Attachment of yeast to modified stainless steel wire spheres, growth of cells and ethanol production

The immobilization of yeast Saccharomyces cerevisiae, their growth and ethanol production were investigated using untreated and modified stainless steel wire spheres (WS) as carriers. The carrier surface was modified by oxidation, by treatment with titanium (IV) chloride (TiCl4,) or by gamma-aminopropyltrietoxysilane (AS) in an attempt to raise the efficiency of the immobilization of the yeast cells. The influence of the cell fixation method on culture growth and ethanol synthesis was investigated. The immobilization of cells to carrier surface was checked by scanning electron microscopy (SEM). More closely attachment of yeast cells was seen on the aminated wire surface. It was established that during fermentation ethanol production by yeast was stimulated using immobilized cells in oxidized WS or treated with TiCl4,. Aminated WS surface stimulated the culture growth but decreased ethanol synthesis. Free yeast cells located in the pores of WS increased the biomass concentration and ethanol production only during the first cycle of batch fermentation. Stable cell growth and ethanol production was observed during subsequent 4-5 repeated fermentation cycles using washing out of free cells from WS before fermentation. The system productivity Q(eth),,, for ethanol synthesis was 0.92-1.25 g/l per h. Cell fixation in WS by lyophilization or convective dehydration improved cell attachment to wire surface but did not influence positively culture growth and ethanol synthesis. The conclusion was made that stainless steel WS filled with paste-like yeast biomass can be used as inoculum for repeated batch ethanol production. The modification method of the stainless steel wire surface significantly influences the immobilization efficiency of yeasts. Oxidized or modified by TiCl4, wire surface and washing out free cells from WS can be recommended for ethanol production by immobilized yeasts. (C) 2000 Elsevier Science Ltd. All rights reserved.