Bioprocess intensification : production of bioethanol from Saccharomyces cerevisiae W303 in monolithic microreactor

Abstract

This study reveals the development of new 3-D support for ethanol production by Saccharomyces cerevisiae W303. The production of bioethanol by immobilising this organism had been demonstrated to have greater advantages over a suspended culture of free cells of S.cerevisiae W303. The production of ethanol is proportional to the growth of yeast, so preparing suitable supports with unlimited spaces that permit the cell proliferation are crucial for a long term continuous operation. The 3-D scaffold prepared from high internal phase emulsion (HIPE) offers good mechanical strength, and has a high surface area for allowing monolayer cell proliferation which was necessary in order to avoid additional stresses for the nutrient and oxygen transfer in the micro-environment. The enhanced porosity of this 3-D scaffold that was characterised by highly interconnected pores, not only promoted the dynamic condition in the monolith, but also facilitated easy flushing of dead cells and metabolic product. This study reveals that sulphonated polyHIPEs, a highly hydrophilic polymer which had pore and interconnect sizes of 45 μm and 16 μm respectively, had shown good bio-compatibility with the model organism, subsequently allowing its growth and glucose conversion. The ethanol productivity in the microreactor was greatly enhanced to 4.72 gL-1h-1, being over 12 times higher than that observed in the suspended shake flask culture (0.41 gL-1h-1) by free cell S.cerevisiae W303, despite only 60.1% of glucose being consumed. Since the remaining sugar must be kept low, the glucose utilization was further enhanced by introducing the two-stage reactor in series. The consumption of glucose was enhanced by 20.1% (compared to single stage reactor), where nearly 72.2 % of the supplied glucose was converted per pass during the pseudo steady state condition. This, on the other hand, increased the ethanol productivity to 5.84 gL-1h-1, which was 14 fold higher than the productivity obtained in the shake flask culture. The increment might be associated with the altered metabolical functions in the immobilised cells. This alteration is attributed to the reduction of the diffusion path of the growth nutrient (e.g: carbon, nitrogen and oxygen) that enhanced the availability and promoted the growth of yeast in the microreactor, thus enhancing the catalytic conversion of glucose to ethanol.