Campos, FranciscoCouto, José AntonioWeiss, Katrin Sophie2015-01-132015-01-132014-04-222014http://hdl.handle.net/10400.14/16238The main purpose of this work is to determine whether there is a correlation between the susceptibility to volatile phenol production of wines and grape variety. Therefore, 11 single varietal red wines from Portugal were heat sterilized, contaminated with Dekkera bruxellensis PYCC 4801 and incubated for at least 10 days at 30°C. Since yeasts did not grow in pure wines due to inhibition by ethanol, the experiments were conducted in diluted samples with similar initial pH and ethanol levels. Yeast growth was monitored and produced levels of 4-ethylguaiacol, 4-ethylphenol and 4-vinylphenol were measured using gas chromatography in order to compare values between varieties. This approach led to the production of 4-ethylphenol and 4-ethylguaiacol in all samples; 4-vinylphenol however, could not be detected in any sample. All values reached or exceeded perception threshold levels established in literature. This suggests that all examined wines are at risk of developing phenolic off-flavour once contaminated with Brettanomyces/Dekkera yeasts, especially considering that samples were diluted and thus contained lower amounts of precursors than are potentially present in pure wines. 4-ethylguaiacol values were generally lower and varied less between samples and varieties compared to 4-ethylphenol. Tinta Roriz and Touriga Franca showed the highest potential in regard to volatile phenol production from the natural precursors available in the wines while Sousão wines appeared to be the least prone and Touriga Nacional wines exhibited intermediate volatile phenol values. Furthermore, 4-ethylphenol:4-ethylguaiacol ratios were calculated and show differences between and similarities within varieties. An attempt was made to estimate the precursor quantities originally present in the wines by comparing the results of spiked and unspiked wines but the obtained results were inconclusive. From the results it can be suggested that the grape variety may have an influence on the production of volatile phenols by predetermining the availability of precursors. However, several other factors including vinification and wine aging methods may have an impact on the production of volatile phenols.engmaster thesis