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February 2015 practical winery & vineyard 59 w i n e m a k i n g Barrel alternatives supply oxygen to wines during élevage A n integral step in the production of quality wine is élevage in oak barrels. Today it is well under- stood that, at the end of élevage, wines aged in contact with wood can show organoleptic properties reflecting both a complexity and an originality forming their personalities. Two physico-chemical phenomena con- tribute to achieving this change in wine: extraction of volatile and non-volatile wood compounds with simultaneous dis- solving of atmospheric oxygen in the wine. Oak is a porous material, permeable to gas, allowing exchanges with ambient air throughout the élevage period. 15 Although the traditional cooperage market is stable, barrel alternatives such as staves, chips, powders, sticks and inserts, used in fermentation or during élevage, continue to become integrated into the everyday practice of modern enology. The choice of the rate at which these alterna- tives are used constitutes a decisive factor in the final quality of the wine. It is undeniable that the olfactory and taste contribution from oak aging, and in particular from contact with an optimal quantity of staves, contributes to the production of quality wine. For example, many users agree that using staves dur- ing the aging of white and red wine can limit the formation of reductive odors, thereby improving the aromatic clarity of their wines. Their use brings oak fla- vors and complexity. However, when an enologist decides to add these alternatives in excessive quantity, the resulting wine can show symptoms of over-oaking: loss of fruit and original "style" accompanied, some- times, by an irreversible change in the wine's aroma (oxidation faults). Mastering the oaky character of oak- aged wines requires knowledge of the compounds associated with these nuances and flavors. Oaky flavors in oak-aged wines come from a large number of aro- matic molecules. They include whiskey lactone, vanillin, eugenol and isoeugenol. Alexandre Pons, 1,2 Andrei Prida, 1 Benoit Verdier, 1 Philippe Darriet 2 and Denis Dubourdieu 2 1 Seguin Moreau, Cognac, 2 Université de Bordeaux, Unité de recherche Œnologie, Institut des Sciences de la Vigne et du Vin, Villenave d'Ornon, France BY These compounds, which come directly from the wood, constitute part of the impact compounds: They contribute sig- nificantly to the oaky character of the wines. 12,14 The change in the taste of wines aged in oak has been the subject of much research, whether focusing on changes in its astringent, bitter character or, on the contrary, in its sweetness. 5,6,7 Research undertaken by the Institut des Sciences de la Vigne et du Vin for many years on premature oxidative development of aromas in red wines has resulted in identification of a new mol- ecule: 3-methyl-2,4-nonanedione (MND). This very aromatic diketone (sensory threshold: 16 ng/L) is reminiscent of the odor of prune. Formed through oxi- dative-type mechanisms, it is found in large quantity (more than 200 ng/L) in wines stored in the presence of oxygen. 11 On the sidelines of this characteriza- tion research, there is an important area of study that has generated very little research: the study of oxygen supply during élevage. This process occurs in two stages. In the first weeks after fill- ing, the wine impregnates the pores of new oak barrels, replacing the air pres- ent in the wood. This phenomenon is accompanied by a rapid and extensive uptake of oxygen. An equilibrium is then established between the wine and the wood, during which slow interac- tion occurs between the outside air and the wine inside the hermetically sealed barrel. Given the complexity of the phenom- ena involved, estimating the oxygen con- centration supplied to the wine during élevage in contact with wood has always been a subject of interest. The first to pursue this field of inves- tigation was Pascal Ribéreau-Gayon, who published the first results of his research on oxygen intake during élevage in 1931. Using a rudimentary system of OENOSTICK barrel insert in 225L barrel. 1,200 1,000 800 600 400 200 0 0 5 10 15 20 25 30 35 O 2 µg/L Days O 2 max Stave removed Phase 1 Phase 2 Phase 3 Figure 1: Example of the kinetics of development in oxygen supplied by a stave of oak wood placed in a model solution with composition similar to wine, according to whether contact continues through the experiment (stave +) or whether the stave is removed once the maximum O 2 value is reached (stave -).