IMPACT OF CLIMATIC CONDITIONS ON THE SEASONING QUALITY OF OAK WOOD FOR OENOLOGICAL USE (QUERCUS PETRAEA)

Abstract

For coopers, seasoning and toasting are considered crucial steps in barrel making during which the oak wood develops specific organoleptic properties. Seasoning, carried out in the open air, allows reducing the moisture content of the staves to between 14 and 18% (compared to 70 to 90% after splitting) while modulating the intrinsic composition of the oak wood. Toasting consists of applying different degrees of heat to a barrel for a specific period of time. As the temperature increases, oak wood produces a wide range of chemical compounds through thermal degradation of its intrinsic composition. Many studies have been conducted to identify the key aroma compounds in oak wood, and in a recent work we re-ported the identification of two new unsaturated aldehydes responsible for the “puff pastry” and “me-tallic” nuances present in toasted oak wood aroma: (2E,4E,6Z)-nonatrienal (I) and trans-4,5-epoxy-(E)-2- decenal (II).1 In foods, these aldehydes are derived from the oxidative degradation of linolenic and linoleic acids, respectively. This degradation is promoted by heat, light and metal ions. However, no data are available on the presence of fatty acids in oak wood for oenological use (Quercus petraea). In this context, this work aimed to study the distribution of fatty acids in oak wood by focusing on the seaso-ning process taking into account the impact of climatic conditions. To do so, we studied in parallel the evolution and distribution of unsaturated aldehydes and fatty acids in seven oak wood staves during the seasoning process (0, 12, 18 and 36 months) depending on the location (Merpins, Châlon-en-Cham-pagne and Beaumes-de-Venise). They were selected for their climatic diversity (average temperature and rainfall). Based on this experimental protocol, 84 samples were analyzed. The study of unsaturated aldehydes was carried out by GC-NCI-MS (NH₃) analysis, while the study of fatty acids required the de-velopment of a quantification method by GC-TOF MS analysis after liquid-liquid extraction and deriva-tisation. The results show a significant impact of climatic conditions on the distribution of unsaturated aldehydes and fatty acids. For example, the highest levels of unsaturated aldehydes (1.5 ng/g wood (I) and 13.2 ng/g wood (II)) were detected in oak wood seasoned in Merpins and Beaumes-de-Venise (southern France) compared to that seasoned in Châlon-en-Champagne (northern France). Conversely, linolenic acid was detected at a higher level in seasoned oak wood in Châlon-en-Champagne (9.5 µg/g wood). It is likely that “warm” climates lead to a degradation of fatty acids in favor of the formation of unsaturated aldehydes. These new results underline the potential effect of global warming on the quality and sensory identity of oak wood and barrels. To go further, these samples were also toasted. The impact on the aroma of red wine will be discussed.