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

AWRI: Identification Of The Major Drivers Of ‘Phenolic’ Taste In White Wines
8.5.2 Phenolic Measures
Absorbances in the 230 to 430 nm range spectra of the 2010 wines are shown in Figure 8-3. To simplify
the figures, only duplicate wines with the lowest and highest phenolic composition are shown. They are
typical of white wines in having local maxima at 265–280 nm, representing phenolic acids as well as
flavanols, and at around 325 nm representing hydroxycinnamic acids (HCA’s) and grape reaction
products (2-S-glutathionyl caftaric acids and some other analogues). The absorbance patterns of wines
produced in the subsequent vintage were similar (Figure 8-4). Individual values of Somers’ measurements
are listed in Appendix B, Table B-6 and Table B-10 and are shown in Figure 8-5 (2010 wines) and Figure
8-6 (2011 wines).
The free run treatment represents a logical standard to which to compare others as it is the most widely
used method of white wine production.. A percentage deviation from the free run wines for total
Phenolics and total HCA are given in Figure 8-7. Whole bunch pressed wines from all three varieties had
much less phenolic material compared to free run, and maceration resulted in much higher values. The
HCA’s of the light and heavy pressings are only slightly higher in Chardonnay and Riesling but the HCA
of Viognier was 20% lower.
The hyperoxidised wines represented a probable extreme. Here, the musts were subjected to an excess of
oxygen resulting in the consumption of most of the phenolic substrates. In typical commercial practice of
hyperoxidation, not as much oxygen is used as in this experimental trial and this may result in residual
phenolics.
Naturally occurring glutathione in grapes acts as an antioxidant by reacting with o-quinones to form GRP,
thus preventing further oxidation. Hyperoxidation of both free run and pressings juices reduced total wine
phenolics and total wine HCA’s of all three varieties. The relative decreases in total phenolics and total
HCA of wine following juice hyperoxidation compared with before treatment are given in Figure 8-8.
Hyperoxidation of free run juice reduced the total phenolics of Viognier wines the most and Chardonnay
wines the least. On the other hand, the decrease in total phenolics after hyperoxidation of pressings was
less pronounced for Viognier than the other two varieties.
The decrease in HCA of hard pressings is very similar for all varieties suggesting that natural
preservatives such as glutathione have already been pressed from the grape flesh and do not provide any
protection to press juice.
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