Methods to Predict Phenolic Extraction from Berries during Winemaking

During the 2004 season we analyzed 21 commercial wines at pressing for tannin, total
phenolics, anthocyanins, large polymeric pigments (LPP), and small polymeric pigments
(SPP). We then removed 750 ml subsamples of each one and stored them at 5, 10, 20
and 28º C. Each subsample was reanalyzed for each of the parameters at 21-day intervals
so that the rate of anthocyanin disappearance, LPP and SPP formation could be measured
along with any changes that occurred in the level of tannin and total phenolics. The
experiment was carried out for 105 days after pressing. The tannin and total phenolic
level remained fairly constant during the experiment, but the anthocyanin and polymeric
pigment showed large changes that were dependent on the temperature at which the
wines were stored. The purpose of this experiment was to determine if elevated
temperatures could be used to predict how a wine might change during aging with regard
to the evolution of the phenolic composition. The results from this year?s work make this
a promising possibility. We believe that if samples of wines were taken at pressing and
incubated at elevated temperature (e.g. 28 C) we could identify wines that may exhibit
poor color stabilization during aging. The next step in this research would be to find
ways to intervene in cases where poor color stabilization is suspected, perhaps by timely
tannin additions, so as to favor anthocyanin reactions that form polymeric pigments
rather than those that lead to loss of color.

During the 2004 season we conducted experiments to determine if the glutathionyl
conjugate of cinnamic acids give a color reaction with ferric chloride in 5%
triethanolamine buffer pH 9.4. We synthesized the glutathionyl conjugate of chlorogenic
acid, purified the glutathione reaction product (2-S-glutathionyl-chlorgenic acid) by
preparative HPLC, and verified the identity of the product by HPLC/MS. We had
previously determined that chlorogenic acid gives a good color reaction with ferric
chloride and this year we were able to show that the glutathione conjugate also reacts.
The extinction coefficient for the glutathionyl conjugate was nearly the same as
chlorogenic acid itself. This is a very important result because it shows that ferric
chloride reacts with all classes of phenolics in grapes and wines except the anthocyanins,
and that the assay we have developed for total phenolics based on ferric chloride detects
dihydroxy cinnamates even when present as the glutathione conjugate.

Sensory experiments were conducted to establish the relationship between astringency as
perceived by a taste panel and the amount of tannin in a wine measured by protein
precipitation. The r2 value of 0.68 indicates a good correlation between astringency as
rated by the panel and the amount of tannin we measure in the wine using our tannin
assay. This experiment answers a very important question and strongly supports the idea
that our tannin assay gives values that are related to perception of astringency.