Glycosylation of smoke-derived volatile phenols in grapes as a consequence of grapevine exposure to bushfire smoke

Impact of Bottle Aging on Smoke-Tainted Wines from Different Grape Cultivars

Smoke taint is the term given to the objectionable smoky, medicinal, and ashy characters that can be exhibited in wines following vineyard exposure to bushfire smoke. This study sought to investigate the stability of smoke taint by determining changes in the composition and sensory properties of wines following 5 to 6 years of bottle aging. Small increases in guaiacol and 4-methylguaiacol (of up to 6 μg/L) were observed after bottle aging of smoke-affected red and white wines, while syringol increased by as much as 29 μg/L. However, increased volatile phenol levels were also observed in control red wines, which indicated that changes in the composition of smoke-affected wines were due to acid hydrolysis of conjugate forms of both naturally occurring and smoke-derived volatile phenols. Acid hydrolysis of smoke-affected wines (post-bottle aging) released additional quantities of volatile phenols, which demonstrated the relative stability of glycoconjugate precursors to the mildly acidic conditions of wine. Bottle aging affected the sensory profiles of smoke-affected wines in different ways. Diminished fruit aroma and flavor led to the intensification of smoke taint in some wines, but smoke-related sensory attributes became less apparent in smoke-affected Shiraz wines, post-bottle aging.

Meta-Analysis of the Core Aroma Components of Grape and Wine Aroma

Wine aroma strongly influences wine quality, yet its composition and its evolution during the winemaking process are poorly understood. Volatile compounds that constitute wine aroma are traditionally divided into three classes according to their origin: grape, fermentation, and maturation aroma. We challenge this view with meta-analysis and review of grape and wine volatiles and their precursors from 82 profiling experiments. We compiled a list of 141 common grape and wine volatiles and quantitatively compared 43 of them. Our work offers insight into complex relationships between biosynthesis of aroma in grapes and the changes during the winemaking process. Monoterpenes are one of the largest and most researched wine aroma compounds. We show that their diversity in wines is mainly due to the oxidative metabolism of linalool in grapes. Furthermore, we demonstrate that most of the linalool produced in grapes is converted to these oxidized derivatives.

Isolation and identification of oxidation products of guaiacol from brines and heated meat matrix

In this study we investigated the formation of the oxidation products of guaiacol in brines and heated meat matrix: 6-nitrosoguaiacol, 4-nitroguaiacol and 6-nitroguaiacol. For this purpose we applied a newly developed HPLC-UV and LC-MS method. For the first time, 6-nitrosoguaiacol was determined in brine and meat (containing guaiacol and sodium nitrite), which had been heated to 80°C and subsequently subjected to simulated digestion. Application of 500mg/L ascorbic acid to the brines reduced guaiacol degradation at pH3 and simultaneously inhibited the formation of 6-nitrosoguaiacol compared to brines containing only 100mg/L of ASC. The oxidation products were isolated with a new extraction method from meat samples containing 400mg/kg sodium nitrite at pH3.6 following simulated digestion. When oxygen was added, 6-nitrosoguaiacol was determined even at legally allowed levels (150mg/kg) of the curing agent. Finally, we developed a new LC-MS method for the separation and qualitative determination of the four main smoke methoxyphenols.

The effect of phenol composition on the sensory profile of smoke affected wines

Vineyards exposed to wildfire generated smoke can produce wines with elevated levels of lignin derived phenols that have acrid, metallic and smoky aromas and flavour attributes. While a large number of phenols are present in smoke affected wines, the effect of smoke vegetation source on the sensory descriptors has not been reported. Here we report on a descriptive sensory analysis of wines made from grapes exposed to different vegetation sources of smoke to examine: (1) the effect vegetation source has on wine sensory attribute ratings and; (2) associations between volatile and glycoconjugated phenol composition and sensory attributes. Sensory attribute ratings were determined by a trained sensory panel and phenol concentrations determined by gas chromatography-mass spectroscopy. Analysis of variance, principal component analysis and partial least squares regressions were used to evaluate the interrelationships between the phenol composition and sensory attributes. The results showed that vegetation source of smoke significantly affected sensory attribute intensity, especially the taste descriptors. Differences in aroma and taste from smoke exposure were not limited to an elevation in a range of detractive descriptors but also a masking of positive fruit descriptors. Sensory differences due to vegetation type were driven by phenol composition and concentration. In particular, the glycoconjugates of 4-hydroxy-3-methoxybenzaldehyde (vanillin), 1-(4-hydroxy-3-methoxyphenyl)ethanone (acetovanillone), 4-hydroxy-3,5-dimethoxybenzaldehyde (syringaldehyde) and 1-(4-hydroxy-3,5-dimethoxyphenyl)ethanone (acetosyringone) concentrations were influential in separating the vegetation sources of smoke. It is concluded that the detractive aroma attributes of smoke affected wine, especially of smoke and ash, were associated with volatile phenols while the detractive flavour descriptors were correlated with glycoconjugated phenols.

Influence of fruit maturity at harvest on the intensity of smoke taint in wine

Bushfire smoke can affect the composition and sensory properties of grapes and wines, in some cases leading to wines which exhibit undesirable "smoky", "ashy" and "medicinal" characters. This study investigated the extent to which fruit maturity (i.e., ripeness) influences the perception of smoke taint in wine. Two white grape varieties (Chardonnay and Sauvignon Blanc) and two red grape varieties (Merlot and Shiraz) were exposed to smoke under experimental conditions, at approximately seven days post-veraison. Fruit was then harvested at two levels of maturity: Harvest A, when total soluble solids were 16-20 °Brix, i.e., the berry ripeness typically required for production of sparkling or light-bodied wines; and Harvest B, when total soluble solids were 22-25 °Brix, i.e., the berry ripeness typically required for production of full-bodied wines. The intensity of smoke taint in resulting wines was found to be influenced by fruit maturity, but differed between grape varieties. Smoke-related sensory attributes were apparent in Sauvignon Blanc wine made from early-harvested fruit and in Chardonnay wine made from late-harvested fruit, only; whereas Merlot and Shiraz wines exhibited smoke taint irrespective of fruit maturity. Smoke-derived volatile phenols, and various alcohols, esters and acids, were also quantified to determine the impact of smoke exposure and fruit maturity respectively, on wine composition.

Determination of the importance of in-mouth release of volatile phenol glycoconjugates to the flavor of smoke-tainted wines

The volatile phenols guaiacol, 4-methylguaiacol, syringol, 4-methylsyringol, o-, m-, and p-cresol, as well as their glycoconjugates, have previously been shown to be present in elevated concentrations in smoke-tainted wine. Sensory descriptive analysis experiments, with addition of free volatile phenols in combination with their glycosidically bound forms, were used to mimic smoke taint in red wines. The addition of volatile phenols together with glycoconjugates gave the strongest off-flavor. The hydrolysis of glycosidically bound flavor compounds in-mouth was further investigated by in vitro and in vivo experiments. The results indicate that enzymes present in human saliva are able to release the volatile aglycones from their glycoconjugates even under low pH and elevated ethanol conditions, confirming that in-mouth breakdown of monosaccharide and disaccharide glycosides is an important mechanism for smoke flavor from smoke affected wines, and that this mechanism may play an important general role in the flavor and aftertaste of wine.

Assessing the impact of smoke exposure in grapes: development and validation of a HPLC-MS/MS method for the quantitative analysis of smoke-derived phenolic glycosides in grapes and wine

Bushfires occur frequently in the vicinity of grape growing regions, resulting in smoke drifting over the vineyards. Wine made from smoked grapes is often downgraded or unfit for sale due to negative sensory characters. To manage or avoid the risk of producing smoke-affected wine, a diagnostic assay was developed for assessing the extent of smoke exposure in grapes and the resulting wines. The method relies on the quantitation of the glycosidic grape metabolites that are formed from major volatile phenols present in smoke. Using HPLC-MS/MS with APCI, a quantitation method for phenolic glycosides as smoke marker compounds was developed and validated. The method was confirmed to be of sufficient sensitivity and reliability to use as a diagnostic assay. On the basis of phenolic glycoside concentrations, grapes or wine can be assessed as smoke exposed or not, and the relative intensity of smoke exposure can be determined.

Glycosidic aroma precursors of Syrah and Chardonnay grapes after an oak extract application to the grapevines

Syrah and Chardonnay grapevines were treated with an oak extract in order to determine the effect on glycosidic aroma precursors. Grapevines were treated at three different timings of the veraison (treatment 1, 2 and 3). Aglycons were obtained by enzymatic hydrolysis, and these were identified and quantified by means of gas chromatography-mass spectrometry (GC-MS). Results suggest that after the applications the majority of compounds from the oak extract were assimilated and stored as glycosidic forms in both cultivars. Also, other compounds not present in the extract were affected, with a different behaviour observed depending on the timing of application and the variety. In general, C6 compounds, alcohols, terpenes, phenols and C13-norisoprenoids in Syrah showed a decrease and in Chardonnay an increase. Thus, this study proved a change in the glycosidic aroma profile in grapes after the oak application, so these treated grapes could produce wines with different aromatic quality.

Exposure of grapes to smoke of vegetation with varying lignin composition and accretion of lignin derived putative smoke taint compounds in wine

Smoke taint in wines from bushfire smoke exposure has become a concern for wine producers. Smoke taint compounds are primarily derived from pyrolysis of the lignin component of fuels. This work examined the influence of the lignin composition of pyrolysed vegetation on the types of putative smoke taint compounds that accrue in wines. At veraison, Merlot vines were exposed to smoke generated from five vegetation types with differing lignin composition. Smoke was generated under pyrolysis conditions that simulated bushfire temperature profiles. Lignin and smoke composition of each fuel type along with putative smoke taint compounds in wines were determined. The results showed that, regardless of fuel type, the commonly reported guaiacyl lignin derived smoke taint compounds, guaiacol and 4-methylguaiacol, represented about 20% of the total phenols in wines. Quantitatively, syringyl lignin derived compounds dominated the total phenol pools in both free and bound forms. The contributions of p-hydroxyphenyls were generally similar to the guaiacyl sources. A further unexpected outcome of the study was that pine smoke affected wines had significantly elevated levels of syringols compared to the controls although pine fuel and its smoke emission lacked syringyl products.

Contribution of several volatile phenols and their glycoconjugates to smoke-related sensory properties of red wine

Guaiacol and 4-methylguaiacol are well-known as contributors to the flavor of wines made from smoke-affected grapes, but there are other volatile phenols commonly found in smoke from forest fires that are also potentially important. The relationships between the concentration of a range of volatile phenols and their glycoconjugates with the sensory characteristics of wines and model wines were investigated. Modeling of the attribute ratings from a sensory descriptive analysis of smoke-affected wines with their chemical composition indicated the concentrations of guaiacol, o-cresol, m-cresol, and p-cresol were related to smoky attributes. The best-estimate odor thresholds of these compounds were determined in red wine, together with the flavor threshold of guaiacol. Guaiacol β-D-glucoside and m-cresol β-D-glucoside in model wine were found to give rise to a smoky/ashy flavor in-mouth, and the respective free volatiles were released. The study indicated that a combination of volatile phenols and their glycosides produces an undesirable smoke flavor in affected wines. The observation of flavor generation from nonvolatile glycoconjugates in-mouth has potentially important implications.

Classification of smoke tainted wines using mid-infrared spectroscopy and chemometrics

In this study, the suitability of mid-infrared (MIR) spectroscopy, combined with principal component analysis (PCA) and linear discriminant analysis (LDA), was evaluated as a rapid analytical technique to identify smoke tainted wines. Control (i.e., unsmoked) and smoke-affected wines (260 in total) from experimental and commercial sources were analyzed by MIR spectroscopy and chemometrics. The concentrations of guaiacol and 4-methylguaiacol were also determined using gas chromatography-mass spectrometry (GC-MS), as markers of smoke taint. LDA models correctly classified 61% of control wines and 70% of smoke-affected wines. Classification rates were found to be influenced by the extent of smoke taint (based on GC-MS and informal sensory assessment), as well as qualitative differences in wine composition due to grape variety and oak maturation. Overall, the potential application of MIR spectroscopy combined with chemometrics as a rapid analytical technique for screening smoke-affected wines was demonstrated.