The Effect of Pre-Veraison Smoke Exposure of Grapes on Phenolic Compounds and Smoky Flavour in Wine

Abstract

Background and Aims. Smoke exposure occurred in the Adelaide Hills region in December 2019 due to a wildfire, when wine grapes were peppercorn-size green berries. Previously, pre-veraison smoke exposure had been identified through model experiments as unlikely to affect grape composition, whereas smoke exposure after veraison can have a major effect on wine flavour. Hence the effects of pre-veraison smoke on grape and wine composition, and smoky sensory properties of wine were investigated. Methods and Results. Chardonnay, Pinot Noir and Shiraz were investigated and eight blocks with varied smoke exposure were selected for each cultivar. Berries were sampled initially four weeks after the fire and at harvest, and mature grapes were made into unoaked wines. Established smoke exposure markers, phenolic glycosides, were found in berries at pre-veraison and at harvest from the high smoke exposure sites, with concentrations well above those found in non-smoke exposed fruit. Volatile phenols were also elevated in grapes at harvest. The resulting red wines from some exposure vineyards were high in volatile phenols, glycosides and smoky flavours. However, most of the Chardonnay wines expressed much less smoky flavours, despite similar levels of smoke exposure of grapes. Conclusions. Pre-veraison smoke exposure can result in elevated concentrations of volatile phenols and their glycosidic metabolites in grape berries and wine and cause strong smoky flavour in wine. Significance. The wine sector and land management agencies responsible for controlled burns need to consider the effect of smoke from fires near vineyards even very early in the growing season.

Modelling Smoke Flavour in Wine from Chemical Composition of Smoke-Exposed Grapes and Wine

“…Compositional data for all control grape and control wine samples were consistent with data reported from nonsmoke-exposed samples [22] for all phenolic smoke markers measured, with the exception of one Shiraz sample (SHI-03-Control) which had a slightly elevated concentration of guaiacol (Table S3 and [28]). Tese observations provide further confrmation of background levels of phenolic compounds typically found in grapes and wine without a history of known smoke exposure.…”

“…Winemaking. Details for Study A have already been described [28]. Importantly, grapes in this study were exposed to smoke from a single wildfre, ca.…”

“…Te basic wine compositional measures are provided in Table S1 and for Study A have been previously reported [28]. Chardonnay wines ranged in alcohol concentration from 12.0-15.8% (v/v).…”

“…Eight samples were collected for each cultivar including samples from vineyards that had no evidence of smoke exposure (control samples); that had mild smoke exposure based on grape maturity smoke compound analysis; and samples from vineyards with evidence of mild fre activity within the block, such as burnt grass undervine and in the midrow. All details of the grapes, wines, and sensory analysis are described elsewhere [28]. Frozen grapes were thawed at 4 °C over the weekend prior to commencement of winemaking.…”

“…Formal sensory analysis was performed on the wines six weeks after bottling. Te sensory assessment for Study A has been described previously [28] and a closely similar procedure was followed for Study B. A panel of screened, qualifed, and experienced assessors was convened to evaluate each of the wine sets.…”