Roger Boulton

Stephen Sinclair Scott Professor of Enology and Chemical Engineering
Department of Viticulture and Enology UC Davis

Interview by Diana H. Stockton

Barrels not only facilitate storage and transport of wine, but Roger Boulton, whom we interviewed by telephone, assures us that oak barrels contribute significantly to the character of thewineinside.Thefinishof awine,that sensation at the back of one’s tongue after a swallow or spit, comes from wood: the oak, and its flavors of caramel, coconut, vanilla, clove or cinnamon, toast, and almond. After twelve months in barrel, oak has affected the aroma, flavor and texture of the wine as well as its color and clarity, yet compounds producing these qualities are but one to five percent of wine. 85 to 90 percent of wine is water and 10 to 14 percent alcohol, with a remainder of tannins, salts and acids.

Changes in wine are brought about by oxidation, and Roger says it is at the bunghole that the most opportunity for oxidation occurs. Much more oxygen enters there when filling, emptying or sampling the barrel than creeps in through the porous staves.True, there is constant evaporation of water and alcohol out through the wood (its rate dependent on temperature and humidity), leaving an ullage or emptiness in the barrel that requires a periodic addition of topping wine, yet for the first three months wine is pretty much the same as when it went into the barrel.Then the phenolics in the oak and wine—large organic molecules of carbon and hydrogen—that contribute to the color, taste, and feel of the wine begin to dissolve off the barrel walls as well as change within the wine inside. Extractions of changing tannins and other compounds diffuse slowly, so the character of the volume of wine along the walls is quite different from that in the middle. As a wine’s tannins break down by forming longer chains or sheets and even dropping out of suspension, sediment of the chains (polymers) of compounds and microscopic dead yeast cells settles down into the curved bilge of the barrel. Racking wine by pumping or siphoning it off this sediment, or lees, and stirring the wine makes it uniform. Any splashing provides an opportunity for oxygen pickup. After wine goes back into barrel the process repeats until the next racking. Laboratory analysis can provide percentages of the various compounds contributed by barrel aging and a flavor portrait of the wine:

Roger says the rate of extraction depends on the structure of the wood grain—its potential for pore intrusion (wine entering the wood) and capillary action in the barrel (wicking water and alcohol to the outside), although whole wine penetrates only about one quarter of aninchintothestaves.Therateof extraction is greatest at the beginning, and then slows with the accumulation and aggregation of compounds. (Wine barrels are thought to be virtually inert, with respect to extraction, after five years of use.) After twelve months in barrel, wines are said to have softened and become less astringent. Although every harvest, every vintage is different, Roger points out that through constant sampling and assessment the winemaker gets to say “when” for each step that is taken to create a finished wine.The winemaker is in control; how a wine is remembered is a balance of oak, terroir and varietal character. [And, if you have the chance to inhale a freshly toasted barrel, as we did at Demptos Napa, you will immediately understand the excellent, intimate pairing of wine with oak.