Methods to predict Phenolic Extraction from Berries during Winemaking and the Chemical Changes in Phenolic Composition of a Wine during Aging
This research has provided significant results related to the analysis of phenolics in grape extracts and wines, and extraction of phenolics during fermentation. We have devised a new procedure for measuring grape and wine phenolics that is based on their reaction with ferric chloride. We showed that ferric chloride gives a good color reaction with every class of phenolic present in grapes and wines except anthocyanins. This addition to our phenolics analysis panel is important because it allows us to easily measure phenolics and tannin in wine using the same chemical reaction (i.e. reaction with ferric chloride under basic conditions). The significance of this is that it is now very easy determine what fraction of the iron-reactive phenolics (IRP) in a wine are actually tannins that bind to protein. We previously established that our tannin assay measures the same amount of tannin in wines that bind to salivary protein. Measurement of iron reactive compounds combined with a simple assay for anthocyanins now gives us a simple but comprehensive system for evaluating all phenolics in grapes and wines. We believe our results raises the bar for analytical procedures claiming to measure tannins, because no other analytical methods for tannins have been shown to measure the amount of tannins that bind to salivary proteins. We have also shown that our measurement of total iron reactive phenolics gives an excellent correlation with total phenolics measured with the familiar Folin reagent.
By studying a population of 65 commercial Cabernet Sauvignon we have been able to demonstrate that there is no correlation between copigmentation of anthocyanins measured at pressing and the amount of polymeric pigment that forms during 100 days of barrel aging. Several researchers suggested that increased copigmentation might lead to enhanced polymeric pigment formation during aging. We were successful in testing this hypothesis in a group of 65 Cabernet Sauvignon wines and we found no relationship between copigmentation and polymeric pigment formation. Nevertheless our data seem to indicate that the amount of small polymeric pigment that forms during aging is related to the fraction of the total phenolics that are iron-reactive but that do not precipitate proteins (non-tannin phenolics).
We found that heating a wine for 8 days at 37 ?C led to an increase in large and small polymeric pigment be had no effect on tannin. We hope to be able to use this method to help predict how the phenolic profile of a wine will change during aging.
