Identification of the major drivers of 'phenolic' taste in ... - GWRDC

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AWRI: Identification Of The Major Drivers Of ‘Phenolic’ Taste In White Wines 2.2.4 Tasting Panel Five of the six tasters were winemakers, each with over 25 years of winemaking experience. The sixth taster had undertaken vintage experience both in Australia and Europe and had over ten years of wine tasting experience as part of his profession. 2.2.5 Tasting Methodology The assessors undertook a general discussion as to what they felt constituted ‘phenolic taste’ in white wine just prior to the tasting. The tasters were presented with duplicate sets of three wines in the orders dictated by the design. They were instructed to taste the three samples in each set in the order in which they were presented and rank them for overall ‘phenolic taste’ with a ranking of 1 indicating highest ‘phenolic taste’ and 3 lowest. While repeat tastings were not forbidden, the assessors were discouraged from doing so. 2.2.6 Statistical Analysis Friedman’s two way analysis of rank data was used to assess overall treatment effects. A rank separation method (Meilgaard et al. 1991) was used to compare treatment ranks. 2.3 Results and Discussion The 2009 Riesling wines were analysed before a comprehensive HPLC analytical tool had been fully developed, so spectral UV analysis as described by Somers and Pocock (1991) were used to determine the approximate phenolic composition of the wines. Under this method, the ‘total phenolics’ values estimates all compounds containing phenol rings absorbing at 280 nm (A280) corrected for the presence of peptides. ‘Total hydroxycinnamic acids’ (HCA) estimates C6-C3 compounds including GRP molecules absorbing at 320 nm (A320), and the ‘flavonoid extractibles’ is an estimate of flavonol-like compounds represented by A280 less the contribution from hydroxycinnamates. As expected, the free run wine contained lower total phenolics, total HCA and flavonoid extractables than did either of the pressings wines (Table 2-1). Unexpectedly, the light and heavy pressings wines did not differ significantly in total phenolics. However the light pressings wines contained more total HCA and fewer flavonoid extractibles than did the heavy pressings wines. Fining juices with a gelatine based fining agent at either level did not affect a practically significant change in any phenolic measure. Fining at a commercially realistic level of 200 ppm only reduced the total phenolic content by at best 5%, and heavy fining at 1000 ppm only resulted in a decrease in total phenolics in the order of 8%. Page | 22
AWRI: Identification Of The Major Drivers Of ‘Phenolic’ Taste In White Wines Table 2-1: Somers’ measurments for 2009 experimental wines (Mean of duplicate spectral measurements for each wine triplicate ferment, Standard errors in parentheses). Press fraction/fining rate Total Phenolics (au) Total HCA (au) Flavonoid Extract. Free Run (FR) 5.97 (0.086) 6.35 (0.114) 1.75 (0.016) Light pressings (LP) 10.19 (0.004) 10.85 (0.004) 2.97 (0.004) Heavy pressings (HP) 10.22 (0.029) 8.39 (0.024) 4.63 (0.012) HP + 200 ppm Liquifine 9.66 (0.008) 8.04 (0.008) 4.30 (0.004) HP + 1000 ppm Liquifine 9.34 (0.016) 7.80 (0.016) 4.14 (0.004) The consensus view amongst the experienced winemaker panel was that ‘phenolic taste’ was a multivariate sensory attribute that comprised aspects of bitterness, astringency (or ‘grip’), and oiliness/viscosity. Some tasters also suggested that high phenolic wines displayed a specific flavour character but were unable to adequately describe it. Others felt that some phenolic wines also displayed hotness or burning sensation experienced at the back of the throat that detracted from overall wine quality. Table 2-2: Rank totals of perceived ‘phenolic taste’ (lower rank total implies higher intensity). Treatment Free Run Light Pressings Heavy Pressings p LSD Rank totals 32 a 22 b 18 b 0.013 9.6 Treatment HP HP200G HP1000G p LSD Rank totals 21 a 27 a 24 a 0.472 n/a Singleton et al. (1975) fermented six different white grape varieties on total skins for one to five days before separating the skins from the fermenting must. The result of this highly unconventional white winemaking practice was to double total phenolics after five days of maceration. Making white wine as one would a red wine represents an extreme variant of traditional white winemaking with the potential to maximise total phenolic content. So although it was shown that both the astringency and bitterness of the wines were moderately correlated with total phenolic content by an expert tasting panel, the practical relevance of the results remain in question. In this study, the wines being compared were a reasonable representation of commercial white winemaking practice. Despite being unable to fully agree as to the nature of ‘phenolic taste’ in white wine, the experienced winemakers were able as a group to differentiate the more phenolic wines made from light and heavy pressing juices from the free run wine (Table 2-2). They were unable to Page | 23
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