When added to wort, long chain fatty acids were incorporated into the lipids of Sacch. cerevisiae during fermentation. Linoleic or oleic, but not stearic acid significantly suppressed the formation, by yeast, of ethyl and iso-amyl acetates. Synthesis of these acetate esters was also suppressed when inocula enriched with linoleyl residues were used. Further effects of addition of linoleic acid were: inhibition of unsaturated but stimulation of saturated fatty acyl residues; a shift from synthesis of medium towards long chain acyl residues; enhancement of the conversion of squalene to lanosterol. Syntheses of lipids (sterols; unsaturated and saturated fatty acyl residues) uere most active during the initial two hours of fermentation when oxygen was present. Following this phase the specific rate of ethyl acetate synthesis increased to a constant level. Saturated fatty acyl residues and squalene continued to be formed but only until the mid-point of fermentation. At this point a further significant increase in the specific rate of ethyl acetate synthesis was observed. Increased rates of ester synthesis were not sustained but contributed significantly to the overall production of esters. The ratio of acetyl-CoA to CoASH (the acetyl-charge) increased and declined in parallel with specific rates of ester synthesis. It appears that specific rates of lipid syntheses determined the acetyl-charge of the cell. It is proposed that when lipid syntheses cease, a condition of 'acetyl-pressure' (i.e. a high acetyl-charge) develops and the cell responds by excreting acetate units (acetate esters). Studies using cerulenin, an inhibitor of fatty acid synthesis, support this view. The suppression of ester synthesis by linoleic acid was found to be independent of intracellular availability of acetyl-CoA and seems likely to be a consequence of altered membrane lipid composition. The results of these investigations suggest possible new procedures for control of ester synthesis in brewery fermentations.
|Date of Award||1982|
Lipid metabolism of Saccharomyces cerevisiae and control of volatile ester synthesis in wort fermentations.
Thurston, P. A. (Author). 1982
Student thesis: Doctoral Thesis › PhD