Impact of Oak on γ‐Nonalactone Concentration During Fermentation and Maturation of Chardonnay Wines

Background and Aims: γ‐Nonalactone is a commonly occurring aliphatic lactone imparting desirable aromas of stone fruit and coconut to white wines. Formation of γ‐nonalactone in wine is linked to its biosynthesis by wine yeast during fermentation from fatty acid precursors, particularly linoleic acid. However, there are also reports of γ‐nonalactone being derived from the use of oak during winemaking and ageing. The current evidence regarding the extraction of this compound from oak is conflicting; hence, this work aimed to determine the influence of oak exposure, using three different oak chip toast levels (medium, medium‐plus and heavy), on the formation of γ‐nonalactone during alcoholic fermentation, malolactic fermentation (MLF) and a three‐month long maturation of Chardonnay wines. Methods and Results: γ‐Nonalactone was quantitated in the wines using solid‐phase extraction (SPE) followed by gas chromatography‐mass spectrometry (GC‐MS). The inclusion of oak during alcoholic fermentation and/or maturation increased γ‐nonalactone concentrations in Chardonnay wines compared to wines that had only undergone alcoholic fermentation and MLF with no ageing or oak. The presence of oak during alcoholic fermentation had the greatest impact. Interestingly, simply maturing the wine, without oak influence, also increased the concentration of γ‐nonalactone over the three‐month period. However, this increase in unoaked wines was not to the same extent as those matured in the presence of oak. Medium toast oak chips displayed a slight increase in γ‐nonalactone concentration compared to much larger increases when medium‐plus and heavy toast oak chips were used. Conclusions: We hypothesise that either γ‐nonalactone itself or its precursors are directly extracted from the oak wood, and that conversion to γ‐nonalactone can occur without yeast during maturation, possibly via autoxidation of linoleic acid.