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AWRI: Identification Of The Major Drivers Of ‘Phenolic’ Taste In White Wines Two non flow-through fractions also contained quercetin-3-glucuronide as a significant proportion of the phenolic compounds that absorb maximally at 370 nm. Various flavonol glycosides (of which quercetin- 3-glucuronide is a member) have been reported to elicit astringency at very low concentrations (Scharbert et al. 2004; Brock and Hofmann 2008). However, of the two fractions that contained quercetin-3- glucuronide (F1 Fiano and F1 Riesling), only one was perceived to be more astringent than the low phenolic control, and only marginally so. Therefore quercetin-3-glucuronide does not seem to contribute to the astringency of these fractions. Adding phenolic fractions (regardless of source or type) resulted in a consistent reduction in perceived burning sensation and acid aftertaste. At this stage of the project, tasters were trained to understand the notion of hotness resulting from alcohol, but had not been introduced to the possibility that other forms of back-palate hotness might exist. The base wine had been extensively stripped of its phenolics, so it could be reasonably assumed that any hotness arising from the base wine was alcohol derived. In this context, every phenolic fraction, regardless of its source or type reduced the perception of burning sensation (presumably resulting from alcohol). Of all the fractions, the F3 fraction from the Gerwurztraminer was the most effective in reducing the burning aftertaste (p=0.0074). It contained a higher proportion of caffeic and coumaric ethyl esters that the other fractions amounting to 97% of the compounds active at 320 nm. Phenolic addition generally resulted in a reduction in the perception of acid aftertaste. The three fractions that resulted in the greatest reduction in acid aftertaste had a common feature in that they were all rich in free hydroxycinnamic acids. However, F2 from the Gerwurztraminer was also abundant in these, but its addition only marginally decreased the acid aftertaste. For the first time we show that phenolic addition has little effect on acidity when perceived in mouth, but significantly affects the intensity of acidity once the wine has been expectorated. The reason for this is unclear, but due to its implications on style perception this result should be pursued. The Fiano F2 fraction differed from the others in that it was rich in monomeric flavan-3-ols (epicatechin gallate, catechin and epicatechin). It was also perceived to be more bitter than the control than any other fraction. Epicatechin gallate has recently been shown to react most strongly amongst the common monomeric flavanols found in wine with the human bitter taste receptor hTAS2R39 (Narukawa et al. 2011). It also was deemed to be bitter and astringent in human taste tests, and it elicited a taste that was the least preferred amongst the common monomeric flavanols; epicatechin, epicatechin gallate, epigallocatechin and epigallocatechin gallate (Narukawa et al. 2010). The other two flavanols found in this fraction, catechin and epicatechin are also known to contribute to bitterness (Kielhorn and Thorngate 1999; Peleg et al. 1999). Interestingly, all the above-mentioned authors have reported that the monomeric flavanols elicit astringency as well as bitterness. Hufnagel and Hofmann (2008) concluded that flavanols did not contribute to astringency in red wine, a conclusion supported by the data here. The Fiano F2 fraction whilst being slightly more bitter than the control was not more astringent. Hufnagel and Hofmann (2008) also suggested that the major bitter compounds in red wine were ethyl esters of caffeic, vanillic Page | 52
AWRI: Identification Of The Major Drivers Of ‘Phenolic’ Taste In White Wines and syringic acid rather than the flavan-3-ols. Here, of the two fractions that contained reasonable quantities of caffeic acid ethyl ester, one was more bitter than the control (Fiano F2) and one was less bitter (Fiano F1). Furthermore, the interpretation is confounded by the fact that Fiano F2 also contained significant quantities of monomeric flavanols as well as hydroxycinnamic ethyl esters. In a previous trial (Chapter 3), adding whole phenolics extracted from wines and added to commercial base wines were shown to result in increases in most sensory attributes related to phenolic taste. However in this study where phenolic fractions were added, we mainly saw a suppression of phenolic tastes. In previous work we have used wines with a reasonable amount of phenolics as the base, while in this study we have utilised a base wine that had been stripped of most of its phenolics. While the possibility that phenolic compounds may synergise to accentuate phenolic tastes cannot be ruled out, to our knowledge this has not previously been reported. 6.3.2 Modelling Taste and Textures on Absorbance Measures at 280, 320 and 370 nm PLS modelled the burning sensation on A280, A320 and A370 very well (p model fit = 0.048, Figure 6.2). The burning sensation was heavily negatively weighted on A320 (i.e. total hydroxycinnamates), and strongly positively weighted on A370. HPLC showed that quercetin-3-glucuronide made up 90% of the active compounds at this wavelength in most of the fractions, which suggests that this compound may play a role in the perception of this sensation. However, the relationship between the total absorbance at a particular wavelength and the relationship between that and specific compounds is tentative. Page | 53 PLS Coefficient 2.5 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 Astringency (0.676) Viscosity (0.631) Bitterness (0.431) Burning (0.048) A280 A320 A370 Figure 6-2: PLS regression coefficients modelling sensory attribute ratings on analytical parameters. Significance of model fit given in parenthesis.
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