Enzymatic hydrolysis of rice straw and glucose fermentation using a Vertical Ball Mill Bioreactor (VBMB): Impact of operational conditions

The effects of agitation speed (100-200 rpm), number of glass spheres (0-30 units) and temperature (40-46 °C) on both enzymatic hydrolysis of rice straw (8% w/v) and glucose fermentation (50 g/L) by Kluyveromyces marxianus NRRL Y-6860 were evaluated using a Vertical Ball Mill Bioreactor (VBMB). This bioreactor was equipped with adjustable flat round plate impellers, allowing its operation with glass spheres as shear agent. For enzymatic hydrolysis, the spheres were the only variable with significant impact on the results, being achieved 87% cellulose conversion after 24 h when using the highest level of spheres at 46 °C. For glucose fermentation, none of the variables influenced the ethanol yield from glucose, which achieved 80% on average. However, for ethanol volumetric productivity, the spheres showed an interactive effect with temperature, being obtained 2.16 g/L.h when both variables were used in the lowest level. These results revealed that the glass spheres played an important role during biomass’ fiber liquefaction and yeast’s fermentative performance. Nevertheless, the shear effect caused by the spheres was positive for enzymatic hydrolysis and negative for glucose conversion into ethanol. By applying the needed adjustments on the levels of the variables for each process (hydrolysis and fermentation), the VBMB can be efficiently used for rice straw bioconversion into ethanol. In addition, the design of this bioreactor would allow its use in different processes, such as simultaneous saccharification and fermentation, in batch or fed-batch configurations, and with possibilities of operating at high solids content.
Acknowledgments: FAPESP (2013/13953-6 and 2015/24813-6) and CNPq.