Mango Parker

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

The presence of glycosides of smoke-derived volatile phenols in smoke-affected grapes and the resulting wines of Chardonnay and Cabernet Sauvignon was investigated with the aid of high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS). All volatile phenols studied (phenol, p-, m-, and o-cresols, methylguaiacol, syringol, and methylsyringol) could be detected as glycosylated metabolites in smoke-affected grapes in a similar fashion to that previously reported for guaiacol. These phenolic glycosides were found in smoke-affected grapes and wines at significantly elevated levels compared to those in non-smoked control grapes and wines. The extraction of these glycosides from grapes into wine was estimated to be 78% for Chardonnay and 67% for Cabernet Sauvignon. After acid hydrolysis, a large proportion of these phenolic glycosides in grapes (50%) and wine (92%) disappeared but the concentrations of volatile phenols determined by gas chromatography-mass spectrometry (GC-MS) were lower than expected. In the case of wine, the majority of the glycosides of phenol, cresols, guaiacol, and methylguaiacol were decomposed upon acid hydrolysis without releasing their respective aglycones, while syringol and methylsyringol were more effectively released.

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.

Phenolic Compositions of 50 and 30 Year Sequences of Australian Red Wines: The Impact of Wine Age

The phenolic composition of red wine impacts upon the color and mouthfeel and thus quality of the wine. Both of these characteristics differ depending on the age of a wine, with the purple of young wines changing to brick red and the puckering or aggressive astringency softening in older wines. This study investigated the color parameters, tannin concentrations and tannin composition of a 50 year series of Cabernet Sauvignon wines from a commercial label as well as 30 year series of Cabernet Sauvignon and Shiraz wines from a separate commercial label to assess the impact of wine age on phenolic composition and concentration. The wine color density in wines of 40 to 50 years old was around 5 AU compared with 16 AU of wine less than 12 months old, which correlated well with the concentration of non-bleachable pigments and pigmented polymers. Conversely, the anthocyanin concentrations in 10 year old wines were substantially lower than that of recently bottled wines (around 100 mg/L compared with 627 mg/L, respectively), adding further evidence that non-bleachable pigments including pigmented polymers play a much larger role in long-term wine color than anthocyanins. No age-related trend was observed for tannin concentration, indicating that the widely noted softer astringency of older red wines cannot necessarily be directly related to lower concentrations of soluble wine tannin and is potentially a consequence of changes in tannin structure. Wine tannins from older wines were generally larger than tannins from younger wines and showed structural changes consistent with oxidation.

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.

Aroma Precursors in Grapes and Wine: Flavor Release during Wine Production and Consumption

Pioneering investigations into precursors of fruity and floral flavors established the importance of terpenoid and C13-norisoprenoid glycosides to the flavor of aromatic wines. Nowadays flavor precursors in grapes and wine are known to be structurally diverse, encompassing glycosides, amino acid conjugates, odorless volatiles, hydroxycinnamic acids, and many others. Flavor precursors mainly originate in the grape berry but also from oak or other materials involved in winemaking. Flavors are released from precursors during crushing and subsequent production steps by enzymatic and nonenzymatic transformations, via microbial glycosidases, esterases, C-S lyases, and decarboxylases, and through acid-catalyzed hydrolysis and chemical rearrangements. Flavors can also be liberated from glycosides and amino acid conjugates by oral microbiota. Hence, it is increasingly likely that flavor precursors contribute to retronasal aroma formation through in-mouth release during consumption, prompting a shift in focus from identifying aroma precursors in grapes to understanding aroma precursors present in bottled wine.

The contribution of wine-derived monoterpene glycosides to retronasal odour during tasting

This study investigated the sensory significance of monoterpene glycosides during tasting, by retronasal perception of odorant aglycones released in-mouth. Monoterpene glycosides were isolated from Gewürztraminer and Riesling juices and wines, chemically characterised and studied using sensory time-intensity methodology, together with a synthesised monoterpene glucoside. When assessed in model wine at five times wine-like concentration, Gewürztraminer glycosides and geranyl glucoside gave significant fruity flavour, although at wine-like concentrations, or in the presence of wine volatiles, the effect was not significant. Gewürztraminer glycosides, geranyl glucoside and guaiacyl glucoside were investigated using a sensory panel (n=39), revealing large inter-individual variability, with 77% of panellists responding to at least one glycoside. The study showed for the first time that grape-derived glycosides can contribute perceptible fruity flavour, providing a means of enhancing flavour in wines, and confirms the results of previous studies that the effect is highly variable across individuals.

Monitoring red wine fermentation in Australia: a novel application of visible and near infrared spectroscopy

Introduction F ermentation is a complex process in which raw materials are transformed into high value product (e.g. grape juice into wine). In standardised batch fermentation processes, each batch should ideally have the same duration and the monitored variables should closely follow some predetermined optimal trajectories with respect to time. However, in the real world the duration of industrial fermentation processes often varies signifi cantly from batch to batch owing to unavoidable biological and compositional fl uctuations. The modern wine industry needs both fast and reliable process quality control methods and techniques that provide real-time information in order to assure the quality and consistency of a fi nal product. Available methods of analysis require sample preparation (e.g. several steps of purifi cation) and costly equipment, which precludes the capture of results in real time in an industrial environment. Red wine colour is attributed to the presence of grape-derived anthocyanins such as malvidin-3-glucoside (M3G) as well as other anthocyanin-derived pigments and “pigmented polymers” (PP) which are formed during fermentation and aging. These compounds are also thought to be largely responsible for colour changes in red wine and undergo different oxidation, condensation and polymerisation reactions. Established methods for determination of anthocyanins and PP are generally based on either colorimetric or chromatographic techniques such as high performance liquid chromatography (HPLC). These techniques can be relatively complex, requiring sample preparation and even chemical manipulations, making them time-consuming, laborious and costly to use routinely during winemaking. Previous work at the Australian Wine Research Institute (AWRI) demonstrated that it is possible to quantitatively predict M3G, PP and tannins during wine fermentation by visible (vis) and near infrared (NIR) spectroscopy and to monitor the time course of anthocyanin degradation and PP formation by HPLC. Over the last few years, the AWRI has been searching for new applications of spectroscopic techniques in the vis, NIR and mid-infrared (MIR) wavelength regions of the electromagnetic spectrum to be used for the analysis and quality control (QC) of both grapes and wines by the Australian wine industry. As part of this development, the feasibility of using vis and NIR spectroscopy combined with chemometrics to monitor fermentation during red wine production as a process analytical technology tool for the wine industry was explored.