Decanting - Day 9
This topic has been allowed to breathe for too long! The previous post in this series, Day 8, on March 23rd provided a link to a Q&A in Scientific American. “How does decanting red wine affect its taste? And why is it suggested for red wine, but not white?” The answer was provided by Andrew L Waterhouse, a Professor in the Department of Viticulture and Enology at the University of California at Davis and author of the book Red Wine Color: Revealing the Mysteries. Professor Waterhouse notes “From the perspective of modifying the taste or appearance of a wine, the decision about whether or not to decant is based largely on two criteria, although the amount of published literature on the topic is very limited.” After searching far and wide for any literature I can agree with the last part of this statement!

In answering the question Professor Waterhouse makes sure that the reader is aware that his response will include several terms “that are not based on measurements, but are descriptive terms conventionally used by wine drinkers”. Those terms are “closed”, “breathing”, “softens”, and “bottle bouquet”. This is an important point. As much as you (or I) may think we experience these phenomena, our description does not provide any measurement. And even though they may seem like simple terms their (subjective) measurement may be extremely complicated to quantify.

The two criteria that Professor Waterhouse comments on are decanting to accelerate the “breathing” process to increase the wine's aromas, and decanting to “soften” the taste of the tannins that cause harshness and astringency in young wines. Let’s talk about the second point first, and we’ll get to the “breathing” issue in another post. As Professor Waterhouse notes chemical analysis has determined no changes in tannins after decanting, and yet it is often noted that a wine “softens” with time once exposed to air. So what explains the phenomena? Well let’s backtrack a little here. What actually happened with this story is that I sent out a question which asked “do you know of any published work that supports the notion that decanting for several hours softens tannins in bottled wine?” to several academics who I thought might be able to provide an answer. Professor Waterhouse was one of the individuals. He was kind enough to reply that there is “Little evidence of actual changes in tannins.” And provided a PDF of the Scientific American commentary. (I found the Scientific American piece independently during a literature search. Oddly enough the direct link to the magazine is a more extensive answer than the material in the PDF.)

I also asked Professor Bruce W. Zoecklein of the Department of Food Science and Technology at Virginia Tech. Professor Zoecklein, who oversees The Wine/Enology - Grape Chemistry Group, replied “I do not know of any publication which reports that aeration of bottled wine softens tannins. That could happen as a result of oxidative polymerization. Naturally, how much polymerization and the sensory impact would be dictated by the phenol make-up of the wine. Oxidative impact is much greater in fermenting and very young wines which contain a higher concentration of monomeric phenols-notable pigments.” So it’s a possibility that oxidation might affect tannins in very young wine, but there is no published experimental evidence for bottled wine.

OK, so let’s think about this in another way. Why does a softening of the tannins in wine have to involve an effect on components in wine? After all, astringency is detected in the mouth. What is astringency? All of the explanations I have found are essentially similar. The most interesting comes from Simon et al (Biochemistry 42 (2003) p10385-10395). “This sensation, earlier considered by Aristotle (384-322 B.C.) as a taste, was described in physicochemical terms by Joslyn and Golstein in 1964 as the result of a strong interaction between tannins and salivary proteins. The formed tannin-protein complex aggregates, thus reducing the lubricating property of saliva. A dry, rough, and pucker sensation is then perceived as a diffuse stimulus in the entire mouth.” In essence polyphenols (tannins) interact with proline-rich proteins (salivary proteins).

In reviewing polyphenols, Lesschaeve and Noble (American. J. Clinical Nutrition 81 (2005) 330S-335S) make a number of interesting points. The tactile sensation of astringency is thought to be perceived through touch, while an astringent is defined chemically as a compound that precipitates protein. Alcohol levels can affect bitterness but not astringency. Astringency increases and bitterness decreases with the degree of polymerization. Adding acid to wine increases astringency. Eating chocolate increases perceived astringency of red wine, with dark chocolate producing the largest effect. Although studies are conflicting there is evidence that astringency perception may be influenced by salivary flow rate. The faster you can restore saliva (salivary protein) the less astringency you will experience. Wine consumers often confuse bitterness and astringency; bitterness is used to express dislike and often associated with acid or astringent characters.

How does any of this explain how decanting “softens” the taste of the tannins? Well I doubt that there is any experimental work that addresses the question directly but my point is quite simple. As a research scientist I’m certain I could make a much more credible case for salivary protein levels influencing tannin perception, as opposed to oxidation affecting tannin chemistry. More to the point, tasting a wine and perceiving changes in the qualities of that wine over time is more complex than just the interaction of air with the wine. There are so many variables to be considered that they render anecdotal description of the effects of aeration meaningless.

If the question is ever completely resolved it is likely that the final answer may well involve chemical (oxidation) and physiological (astringency perception) explanations, but as it stands right now there is no unequivocal evidence that aeration due to decanting “softens” the taste of the tannins.

Hmm, I wonder if the perceived increase in a wine's aroma during aeration might be explained by physiological responses as well.

I want to acknowledge the kind assistance of Professors Waterhouse and Zoecklein in taking time from their busy schedules to answer my questions and permitting me to quote from their replies.