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

AWRI: Identification Of The Major Drivers Of ‘Phenolic’ Taste In White Wines
wines. They also found that the concentrations of gallic and caffeic acid were correlated with the
perceived bitterness of these wines. However, such associations could be the result of co-correlation
with other phenolic compounds.
While the threshold estimates of the hydroxycinnamic acids and their tartaric acid esters differ greatly
between studies summarised in Gawel (1998), they are typically present in white wine at
concentrations near or below their detection threshold in water. This would indicate that individually
they have little or no sensory impact on wine. Using duo-trio difference testing Vérette et al. (1988)
reported that caffeic, coumaric and caftaric acids were undetectable in white wine when present in the
upper range of white wine concentrations. However, the effect of using a small number of tasters
combined with applying a relatively insensitive test method suggests this study should be revisited.
Furthermore, even if the concentrations of individual phenolic species were insufficient to induce an
astringent or bitter sensation in white wine, they may do so in combination. The thresholds of mixtures
of two or three hydroxycinnamic and hydroxybenzoic acids were in most cases found to be lower than
the minimum threshold of each of the components (Maga and Lorenz 1973). This result is not
surprising as the intensity of mixtures of similar tasting bitter substances is generally additive (Keast et
al. 2003). Therefore, although it appears that single species of hydroxycinnamates and their
derivatives are not bitter at the concentrations found in wine, the possibility that the combined impact
of the entire hydroxycinnamate pool can elicit a perceptible bitterness in white wine cannot be ruled
out. Nagel et al. (1997) have suggested that hydroxycinnamates might not be bitter, explaining that
previous sensory assessors might have confused bitterness with the compounds’ sourness. They noted
that after masking the acidity of a hydroxycinnamate with potassium bitartrate, tasters were unable to
discriminate bitterness in the sample. However, the possible masking role of the added salt on the
bitterness of the test sample was not considered.
The oxidation product of caftaric acid, trans-2-S-glutathionyl caffeoyl tartaric acid (grape reaction
product, or GRP) was also not perceptibly bitter when tasted at levels typically encountered in white
wine (Vérette et al. 1988). A study of six wines produced from unoxidised juice and the corresponding
oxidised juice gave inconclusive results regarding the role of GRP and bitterness (Nagel and Graber
1988).
1.2.2 Flavonols
Quercetin glycones are the main flavonols in white wine, although quantitative data in a representative
range of Australian white wines is not available in the published literature (McDonald et al. 1998;
Boselli et al. 2006). The thresholds of flavonol aglycones are around 10-20 mg/L, determined in 5%
aqueous ethanol (Dadic and Belleau 1973), with descriptors such as ‘bitter’, ‘harsh’, and ‘astringent’.
Therefore, it might be that flavonols and their glucosides are important either individually or
cumulatively, to the mouth-feel of white wine.
The impact of quercetin on white wine astringency is unclear. Dadic and Belleau (1973) reported that
it elicits a bitter taste with only a weak astringency when presented in water. However, rutin which is
one common quercetin glycoside has recently been reported to elicit an astringent like sensation even
at extremely low concentrations when found in black tea (Scharbert et al. 2004). The reason for these
conflicting results is unclear. It might have been due to the two studies using free and glycosidically
bound quercetin, as the astringency of quercetin glycosides appears to depend on the structure of the
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