What are the primary volatile aroma compounds produced by Brettanomyces and how can their formation be controlled?

Brettanomyces (often shortened to 'Brett') is a spoilage yeast that can produce several volatile aroma compounds, primarily 4-ethylphenol (4-EP) and 4-ethylguaiacol (4-EG), which can negatively impact wine quality. These compounds are responsible for aromas described as barnyard, horse sweat, medicinal, band-aid, or smoky. The specific aroma profile depends on the concentration and ratio of 4-EP and 4-EG. 4-EP is typically associated with more barnyard-like aromas, while 4-EG contributes smoky and spicy notes. Brettanomyces produces these volatile phenols through a series of enzymatic reactions. First, it converts coumaric acid and ferulic acid (naturally present in wine) into 4-vinylphenol (4-VP) and 4-vinylguaiacol (4-VG), respectively. Then, it reduces these vinyl phenols to 4-EP and 4-EG. The presence of these precursor compounds influences the amount of 4-EP and 4-EG produced. Several strategies can control Brettanomyces and minimize the formation of these volatile phenols. Sanitation is crucial. Maintaining a clean winery environment, including equipment, barrels, and hoses, reduces the risk of Brettanomyces contamination. Sulfur dioxide (SO2) is an effective antimicrobial agent that inhibits Brettanomyces growth. Maintaining adequate SO2 levels in wine can prevent or slow down its development. However, Brettanomyces can become resistant to SO2 over time, so it is not a foolproof solution. Using sterile filtration to remove Brettanomyces cells before bottling is another effective method. This physically removes the yeast from the wine, preventing further spoilage. Maintaining low pH levels in wine can also inhibit Brettanomyces growth. Brettanomyces prefers higher pH levels, so lowering the pH can create a less favorable environment. The use of the antimicrobial agent dimethyl dicarbonate (DMDC) is effective, but regulatory restrictions may vary by region.