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winemaking MAKIN G Functions and benefits of for fermentation/élevage BY Nicolas Tiquet-Lavandier, Marie Mirabel, Taransaud Tonnellerie, Cognac, France W hat is an oak barrel for? The French word "élevage," which means bringing up, can give us a clue. It is to achieve the potential of a wine through maturation and to enrich it with new characteristics that come from oak. This involves a long process that is respectful of the nature of the wine. Barrels can be used for alcoholic and malolactic fermentation prior to élevage, but it is quite unusual to use barrels for fermentation only and not for élevage afterwards. Indeed, most often barrels are used mainly for élevage. What occurs during élevage? What are the functions of barrels and what benefits can we expect from them? Barrels serve to: •Enrich the wine with new compounds, •Play a role as an interface between the wine and the environment, and • Allow specific physical reactions to occur. First function — Enriching the wine with new compounds Which compounds are released into a wine and how do they improve a wine? To answer these questions we need to look more closely at the raw oak and its chemical composition. a. Raw oak composition — Different polymers are clearly identified in oak Figure II shows the chemical structure of these different compounds. Two sugar polymers are present. Cellulose represents 40% of oak's chemical composition and contributes to its strength, and hemicellulose represents 25%. Both of these sugar polymers provide support. There is also a phenol polymer called lignin, which represents 25% This text reprinted from first publication: Nov/Dec 2008. 36 p r acti c al w i ne ry & v i n e yard JANUARY 20 14 of the oak and helps to add structure to the oak. These polymers will not be released in the wine, but they will be transformed during the barrel-making process. Some extractable components are also present, that are releasable. About 10% of the oak components are phenolic compounds (tannins, phenols, coumarins — bitter phenols, etc.). Between 1% to 5% are represented by other components such as minerals, terpenes and aromatic compounds. The geographical origins of the oak, the forest where it comes from, and the type of grain will influence the chemical composition of the wood. b. Compounds released from oak into the wine There are two main categories: Aromatic compounds such as lactones, eugenol and vanillin that will contribute to the flavor of the wine. Wood polyphenols and ellagitannins will contribute to the structure and taste of the wine. First, in terms of flavor, different aromas are identified: •Furfural (almonds and smoke), •Methyloctalactone (coconut), •Eugenol, isoeugenol (cloves and spices), •Vanillin (vanilla), •Maltol, cyclotene (caramel and toffee), •Guaiacol (smoky notes). The bouquet of a wine aged in barrels is therefore richer and more complex than that of unoaked wine. Second, some compounds influence barrel taste and mouthfeel. Tannins from wine grapes represent between 2 and 4 g/L. But oak tannins extracted by ageing in a barrel can be up to 0.2 g/L. These oak tannins have strong antioxidant properties. They make the wine more tannic (enriching its structure) and they also make it tastier. At the same time, other components add some sweetness to the wine. The cooperage processes, such as the seasoning of staves and the different barrel-making steps including toasting, will change the oak composition and, in particular, the nature and concentrations of the above-mentioned compounds. Figure I: Reactions and exchanges with the barrel as membrane.