Abstract
The unwanted modification of wine sensory attributes by yeasts of the species Brettanomyces bruxellensis due to the production of volatile phenols is presently the main microbiological threat to red wine quality. The effects of ethylphenols and other metabolites on wine flavor is now recognized worldwide and the object of lively debate. The focus of this review is to provide an update of the present knowledge and practice on the prevention of this problem in the wine industry. Brettanomyces bruxellensis, or its teleomorph, Dekkera bruxellensis, are rarely found in the natural environment and, although frequently isolated from fermenting substrates, their numbers are relatively low when compared with other fermenting species. Despite this rarity, they have long been studied for their unusual metabolical features (e.g., the Custers effect). Rising interest over the last decades is mostly due to volatile phenol production affecting high quality red wines worldwide. The challenges have been dealt with together by researchers and winemakers in an effective way and this has enabled a state where, presently, knowledge and prevention of the problem at the winery level is readily accessible. Today, the main issues have shifted from technological to sensory science concerning the effects of metabolites other than ethylphenols and the over estimation of the detrimental impact by ethylphenols on flavor. Hopefully, these questions will continue to be tackled together by science and industry for the benefit of wine enjoyment.
Highlights
The yeast genus Brettanomyces has been related with the production and characteristics of English beers since the beginning of the XX century [1]
S. cerevisiae may produce vinyl derivatives that may be later reduced by B. bruxellensis
Levels may be as high as 40 mg/L of free sulfite at pH 3.5, to control D. bruxellensis in wines aged in barrels [24]
Summary
The yeast genus Brettanomyces has been related with the production and characteristics of English beers since the beginning of the XX century [1]. Other products of Brett metabolism like isovaleric and isobutyric acids [11] influence the perception of “horse sweat”, which may explain why the smell of wines spiked with VPs is more objectionable than the smell of wines with the same concentrations but produced naturally. The precursors of 4-EP, 4-EG, and 4-EC are hydroxycinnamic acids (p-coumaric, ferulic and caffeic acids, respectively), which are enzymatically decarboxylated by a cinnamate decarboxylase, leading to vinyl derivatives and reduced by a vinylphenol reductase, originating in the ethyl derivatives [11] In grape juices those hydroxycinnamic acids are esterified, mainly to tartaric acid, in concentrations higher than 10 mg/L. The natural concentrations of hydroxycinnamic acids in wines are high enough to provide substrate for the production of volatile phenols much above the preference thresholds. The key for the prevention of the problem is to understand the ecology and the behavior of Brettanomyces in wines in order to apply the adequate control measures and avoid, or reduce, the conversion of hydroxycinnamic acids to VPs
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