Brettanomyces in Wine
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The Brettanomyces / Dekkera yeasts can be found in fermenting must and in wine. Typically they grow after alcoholic and malolactic fermentation during storage of wine in tank, barrel, or bottle. They contribute characteristic bretty flavors which are described as smoky, barnyard, plastic, burnt plastic, vinyl, Bandaid, and creosote. They can also contribute a metallic bitterness. Compounds which are responsible for brett flavor in wine include 4-ethyl phenol, 4-ethyl guaiacol, isovaleric acid, and unidentified burnt plastic compound. Descriptive sensory evaluations show an inverse relationship between fruity and bretty flavor perception. The brett aromas in some wines are considered a positive attribute, especially when present at low concentration. Often these flavors are considered a defect. The wine's varietal and regional flavor characteristics might be completely masked by these flavors and the wine can be unpleasantly bitter. 4-ethyl phenol is used by some wineries as an indicator compound for the activity of Brettanomyces. Yet some wines having a strong brett flavors do contain none or very little 4-ethyl phenol. Preliminary studies show that 4-ethyl phenol is formed all through growth of Brettanomyces. Thus this compound can be used to confirm the presence of bretty flavors. The search for indicator compounds formed during early stages of growth of B. continues. The Brettanomyces yeasts can produce the characteristic brett flavors when growing at low cell density of several hundred to several thousand cells per mL. The fact that these yeasts are often present at low cell numbers and that they are slow growing makes detection difficult. Genetic analyses of wine isolates have shown that only strains of B. bruxellensis grow in wine. Semi-selective plating is performed prior to genetic analysis on media including cycloheximide. The development of DNA based probes has made it possible to reliably identify the yeast once it is plated on a nutrient agar plate. Further developments of specific probes and of probe application techniques will improve the specificity and speed of detection.
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2000
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Keywords
Brettanomyces; Wine; Yeast