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June-August 2023

Chemical basis of ‘stone fruit’ aromas in white wine

Hundreds of volatile aroma compounds can be present in any one glass of wine, with these compounds coming directly from the grapes, made by the yeast, released from precursors during fermentation, derived from oak contact, or developing during barrel and/or bottle ageing.

‘Stone fruit’ characters are important flavour notes for several white wine varieties, such as ‘fresh peach’ in Chardonnay, ‘apricot’ in Viognier, and ‘dried apricot’ in sweet botrytised Semillon. But which volatile compound(s) cause these stone fruit aromas?

In fresh apricots and peaches, several n-alkyl γ- and δ-lactones are impact aroma compounds; that is, they smell like stone fruit. Some wine aroma studies using GC-olfactometry (GC-sniff) have suggested that they were the cause of peach or apricot aromas in wine. However, one study found only low concentrations of γ-octalactone and γ-nonalactone (<5 µg/L) in just a few dry white wines, well below their aroma detection thresholds (R.C. Cooke et al. J. Agric. Food Chem. 2009, vol. 57(2), pp. 348–52), suggesting they are not likely to be major contributors to stone fruit aroma in dry white wines. However, tenfold higher concentrations were found in the botrytised wines. Even at these moderate concentrations, synergistic effects between the lactones might explain their involvement in the ‘dried fruit’ aroma of botrytised wines (I. Jarauta et al. Dev. Food Sci. 2006, vol. 43, pp. 205–8).

To specifically target stone fruit aromas in dry white wines, Chardonnay and Viognier wines with differing stone fruit aroma intensities were subjected to extensive chemical analyses and sensory quantitative descriptive analysis (T.E. Siebert et al. J. Agric. Food Chem. 2018, vol. 66(11), pp. 2838–50). Notably, the wines studied differed strongly in their rated intensity of peach (defined as fresh white peach) and apricot (tinned apricots) aroma attributes (see diagram). A group of compounds were predicted from statistical models to be important for a wine to have high apricot aroma. These included γ-nonalactone, γ-decalactone and (Z)-6-dodecenolactone, all at very low concentrations; the monoterpenes linalool, geraniol and nerol, at concentrations higher than you might find in a ‘floral’ variety such as Riesling; and the C6 compounds (E)-2-hexenal and (E)-2-hexenol; plus aldehydes nonenal and benzaldehyde. The peach aroma attribute was associated with some ethyl and acetate esters, benzaldehyde and monoterpenes.

Reconstitution sensory experiments confirmed that the monoterpenes were the key aroma compounds responsible for the apricot attribute in white wine. This result was somewhat surprising because these monoterpenes are usually described as ‘floral’ and ‘citrus-like’. A follow-up reconstitution sensory study, looking more closely at the interaction of the monoterpenes with the aldehydes and the γ-lactones, verified monoterpenes as the major contributors for apricot aroma in wine, with an enhancing role from γ-lactones, and a masking effect by aldehydes (D. Espinase Nandorfy et al. Aust. J. Grape Wine Res. 2022, vol. 28(3), pp. 424–38).

As monoterpenes are grape-derived compounds, we ran vineyard studies to assess practical options for wine producers to be able to enhance the levels of these compounds. Large differences were found in monoterpene concentrations between Viognier clones through ripening, with regional and sun exposure differences also observed (T.E. Siebert et al. Food Chem. 2018, vol. 256, pp. 286–95; T.E. Siebert et al. 18th Australian Wine Industry Technical Conference 2022, Poster). Grape compositional differences were related to wine composition and sensory properties in a follow-up winemaking study. Region of origin was the most important factor, but clone and harvest date were also significant, for levels of apricot aroma and flavour in Viognier.

To confirm the compounds that result in peach aroma, the first reconstitution attempt in a Viognier model did not satisfactorily represent this aroma. But by using a Chardonnay model, fermentation-derived esters ethyl octanoate and ethyl hexanoate directed the more subtle peach aroma (Espinase Nandorfy et al. 2022). Also, consumer acceptance testing found that peach aroma in Chardonnay was positively linked to ‘liking’ scores (D. Espinase Nandorfy et al. Aust. J. Grape Wine Res. 2023, submitted). A Chardonnay winemaking study is underway at the Australian Wine Research Institute to assess differences in these peachy esters arising from fermentation variables, comparing yeast selection, degree of juice clarification and region.

Interestingly, these studies have found that the different stone fruit aromas peach, apricot and dried apricot are caused by three separate families of aroma compounds, and each family is formed from distinct parts of the grape-growing and winemaking process!

Tracey Siebert is a research scientist at the Australian Wine Research Institute, South Australia.

Sensory profiles of four of the white wines studied.

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