Winemaking practice affects the extraction of smoke‐borne phenols from grapes into wines

Abstract

Background and Aims: Exposure to smoke and uptake of taint imparting phenols in grapes and wines is a significant problem in bushfire-prone regions of Australia and other countries. The effects of smoke exposure on taint occurrence in wines, however, can be variable. This study assessed the influence of cultivar on uptake and accumulation of smoke-borne phenols in grapes and of subsequent processing and winemaking methods on extraction of phenols into wines. Methods and Results: Smoke-exposure experiments were conducted in commercial vineyards of Chardonnay, Merlot and Sauvignon Blanc 14 days after the onset of veraison. At maturity, grapes were harvested for winemaking, which included malolactic fermentation (MLF) for Merlot. Volatile and glycoconjugated phenols were determined in grapes and the resultant wines. All cultivars had a similar concentration of smoke-derived total phenols in their grapes. The apparent extraction of total phenols from grapes into wines, however, differed markedly among the three traditional winemaking methods. Red winemaking (Merlot) with skin contact extracted 88% of total grape phenols, whereas white winemaking either by crushing before pressing (Sauvignon Blanc) or by whole-bunch pressing without crushing (Chardonnay), respectively, released 39 and 18% of total phenols. For Merlot wines, MLF did not affect the extraction of total smoke-derived phenols. Conclusions: Under standardised exposure conditions (duration, intensity and phenology), the three cultivars studied accumulated a similar concentration of total phenols in grapes. The grape-processing and winemaking methods, however, can bring about a fourfold difference in the concentration of total phenols of wines. The smoke-derived phenols extracted from grapes into wine and the distribution of these phenols between the volatile and conjugated pools were not affected by MLF. Significance of the Study: The key findings reported here have the potential to improve decision-making by grapegrowers and winemakers on the effect of cultivar and winemaking practice on potential smoke taint in wine.

Review of smoke taint in wine: smoke-derived volatile phenols and their glycosidic metabolites in grapes and vines as biomarkers for smoke exposure and their role in the sensory perception of smoke taint

“…It was also suggested that variation in cultivar sensitivity observed in earlier reports may have been a reflection of phenology at the time of exposure and exposure conditions rather than demonstrating inherent cultivar differences (Kelly et al. ). It was also clear, however, that cultivar differences could be observed for the concentration of volatile phenols in fruit, in the order of Sauvignon Blanc >> Merlot > Chardonnay (Kelly et al.…”

“…Work conducted by Kelly et al. () demonstrated that when smoke exposure events occurred at a comparable stage of berry development (14 days post‐veraison) that the cultivar (Sauvignon Blanc, Chardonnay and Merlot) appeared to have no effect on the accumulation of total glycoconjugated phenols. This work highlighted the importance of standardising smoke exposure conditions in model experiments, including duration, intensity and timing in relation to grape development.…”

“…It was also clear, however, that cultivar differences could be observed for the concentration of volatile phenols in fruit, in the order of Sauvignon Blanc >> Merlot > Chardonnay (Kelly et al. ).…”

The Impact of Smoke From Vegetation Fires on Sensory Characteristics of Cabernet Sauvignon Wines Made From Affected Grapes

“…Guaiacol and 4-methylguaiacol (4-MG) have been used as key indicators of smoke exposure and potential taint formation (Kennison et al, 2007;Sheppard et al, 2009). More recently, studies have demonstrated that additional volatile phenolic compounds (cresols, phenols and syringols) may account for taint characteristics in smokeaffected wines (Kelly et al, 2012;2014;Parker et al, 2012). Sheppard et al (2009) reported that methoxyphenols (including guaiacol and 4-MG) arise from the partial pyrolysis of lignin and are key contributors to the aroma of wood smoke.…”

“…Winemaking practices that reduce grape skin contact have been shown to limit the extraction of guaiacol glycosides, resulting in reduced smoky aroma and flavour characteristics (Ristic et al, 2011;Kelly et al, 2014). However, glycosylated precursors may also be present in the flesh of severely exposed grapes (Singh et al, 2011), and even the free-run juice of smoke-tainted grapes can contain high levels of 4-MG and guaiacol (Kennison et al, 2008).…”