| Abstract: | Ethanol fermentation by Saccharomyces cerevisiae was investigated in broth-organic solvent two-liquid phase systems with and without interphase mixing. The effect of interphase mixing with aliphatic alcohols as polar solvents and with alkanes as apolar solvents was determined from investigations in three distinct systems: a solvent-saturated broth, a solvent-broth dispersion system and a dispersion-free solvent-broth system. Microbial activities, measured as a function of solvent volume and inoculum size, revealed that a dispersed polar solvent caused a longer growth lag time and a slower growth rate. Viability measurements showed that yeast cells were gradually killed in a dodecanol-water dispersion system and not in a dispersion-free one. This process was believed to be initiated and enhanced by cell adhesion to the solvent interface as observed under a microscope. Yeast activity was also measured as a function of an apolar solvent volume and inoculum size. A full fermentative activity was observed in an initially alkanesaturated broth unlike in an intermixed biphasic system. As a result of these investigations, it was concluded firstly that microbial activity in initially solvent-saturated broths with and without contact with a stagnant solvent phase can be different and secondly that the process efficiency of a biphasic bioreactor, unlike that of a chemical reactor, can be hampered by interphase mixing. The yeast activities in the presence of the various alcohols and alkanes were correlated with ‘log P’ values of the solvents. Full activity was observed at log P > 5 and almost none at log P < 4. |
