Abstract
The oak barrel maturation step is nowadays strongly rooted in the production of quality wines. Two main physico‑chemical phenomena contribute to the modification and improvement of wine: the solubilisation of volatile and non-volatile wood compounds concomitant with the dissolution of oxygen from the air into the wine. Indeed, wood is a porous material and gas transfer (especially oxygen transfer, expressed as oxygen transfer rate or OTR) through oak barrels, is an intrinsic parameter which ensures wine oxygen supply during maturation. Due to its oenological impact, it has been actively studied over recent decades using several approaches based on the same principle: the monitoring of oxygen in a model wine solution in the barrel. This project aimed at assaying barrel OTR by using a new tool based on the theoretical knowledge of gas transfer through porous materials. An oxygen concentration gradient was created on each side of a barrel kept in an airtight stainless-steel tank. The concentration of the oxygen in the atmosphere around the barrel was monitored in order to quantify oxygen transfer, thus the avoiding common drawbacks of interactions between dissolved oxygen ingress kinetics and the consumption of oxygen in the liquid phase by wood components. This study reports for the first time, the diffusion coefficient of entire oak barrels (Q. sessilis) to be between 10-10 and 10-9 m²/s, and it contributes to increasing knowledge on the complex phenomena driving oxygen ingress during the maturation of wine in barrels kept in cellar conditions. The results highlight the important role of wood moisture content in oxygen transfer, and provides a simple and reliable parameter to monitor it: the weight of the barrel. Following methodology developed by the authors, the OTR of a new oak barrel was found to be 11.4 mg/L per year. Taking into account the oxygen released through the wood pores, a new barrel will contribute 14.4 mg/L per year of oxygen to the wine, of which 46 % in the first three months of aging.
Highlights
Wooden barrels have been used to store, carry and sell wines
This study reports for the first time, the diffusion coefficient of entire oak barrels (Q. sessilis) to be between 10-10 and 10-9 m2/s, and it contributes to increasing knowledge on the complex phenomena driving oxygen ingress during the maturation of wine in barrels kept in cellar conditions
Oxygen ingress or the oxygen transfer rate (OTR) through oak barrels is another important parameter to take into account when interpreting the chemical modification of wines aged in oak barrels
Summary
Wooden barrels have been used to store, carry and sell wines Nowadays, it is well known and described from a chemical point of view that maturation in oak barrels profoundly modifies the intrinsic composition of wines and shapes their quality. It is well known and described from a chemical point of view that maturation in oak barrels profoundly modifies the intrinsic composition of wines and shapes their quality These modifications result from complex phenomena associated with the solubilisation of volatile and non-volatile compounds in the wine, accompanied with the slow ingress of small quantities of oxygen (Boidron et al, 1988; Michel et al, 2011). Oxygen ingress or the oxygen transfer rate (OTR) through oak barrels is another important parameter to take into account when interpreting the chemical modification of wines aged in oak barrels. It is widely accepted that the oxygen absorbed by wine during maturation in barrels comes from the atmospheric air and from the pores of the oak wood barrel (del Alamo-Sanza and Nevares, 2014; Qiu, 2015; Vivas and Glories, 1997)
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