What is the principal volatile compound responsible for the buttery or butterscotch flavor often found in Chardonnay wines?

The principal volatile compound responsible for the buttery or butterscotch flavor often found in Chardonnay wines is diacetyl. Diacetyl is a naturally occurring compound belonging to the chemical group known as vicinal diketones (VDKs). It's produced during fermentation, primarily as a byproduct of yeast metabolism. Specifically, diacetyl is formed when yeast converts alpha-acetolactate, an intermediate compound in the biosynthesis of valine and leucine (amino acids), into diacetyl. The concentration of diacetyl is influenced by several factors, including the yeast strain, fermentation temperature, and the availability of oxygen. The presence of diacetyl is often associated with malolactic fermentation (MLF), a secondary fermentation process where malic acid is converted to lactic acid by lactic acid bacteria. During MLF, bacteria can further metabolize citric acid, which can increase the levels of diacetyl. The buttery or butterscotch flavor imparted by diacetyl is often considered desirable in some styles of Chardonnay, adding complexity and richness to the wine. However, excessive levels of diacetyl can be perceived as an off-flavor, overpowering other aromas and flavors in the wine, therefore winemakers often carefully monitor and control the conditions that influence diacetyl production.