Is there value in adding tannin to wine?

Recently published work conducted in collaboration between grape and wine researcher Dr Mark Downey, from Victoria's Department of Primary Industries (DPI), and research oenologist Dr Jim Harbertson, from Washington State University (WSU) in the US, has found that many tannin additions to wine may be unjustified.
Downey and Harbertson reported that commercially available oenological tannins contained as little as 12 per cent and up to 48% tannin and had little impact on wine when added at recommended addition rates. While recommended addition rates were too low to impact on the measurable amount of tannin in wine, excessive addition rates with a measurable impact on wine had a negative impact on sensory character.
As part of the joint collaboration, renowned sensory scientist Hildegarde Heymann and post-doctoral student Giuseppina Parpinello, from the University of California at Davis (UC Davis) in the US, conducted sensory analysis on Merlot and Cabernet Sauvignon wines made in Washington State with tannin additions. A range of oenological tannins composed of both condensed and hydrolysable tannins were added at different concentrations to Cabernet Sauvignon and Merlot wines.
Exogenous tannins are added to wine for a range of reasons, to stabilise colour, to modify mouthfeel, or to mask green characters or other faults. Recommended concentrations for tannin additions are quite small, around 50-300mg/L compared with the levels of tannin observed in red wines, which can range from 50-1900mg/L. In most cases these exogenous tannin addition rates are low. When considered together with the low proportion of tannin present in the oenological tannins (12-48%), very little tannin is added to wine at the recommended addition rates. Downey and Harbertson found that when added at recommended rates, oenological tannin addition had no effect on measured levels of tannin, anthocyanins, polymeric pigments or sensory character of the wine. In excess, there was a measurable increase in the concentration of tannin in the wine and large polymeric pigments. There was also a decrease in the concentration of anthocyanins, possibly related to the increase in large polymeric pigments.
For sensory characters, the addition of tannin at excessive rates increased the perceived colour intensity for all but one of the added tannins, which had lower colour intensity. Despite an increase in tannin concentration at excessive rates, there was no difference in astringency, but an increase in earthy flavours, bitterness and sour flavours, and a decrease in sweetness and viscosity.
Downey and Harbertson found that the differences observed in the sensory character of the wine were not differences in astringency, suggesting that the threshold for an increase in tannin astringency was quite large, whereas the threshold for aroma difference was quite small. To impact on astringency, a large amount of tannin would need to be added to wine, but this could significantly impact on negative characteristics. However, the wines used in this study had a reasonable base level of tannin at around 500mg/L. The addition of oenological tannins to wines with a low level of tannin may have a more significant impact and benefit
on astringency.
Exogenous tannins are one of the many tools available to winemakers in Australia, but previous research has shown that they are used without a clear understanding of their impact. Some winemakers consider their addition essential, while others consider tannin addition more of an insurance policy. However, the true contribution of exogenous tannin to wine quality is largely unknown and undocumented and tannin addition may be an unnecessary added cost to wine producers.
The majority of tannin in wine is extracted from grapes during the winemaking process, but our understanding of how tannin is extracted from grape berries and the exact contribution of grape tannin to wine quality is still unknown.
Some of the gaps in our understanding of grape tannin are; which tannin structures in grapes are extracted into wine and how do they interact with other compounds in the wine to impact on wine quality? How does tannin structure influence different astringent parameters of mouthfeel? Does tannin structure influence the chemical reactions between tannins and anthocyanins that are important for wine colour and ageing stability?
The ongoing collaboration between Downey and Harbertson aims to tackle some of these questions with the help of collaborators from UC Davis, Constellation Wines US and E&J Gallo Winery, California. One of the challenges the research groups face in addressing these questions is generating pure tannin standards of known structure and composition that can be used to study tannin interactions with other compounds and sensory analysis. The difficulty in generating pure tannin standards is separating the complex mix of grape tannins into individual compounds, but with the help of Dr Mark Kelm, a research chemist from Constellation Wines US, and a series of exchanges to DPI Mildura over the past few years, Harbertson and Downey are working on generating a range of tannin standards. Most recently in May 2012, Harbertson spent three weeks at the laboratories at DPI Mildura to continue experimental work designed to further our understanding of grape tannin interactions. The results from the most recent exchange will soon be published, but in the mean time, the next set of experiments and exchanges are already being planned. The ongoing collaborations between DPI and WSU have led the respective institutions to sign a formal agreement, a Memorandum of Understanding, which allows them to collaborate in a way that pools resources and knowledge with a positive impact on the depth of research.

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