetics analysis. The structured model, kinetic parameters, and achievement of the fermentation strategy shall provide new insights towards whole sugar fermentation via real-time monitoring for process control and optimization.Results: Significant carbon catabolite repression of co-fermentation using glucose/xylose mixture was overcome by replacing glucose with cellobiose, of which the consumption ratio of hexose to pentose was improved dramatically from 10.4:1 to 2.17:1. An outstanding product concentration of 65.15 g·L -1 and productivity of 13.03 g·L -1 ·h -1 were achieved with 50 g·L -1 cellobiose and 30 g·L -1 xylose, at an optimized dilution rate of 0.2 h -1 with gradually increased cell retention time. Among the total lactic acid production, xylose contributed to more than 34% of the mixed sugars, which was close to the related contents in agricultural residuals. The model successfully simulated the transition of sugar consumption, cell growth, and lactic acid production among the batch, continuous process, and CF/CR system.Conclusion: Cell retention time played a critical role in balancing pentose and hexose consumption, cell decay, and lactic acid production in the CF/CR process. With the increase of cell concentration, consumption of mixed sugars increased with the productivity of final products, hence the impacts of substrate inhibiting reduced. With the validated parameters, the model showed highest accuracy simulating the CF/CR process, of which significantly longer cell retention times over hydraulic retention time were tested.
Efficient Homofermentative l-(+)-Lactic Acid Production from Xylose by a Novel Lactic Acid Bacterium,Enterococcus mundtiiQU 25
