Ethanol fermentation using macroporous monolithic hydrogels as yeast cell scaffolds

Abstract

Ethanol fermentation with a hybrid system containing free and immobilized yeast cells on a macroporous monolithic hydrogel scaffold was investigated. Cell immobilization was achieved using N-isopropylacrylamide (NIPA) and methoxy triethyleneglycol acrylate (MTGA) hydrogels having macroporous monolithic structures containing interconnected micrometer-sized macropores. The immobilized cells grow within the hydrogel and can exit through the macropores into the fermentation medium, and thus a hybrid bioreactor consisting of free and immobilized yeast cells is constructed. The transformation of d-glucose to ethanol using Saccharomyces cerevisiae in batch and continuous fermentation processes was examined. Ethanol was successfully fermented using the macroporous hydrogels, whereas the conventional nonporous NIPA hydrogel does not effectively act as a cell scaffold. In continuous fermentation, the hybrid system containing both free yeast cells and those immobilized in the macroporous MTGA hydrogel showed higher ethanol productivity than the control system containing only free cells. Crucially, the macroporous hydrogel allowed the stable growth of immobilized yeast cells, resulting in a high cell density in the reactor. The hybrid system under steady-state fermentation had free and immobilized cell densities of 1.72 kg-free-cell/m3 and 29 kg-immobilized-cell/m3-gel, respectively, and the ethanol productivities 0.421 kg-ethanol/(kg-free-cell·h) and 0.128 kg-ethanol/(kg-immobilized-cell·h), respectively.