Mucor indicus as a biofilter and fermenting organism in continuous ethanol production from lignocellulosic hydrolyzate

The dimorphic filamentous fungus Mucor indicus was used as a biofilter and fermenting microorganism in a bioreactor for continuous cultivation of dilute-acid lignocellulosic hydrolyzates up to dilution rate (D) 0.3 h−1. The hydrolyzate was so toxic that the fungus could not grow in traditional stirred-tank bioreactors even at D 0.1 h−1. On the other hand, the fungus could be manipulated to grow as a mixture of short dispersed filaments and yeast-like cells. This morphology was suitable to create a biological filter, when attached to a specific screen located across the outlet stream. This filter was fully permeable to the liquid, but not the biomass inside the bioreactor. While the biomass concentration in the outflow was between 0.51 and 3.43 g/l at dilution rates 0.3–0.1 h−1, the biomass concentration inside the bioreactor was held at 8.34–9.91 g/l. The cells consumed 95.7%, 94.3%, and 75.3% of hexoses and 64.5%, 25.9%, and 7.6% of xylose present in the hydrolyzate at D 0.1, 0.2, and 0.3 h−1, respectively. The ethanol yields at these dilution rates were 0.37, 0.45, and 0.46 g/g consumed sugars, respectively. The only important by-products were glycerol and xylitol, which at D 0.2 h−1 yielded 0.036 and 0.022 g/g consumed sugars, respectively.