LIPID COMPOSITION OF AEROBICALLY AND ANAEROBICALLY PROPAGATED BREWER'S BOTTOM YEAST

Aerobically grown pitching yeast is very rich in unsaturated fatty acids and sterol esters compared to traditional, anaerobic yeast. The principal fatty acids in aerobic yeast cells are unsaturated palmitoleic and oleic acids, whereas in anaerobic cells saturated palmitic acid predominates. The difference in fatty acid distribution between aerobic and anaerobic cells is most marked in the sterol esters. The fatty acids of phospho-lipids are more stable, although remarkable differences are observed. The sterols of aerobic cells are almost entirely in esterified form and zymosterol is the principal sterol. During the first hours of fermentation a rapid synthesis of palmitoleic acid is observed when anaerobic yeast is used for pitching and the wort is aerated. The synthesis of oleic acid requires more oxygen and time than is available under normal brewing conditions. When aerobic pitching yeast is used no more unsaturated fatty acids are synthesised and the lipid stores of pitching yeast are distributed among the daughter cells. The decrease in acetate ester production by aerobic pitching yeast is concluded to be due to a decrease in acetyl CoA synthesis, which may be caused by the high proportion of unsaturated fatty acids in membrane lipids.