engleski

Phospholipids of recycled brewer's yeast

During industrial processes, yeast cells are exposed to different environmental conditions, such as the toxic effect of ethanol and other metabolites, nutrient and oxygen limitation, pH fluctuation, temperature changes etc. Among them, the toxic effect of ethanol and oxygen depletion are of special importance for brewer’ s yeast, because they act as a potent stress factors to yeast cells. Various physiological adaptations, related to stress protection, occur in yeast. These adaptations range from the alteration in membrane fluidity to the synthesis of detoxification enzymes. A common response of yeast cells exposed to ethanol is the change in phospholipid (PL) composition, as well as the increased unsaturation of membrane fatty acids. On the other hand, biosynthesis of unsaturaed lipids is limited under anaerobic conditions like those during fermentation of bottom-fermenting brewer's yeast. The aim of this work was to investigate the changes in the content and composition of yeast PLs during brewing and recycling of the biomass in subsequent brewing cycle. For that purpose, bottom-fermenting brewer’ s yeast of Saccharomyces cerevisiae strain was used. The pitching yeast (pure yeast culture, aerobically propagated) and three recycled yeast generations consecutively used in industrial beer production were analysed. The content of total lipids was rather low in all generations of brewer’ s yeast (3.2% - 3.6% of dry mass, respectively). Total PL accounted for 33-36% of total cell lipids, making 72-92% of polar lipids. Phosphatidylcholine (PC) was the main PL component in all yeast generations, with statistically significant higher values in recycled generations compared to the pure culture. On the contrary, phosphatidylethanolamine (PE), the second major PL component, had the highest value in the pure culture. C16 and C18 were the most abundant FAs in polar lipid fraction of total lipids, as well as in PC and PE. While saturated FA predominated in the pitching yeast, unsaturated FAs predominated in polar lipid fraction and in PC of all recycled generations. PE was characterised by equal amounts of saturated and unsaturated FAs in the pitching yeast and by significant preponderance of unsaturated FAs in recycled yeasts. Very long chain FAs, C24 and C26 were identified in PC and PE. Changes in the PL composition and the increase in fatty acid unsaturation in all recycled yeast generations, as well as the presence of very long-chain FA represent a part of adaptive response of brewer's yeast to unfavourable conditions during beer fermentation

brewer's yeast; phospholipids; fatty acids

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