Continuous simultaneous saccharification and co-fermentation (SSCF) for cellulosic L-lactic acid production

Continuous fermentation is preferred in industrial bioprocesses by its steady-state operation mode and the significant reduction of switch operation of vessels emptying, cleaning, and sterilizing. General biorefinery fermentation of lignocellulose is conducted in the mode of simultaneous saccharification and co-fermentation (SSCF) thus the continuous operation does not stand well because of the mismatch between the optimal temperatures of cellulase enzymes and microbial cells. This study challenged the continuous SSCF operation for production of high chiral purity L-lactic acid based on the dry biorefinery processing platform. An engineered thermophilic L-lactic acid bacterium Pediococcus acidilactici with complete non-glucose assimilation capacity and antibacterial activity with a nearly perfect match of temperature (42–50 °C) and pH (5.5) with that of cellulase enzyme used (50 °C, pH 4.8). This microbial cell factory allows the enzymatic hydrolysis and fermentation to be conducted simultaneously in one-pot bioreactor and provides the basis for establishing a valid continuous SSCF operation. The continuous SSCF of the biodetoxified wheat straw was established in cascade bioreactors and the performances of L-lactic acid titer, yield, and productivity reached 107.5 ± 1.1 g/L, 0.29 ± 0.01 g/g DM, and 2.69 ± 0.03 g/L/h, respectively. The chiral purity of the cellulosic L-lactic acid reached 99.5 %. The low residual sugar concentration and high-optical purity L-lactic acid product in broth facilitate the subsequent purification of L-lactic acid as the monomer for cyclic lactide or direct PLA synthesis. This multi-stage continuous SSCF technology overcomes the inherent barriers of the batch SSCF operation and provides a prototype for future industrial cellulosic lactic production.