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

AWRI: Identification Of The Major Drivers Of ‘Phenolic’ Taste In White Wines
5.2.4 Statistical Analysis
A three way ANOVA (Factors: phenolics, pH, alcohol) with assessors as a blocking variable was
conducted using MINITAB v14. Means separation was determined with Fisher’s Least Significant
Difference.
5.3 Results and Discussion
The addition of 30% more phenolics to the base wine significantly increased the perception of astringency
(p=0.046) (Table 5–1). However the strength of the effect depended on pH. At a low pH of 3.0,
astringency was perceived to be high, and the addition of phenolics had little further effect. However, at a
more realistic pH level of 3.3, the addition of whole phenolics increased astringency to the same level as
the low pH condition (Figure 5–1A). This suggests that for this particular wine, the 0.3 unit difference in
pH had the same effect on astringency perception as did an additional 30% phenolics (or vice versa). An
estimate of the impact of phenolic content on astringency can be made in a real-world winemaking
context following the winemaking trials conducted in 2010 (Chapter 8). White wines were made from
three varieties using both conventional and non-conventional winemaking processes designed to
maximise the differences in total phenolic concentration in the finished wines. Of the winemaking
treatments selected, a 30% increase in phenolics over those in free run controls (as used in this model
study) was achieved in wines that were made from heavy pressings (measured by total HPLC peak area at
280 nm (Figure 8–10)). While more work is required to quantify the relative importance of pH with total
phenolic content, it would appear that pH as a factor in white wine astringency has been underestimated.
The base wine was a commercial Riesling so it contained phenolics. Therefore, the increased astringency
resulting from pH reduction could either be the result of accentuation of the astringency elicited by the
native phenolics, or that elicited by pH itself. Both effects have been extensively reported. The
astringency of gallic acid (Guinard et al. 1986), catechin (Kallithraka et al. 1997), tannic acid (Peleg et al.
1998), flavanol oligomers extracted from grape seed (Fontoin et al. 2008), and whole grape seed tannin
(Fia et al. 2008) all increased with decreasing pH. Both organic and inorganic acids have also been shown
to be astringent like in the absence of phenolics (Hartwig and McDaniel 1995). The maximum level of
astringency produced by three organic acids declined as pH increased (Lawless et al. 1996) and was also
shown to be a function of pH rather than the acid concentration or anion species present.
While the addition of alcohol did not have a significant effect on astringency perception (p=0.524), higher
alcohol tended to increase astringency particularly at lower pH (Figure 5-1 (A)). This trend is contrary to
other findings whereby the astringency of red wine (Gawel et al. 2007) and model wines with added
oligomeric proanthocyanidins (Fontoin et al. 2008) decreased with increasing alcohol. Ethanol is thought
to weaken the interaction between salivary protein acceptor sites and the polyphenol hydroxyl groups by
disrupting hydrophobic interactions (Pascal et al. 2008). Indeed, fewer monomeric phenolics were
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