Survival, metabolism and production of volatile phenols by Dekkera bruxellensis in monovarietal wines

Abstract

The specific defect caused by the production of volatile phenolic compounds in wines is a majoreconomic concern among wine producers at a global scale. Vinylphenols and, specially,ethylphenols may accumulate in wines imparting undesirable odours and flavours, which maystrongly affect the wine quality. The main organisms responsible for the production of thesecompounds are the yeasts Brettanomyces/Dekkera. Several studies have been carried out on thissubject over the last 20 years, but scientific work concerning the relationship between the grapevarieties and the production of volatile phenols is extremely scarce. Thus, the overall aim of thiswork was to characterise the survival and metabolic behaviour of Dekkera bruxellensis inmonovarietal wines and to evaluate whether certain grape varieties pose a higher risk to developvolatile phenols.The work started by searching for contaminated wines and by isolating strains ofBrettanomyces/Dekkera using selective and differential culture media. The percentage ofcontaminated wines was 37% and all strains isolated were identified as Dekkera bruxellensis.The results support the view that, among the Brettanomyces/Dekkera yeasts, D. bruxellensis isthe main species responsible for the production of volatile phenols in wines. Phenotypicvariation was found among the strains studied regarding the tolerance to ethanol. The mosttolerant strains were able to grow at 14% v/v ethanol and the wine isolates were, in general,more tolerant than the reference strain D. bruxellensis PYCC 4801. Strains, from the mosttolerant group, were selected to perform survival and metabolic studies in real wine conditions.Prior to the metabolic studies, a chemical characterisation of monovarietal red wines from eightselected grape varieties (Cabernet Sauvignon, Syrah, Aragonez, Castelão, Touriga Franca,Touriga Nacional, Trincadeira and Vinhão) was performed, focusing on the volatile phenols andon the respective precursor compounds, both on the free and bond forms. It was found that theprecursors exist mostly as esters of tartaric acid (caftaric, coutaric and fertaric acids). Thepredominant free hydroxycinnamic acid was p-coumaric acid, the highest concentrations beingfound in Syrah and Touriga Franca and the lowest in Touriga Nacional and Trincadeira. TourigaNacional exhibited the highest difference between bound and free forms. Twenty two % of thewines analysed presented levels of volatile phenols above the perception threshold, the highestvivalues being found in Vinhão and Trincadeira. The results show relevant differences amonggrape varieties but the availability of precursors on the free form may not be the only factorexplaining the potential of wines to develop volatile phenols.The survival of D. bruxellensis in monovarietal wines (Touriga Nacional, Cabernet Sauvignonand Syrah) and the metabolism of hydroxycinnamic acids were evaluated. After the inoculation,yeast culturable populations were reduced to undetectable numbers in all wines, but viabilitymeasurement by flow cytometry showed that a significant part of the populations were in aviable-but-not-culturable state (VBNC). VBNC cells were metabolically active, causing thespoilage of the wine, and this physiological state favoured the accumulation of vinylphenolsrather than ethylphenols independently of the grape variety. The survival capacity of D.bruxellensis was higher on Touriga Nacional wines than in Cabernet Sauvignon and Syrah.Wine susceptibility to Dekkera may be correlated with the phenolic composition, specificallythe bound/free molar ratios of hydroxycinnamic acids. As stated above, Touriga Nacionalexhibited the highest difference between bound and free forms and the level of freehydroxycinnamic acids is relatively low in comparison with other grape varieties. Therefore,the documented antimicrobial activity of free hydroxycinnamic acids may be expected to belower in Touriga Nacional wines.Besides the production of volatile phenols, D. bruxellensis showed esterase activity leading tothe increase on the amount of ethyl esters, including ethyl acetate. The fatty acids isovaleric acidand caprylic acid also increased in the inoculated wines. The concentration of monoterpenes analysed increased in the Cabernet Sauvignon wines but not in Touriga Nacional wines. The grape variety effect observed can be related with different compositions of glycosidically boundterpenes subjected to the ß-glycosidase activity of D. bruxellensis.

Date of Award	29 Jul 2019
Original language	English
Awarding Institution	Universidade Católica Portuguesa
Supervisor	José António Couto (Supervisor) & Francisco Manuel Morais Sarmento de Campos (Co-Supervisor)