Current progress in high cell density yeast bioprocesses for bioethanol production

High capital costs and low reaction rates
are major challenges for establishment of fermentation-based production systems
in the bioeconomy. Using high cell density cultures is an efficient way to
increase the volumetric productivity of fermentation processes, thereby
enabling faster and more robust processes and use of smaller reactors. In this
review, we summarize recent progress in the application of high cell density
yeast bioprocesses for first and second generation bioethanol production. High
biomass concentrations obtained by retention of yeast cells in the reactor
enables easier cell reuse, simplified product recovery and higher dilution
rates in continuous processes. High local cell density cultures, in the form of
encapsulated or strongly flocculating yeast, furthermore obtain increased
tolerance to convertible fermentation inhibitors and utilize glucose and other
sugars simultaneously, thereby overcoming two additional hurdles for second
generation bioethanol production. These effects are caused by local
concentration gradients due to diffusion limitations and conversion of
inhibitors and sugars by the cells, which lead to low local concentrations of
inhibitors and glucose. Quorum sensing may also contribute to the increased
stress tolerance. Recent developments indicate that high cell density
methodology, with emphasis on high local cell density, offers significant
advantages for sustainable second generation bioethanol production.