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58 WINES&VINES December 2015 WINEMAKING PRACTICAL WINERY & VINEYARD A ging wine and other alcoholic beverages in oak barrels is a com- mon practice in most wine regions of the world. The interactions between wood compounds and wine have been extensively stud- ied. However, information regarding the supply of oxygen into wine in barrels is limited. Oxygen directly or indirectly determines the phenomena experienced by wine during élevage, such as polymerization of tannins and anthocyanins, which improve the stabil- ity and quality of astringency and color, consumption of free sulfur dioxide, oxidation of ethanol to acetaldehyde and general modi- fications of the phenolic wine profiles. These phenomena reflect changes in wine com- pounds and their interactions with wood-ex- tracted compounds. In addition, these changes are often led by oxygen and may be beneficial or harmful. In general, the more controlled these reactions are, the better the results. Therefore, to control this process, it is important to know how much oxygen is entering a barrel of wine, the mecha- nisms that govern this process and the factors that affect oxygen transfer rate. Different theories have developed in the past 100 years regarding how oxygen enters wine barrels. In 1931 J. Ribereau-Gayon con- firmed that O 2 entered the barrel by assessing the formation of SO 4 -2 after filling a barrel with an aqueous SO 2 solution. Ribereau-Gayon found that the oxygen transfer rate (OTR) through the oak wood was limited (2 to 5 ml/L per year) and that the main route of O 2 entry occurred where the wine was in contact with the gas in the headspace of the barrel. The total rate at which oxygen enters wine barrels varies with the time of year from 11.08 to 14.77 mg/L per year (based on 1 mg ≈ 1.35 ml at 15º C) in hermetically sealed barrels and from 1.48 to 3.69 mg/L per year in unsealed barrels. 9 M. Moutounet wrote: "If we examine the results of the studies of Ribereau-Gayon (1931) on the penetration of oxygen, we observe that in a situation of permanent topping-up at at- mospheric pressure, the oxygen transferred is evaluated at between 2 and 5 ml/L per year, whereas in barrels with an airtight bung the oxygen that penetrates is around 15 to 20 ml per year. This data can be explained by the existence of negative pressure inside the barrel, which favors gas entry." R. Peterson worked on this topic and argued that the oxygen present in wine barrels was mainly introduced during barrel filling and racking, which adds approximately 2.8 to 7.0 ml/L per year to the oxygenation capacity of the barrels based on the previous work from Prillinger in full 50-gallon barrels. 6 V. Singleton postulated that O 2 primarily enters the barrel through dry wood. 4 There- fore, wine only would receive oxygen from the air in the headspace at the top of the barrel during élevage. In addition, wine would receive Oxygen Transfer Rate in Oak Barrels Annual evaluation for dynamic oxygen intake and entry By Maria Del Alamo-Sanza and Ignacio Nevares Four American oak barrels were tested 23 times within one year to determine the evolution of oxygen uptake velocity, which demonstrated in a slowdown of the oxygen transfer rate. An airtight bung (inset photo) was created to introduce two optical submersible dissolved-oxygen probes, a gauge for vacuum measurement, inert gas inlet (N 2 ) and gas outlet to evacuate dissolved-oxygen content in a synthetic wine before every test.