Relationship of alcohol production to lipid composition of yeast in a continuous flow bioreactor

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Abstract
Saccharomyces cerevisiae was cultivated in a controlled aerated, dual-stage (column), continuous flow bioreactor in a hybrid free-cell and immobilized-cell state. The yeast cells maintained an ethanol concentration of 58–64 and 91–98 g/L in stages I and II, respectively. The lipid composition of the cells cultivated under these conditions was correlated to the effects of aeration by interrupting the aeration on days 113 and 266 of continuous operation. Under conditions of aeration or nonaeration, an alternating increase and decrease in the contents of squalene, sterols, and fatty acids of the respiratory-competent and -deficient unattached free cells was observed. The cellular free lipid compositions of the immobilized cells in the aerated and nonaerated conditions were similar and characteristic of respiratory-deficient cells with the exception of the immobilized cells exposed to a higher ethanol concentration (stage II). These cells contained a broader range of sterol components and increased levels of unsaturated fatty acids than immobilized cells at a lower ethanol concentration (stage I). The neutral lipid to phospholipid ratio decreased for respiratory-deficient cells with phosphatidylethanolamine and phosphatidylinositol being the principal phospholipids. The data demonstrated the essentiality of the hybrid bioreactor design for continuous long term performance and the importance of maintaining specific yeast lipid constituents for continuous high alcohol productivity.Key words: yeast immobilization, lipids, fatty acids, sterols, phospholipids, continuous flow, aeration, ethanol.