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AWRI: Identification Of The Major Drivers Of ‘Phenolic’ Taste In White Wines 9 Outcomes, Conclusions and Recommendations Significant progress has been made in finding the molecular drivers of phenolic tastes in white wine. The task was, and remains, challenging as white wine contains over 100 different phenolic compounds spanning a dozen or more structural classes. While the direct effect of only some of the most abundant phenolic compounds were investigated, a further body of correlative work has allowed us to direct our focus on certain phenolic classes as candidates for either causing or suppressing phenolic tastes in white wine. Using ‘total phenolics’ isolated from commercial wines we, for the first time, demonstrated that wines with different phenolic composition, when presented at wine like concentrations, can display different textures when tasted in the same matrix (alcohol, pH, TA etc). This suggests that differences in phenolic composition influences textural differences in white wines. Phenolics were shown to be important in defining wine style. Total phenolics were one of the major factors that differentiated the two recognised styles of the commercially important variety, Pinot G. Lower total phenolics and a higher proportion of total hydroxycinnamates were shown to be strongly associated with the perception of quality in commercial Rieslings by Australian winemakers. On the other hand the acceptability of commercial white wines by Sydney wine consumers was less affected by phenolic levels and more by residual sugar or alcohol concentration. We also established that alcohol concentration enhanced four major taste/textural attributes (astringency, viscosity, bitterness and hotness) in white wine, and that phenolics and alcohol contributed in an additive way to these attributes. Interestingly, research into stylistically different whole wines demonstrated that the astringency of Pinot Gris/Grigio wines was mostly associated with low pH. In order to further explore the influence and interactions of phenolics with the key matrix elements of alcohol and pH on taste/textural attributes, we demonstrated that variation in white wine tastes and textures could be attributed to both phenolic composition and their interaction with the wine matrix. These were important steps towards an understanding the molecular basis of textural perception in white wine; the identity of the phenolic molecules (or groups of molecules) that caused differences in tastes and textures remains unclear. To tackle this, the two most dominant phenolic molecules in Australian white wines were isolated and then tasted. The first was caftaric acid. It is usually the most abundant hydroxycinnamate in both juice and wine. The second was 2-S-glutathionyl caftaric acid (better known as Grape Reaction Product or GRP). It is a derivative of caftaric acid and its levels can be increased in white wine by using oxidative 94
AWRI: Identification Of The Major Drivers Of ‘Phenolic’ Taste In White Wines juice handling, but only at the expense of caftaric acid. Caftaric acid was shown to reduce the burning hotness from alcohol and GRP was shown to increase palate oiliness. The discovery that these two phenolics decrease the dry mouth-feel produced by alcohol and acidity, and caftaric acid suppressed alcohol hotness was unforeseen. It has long been assumed that most ‘phenolic taste’ elements came from phenolics and that the wine matrix (e.g. acid, alcohol) modulated these perceptions, so the finding that some principal phenolic taste characteristics come from the matrix and are then modulated by some phenolics was unexpected. This has significant practical implications because these two compounds can be varied in winemaking through oxygen exposure and so variations in their concentrations, and that of similar compounds, may allow modulation of the significant sensory effects from alcohol and pH. An advanced HPLC method has been developed and now allows separation of > 80 identified (40 of which can currently be quantified) phenolics in white juices and wines. This is a notable advancement on the previous methods available, in particular because it requires minimal sample preparation. This was a challenging aspect to the project that was critical to analysing the winemaking experiments described next. We produced a set of white wines that varied greatly in phenolic composition, so as to elicit measurable differences in both taste and texture. A mix of both conventional and less practiced white wine making techniques were used over three vintages and most winemaking treatments were repeated over two of the years (2010 and 2011). The winemaking experiments, together with the outcomes of the previous experiments in the project, allow insight into the molecular basis of ‘phenolic’ taste which is summarised below. ‘Astringency’ ratings in white wines were found to be strongly negatively correlated with pH (i.e. lower pH gives higher astringency). Aggregate measures of phenolics, various complex mixtures of phenolic compounds, and the two major individual phenolics (caftaric acid and GRP) were, in general, not particularly associated with astringency. A striking example of this is the minimal difference in astringency between low phenolic hyper-oxidised wines and the high phenolic macerated and skin contact wines made in 2011. These are among several findings of significance that contradict the widely held assumption that phenolics are the main cause of astringency in white wines. ‘Viscosity’ ratings in white wines were found to be strongly positively correlated with pH (i.e. lower pH gives lower viscosity). This new discovery further emphasises the importance of this matrix effect on the perception of mouth-feel in white wines. ‘Hotness’ and ‘burning after taste’ are most highly associated with alcohol concentration, not aggregated phenolic measurements or caftaric acid. Phenolics have anecdotally been implicated in these sensory characteristics, but mostly phenolics seem not to be positively related to these heat attributes, with the exception of some small effects from GRP and GRP-like compounds. In a further discovery, these 95
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Identification of the major drivers of 'phenolic' taste in ... - GWRDC

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