ƴ-Nonalactonehas been identified as a significant contributor to the aroma profile of a range of wines and is associated with stonefruit and coconut descriptors.1 The exact route of ƴ-nonalactone biosynthesis in wine has not been fully elucidated; however, precursors including linoleic acid, 13-hydroxyoctadeca-9,11-dienoic acid (13-HODE) and 9-hydroxyoctadeca-10,12-dienoic acid (9-HODE) have been identified in incubation experiments.2 Wines produced from grapes infected with “noble rot” caused by Botrytis cinerea fungus generally show higher concentrations of ƴ-nonalactone compared to non-botrytized white wines, but the relative contribution of potential formation pathways has not been elucidated.3
To assess the effect of linoleic acid on the production of g-nonalactone in wine, fermentations with and without added linoleic acid were carried out in synthetic grape must (SGM) at 28 °C using commercial Saccharomyces cerevisiae EC1118. Prior to ƴ-nonalactone quantitation in the finished wines and in a subset of six Australian and New Zealand commercial wines, several routes for the synthesis of a deuterated analogue of ƴ-nonalactone were attempted, before the deuterated d6-analogue of ƴ-nonalactone from its non-deuterated analogue was produced successfully. Subsequently, attempts were made to utilise the d6–ƴ-nonalactone analogue as an internal standard for the measurement of g-nonalactone using gas chromatography-mass spectrometry. However, the synthetic d6–ƴ-nonalactone analogue proved to be an inappropriate internal standard for this purpose, due to back-exchange of deuterium atoms in wine. 2-Octanol was instead utilised as a surrogate internal standard for the measurement of ƴ-nonalactone. ƴ-Nonalactone was successfully identified (above the limit of detection, 4.12 g L-1) in two commercial New Zealand botrytized wine samples, and one fermentation sample to which linoleic acid (132 mgL-1) had been added. This suggests a possible link between the effect of Botrytis cinerea and/or linoleic acid, and increased levels of ƴ-nonalactone in wine. Further research is needed in this area to determine the mechanism of ƴ-nonalactone biosynthesis and to more accurately quantify ƴ-nonalactone in wine, using a more effective internal standard.
Authors: Gillean Miller – School of Chemical Sciences, University of Auckland,Lisa PILKINGTON, School of Chemical Sciences, University of Auckland Bruno FEDRIZZI, School of Chemical Sciences, University of Auckland David BARKER, School of Chemical Sciences, University of Auckland Rebecca DEED, School of Chemical Sciences and School of Biological Sciences, University of Auckland
Keywords: botrytized wines, botrytis cinerea, gc-ms, lactones