Continuous ethanol production in a two-stage, immobilized/suspended-cell bioreactor

Abstract

A two-stage, continuous-flow bioreactor, containing both immobilized and suspended free yeast cells, was constructed and tested for its capacity to produce ethanol over an extended period of time from a feedstock containing sucrose. The reactor has now been in continuous operation for 2 years. The operational conditions required to maintain an ethanol production level of 98 (±3.3) g l −1 and total sugar consumption of 95.8% was an adequate feedstock (synthetic sugar-cane juice with 200 g l −1 sucrose, mineral salts, and 5 g l −1 yeast extract), a nutrient supplement of yeast extract, peptone, and mineral salts to the second reactor stage, continuous aeration to both stages at 10–30 ml min −1, and a liquid recycle ratio of 20:1 within each reactor stage. The controlled aeration was necessary to maintain an active and stable yeast population. The dilution rates studied varied from 0.1 to 0.25 h −1. The greatest ethanol productivity was achieved at the highest dilution rate tested (0.25 h −1, but the most complete conversion of sugar to ethanol occurred at the lowest dilution rate. The most effective nutrient supplement for supporting maximal ethanol production was obtained with a combined addition of yeast extract, peptone, and mineral salts. The effectiveness of the nutrient supplement could not be equalled by substituting vitamins, inositol, acetic acid, or trace minerals either alone or in combinations. High-test molasses (commercially concentrated, natural sugarcane juice) was a suitable feedstock only when it was fortified with ammonium salts. It was concluded from this study that the combination of the immobilized cells and high concentration of suspended free cells within the air-supplemented reactor stabilized the continuous ethanol fermentation for long-term performance, eliminated the necessity for cell conditioning and recycle, and effectively suppressed infection from extrinsic organisms.