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 glycosidic moiety (Scharbert et al. 2004), or alternatively, the differences might have been due to the studies employing different media (i.e. water versus black tea) (Dadic and Belleau 1973). It is also worthwhile to note that the presence of rutin in grape skins of Vitis vinifera has been well documented; however, its existence in wine has been debated amid possible confusion with quercetin-3- glucuronide. In a recent survey of Australian wine, and contrary to literature reports of rutin in wine, rutin was not found in any of the wines analysed, and further experiments showed that rutin was rapidly degraded in wine to yield the aglycone quercetin (Jeffery et al. 2008). No sensory data on the effect of glycosylated flavanonols have been reported. 1.2.3 Flavanols While the concentration of flavanols in white wines is low, so too are their detection thresholds (summarised in Gawel 1998), suggesting that they might play a role in eliciting phenolic tastes. The major flavanol in wine, catechin has been described as being both astringent and bitter (Robichaud and Noble 1990). However, aqueous catechin solutions at (up to 200 mg/L) could not be distinguished from water when applied to the surface of the mouth while being restricted from contacting bitterness receptors in the tongue (Gawel, unpublished data). This result suggests that catechin does not elicit astringency. It is possible that catechin has been perceived as astringent in other studies when the compound came in contact with the tongue as well as the surface of the mouth due to a carryover effect from the perception of bitterness. Alternatively, the catechin used in some model studies might have been contaminated with tannins. If catechin is not astringent, then its reported detection thresholds are most likely to represent bitterness thresholds, rather than astringency. Furthermore, the bitterness of the other common monomeric flavan-3-ols such as epicatechin, epicatechin gallate, and epigallocatechin are likely to further contribute to the bitterness of white wines, particularly if they are made with intentional or unintentional skin and seed contact prior to fermentation. 1.2.4 Tyrosol The ‘tannin taste’ of Riesling wines from the same juice with different levels of must solids correlated most strongly with their tyrosol concentration, while being poorly correlated with hydroxycinnamic acid and flavan-3-ol concentration (Konitz et al. 2003). 1.3 Matrix Effects When considering the taste or mouth-feel of white juices or wines, we need to examine not only the phenolic compounds themselves, but also the matrix of the wine. Alcohol is known to be bitter, hot and to a lesser extent viscous when at wine concentration (Scinska et al. 2000; Gawel et al. 2007). Higher alcohol also enhanced the perception of a metallic character (Jones et al. 2008), and reduced the astringency (Fontoin et al. 2008) of model wine. From these studies it appears that alcohol can directly affect wine textures, some of which are normally associated with the presence of phenolics. Page | 16
AWRI: Identification Of The Major Drivers Of ‘Phenolic’ Taste In White Wines Organic acids have been shown to be astringent in their own right (Rubico and McDaniel 1992, Hartwig and McDaniel 1995). The astringency of equi-normal binary mixtures of organic acids increased as pH decreased, but remained unchanged when the normality of equi-pH solutions was increased. This result suggests that pH rather than titratable acidity influences perceived astringency (Lawless et al. 1996). Furthermore, the astringency of organic acids has been shown to be independent of anion species again highlighting the influence of pH. The increased astringency of phenolics in lower pH wines may be due to direct changes in salivary rheology (Nordbo et al. 1984) or through enhancing interactions between lubricating salivary proteins and phenolics (Obreque-Slier et al. 2011). Glycerol is an important compound in white wine, being the third most abundant after water and ethanol (Gawel et al. 2007). It is equi-sweet to glucose but despite its physical viscosity when in a pure form, it does not increase the perceptual viscosity of dry table wine (Noble and Bursick 1984; Gawel and Waters, 2008). However, due to their sweetness, both glycerol and residual sugar may play a role in moderating astringency (Lyman and Green 1990, Smith et al. 1996). 1.4 Winemaking Prior to fermentation, juice for white wine production is typically expressed and separated from grapes by a sequence of unit processes including destemming, crushing, draining and pressing. The final phenolic composition of a wine depends not only on grape composition but also on the conditions that influenced their extraction into the must, and on subsequent fining, and pre and post bottle aging. The compartmentalisation of particular phenolics in different parts of the grape bunch present winemakers with the ability to tailor the phenolic profile of their juices with the view to creating wines of a specific style. Free run juice is largely comprised of the hydroxycinnamates from the pulp, whereas pressings contain more flavonoids because of their greater abundance and consequent extraction from seeds and skins (Patel et al. 2010). They also showed that pressings contain less glutathione and caftaric acid, but more GRP than free run juice. This change in phenolic profile following pressing can be attributed to oxygen uptake during the press cycle. Extraction of flavonoids is increased by longer skin contact times at higher temperatures and the length and pressure applied during the press cycle. The effect on individual phenolic compounds in white wines with different pre-fermentation maceration conditions, including contact time (2 to 24 hours) and temperature (5, 10 or 20ºC), has been studied both on both an experimental and industrial- scale (Gomez-Miguez et al. 2007; Hernanz et al. 2007). These studies showed that individual compounds and classes of phenolics vary widely in their extraction dynamics and response to both temperature and time. Pressings also have higher pH values resulting from greater extraction of potassium, and slightly lower titratable acidities and slightly higher total soluble solids than do free-run juices (Singleton et al. 1975). Once the grapes are pressed, there are many chemical reactions that can occur, with enzymatic and non-enzymatic oxidation resulting in major changes to phenolics. Hyperoxidation is a technique practiced on must prior to the start of fermentation. It involves saturating an unsulfited must with Page | 17
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