The aim of the master's thesis was to study the influence of the serial repitching of yeast biomass for the expression of genes involved in the metabolic pathway of valine and compare this with the concentration of diacetyl in beer and other important aromatic components in beer. From 31 successive beer fermentations carried out in a micro-fermentation system that mimicked industrial conditions, yeast biomass was taken for RNA isolation and was transcribed to cDNA. With quantitative PCR, changes in the expression of the BDH1, BDH2, ILV2 and ILV6 genes were analysed. These genes encode enzymes that are important in the valine metabolic pathway and consequently influence the production of diacetyl which concentration varies between successive use of yeast bimass. Compared to the first fermentation, we observed an increasing trend in the expression of all investigated genes in subsequent successive fermentations. The greatest changes occurred in the expression of the BDH1 gene, slightly smaller for genes ILV2 and ILV6. The smallest changes in expression were observed in the BDH2 gene. The concentration of diacetyl and other important aromatic compounds, acetaldehyde, dimethyl sulphide, propanol, isobutanol, ethyl acetate and isoamyl acetate determined by gas chromatography showed that changes in the production of aromatic beer components occur in successive fermentations. As a result of excessive successive use, diacetyl exceeded the sensory threshold. Correlation between the investigated genes and the concentration of diacetyl in beer could not be confirmed.
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