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A Technical Guide for Wine Producers
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Oxygen ingress in bottled wines
A wine that is bottled with different closures and filling heights develops into different wines that also age differently. An important cause of this phenomenon is that different bottle closures have varying oxygen permeability. The consequences for wine quality may surpass the influence of different viticultural and oenological practices.
Wine is exposed to oxygen at various stages in the course of the vinification process. The stages with the greatest possibility of oxygen exposure are during pumping over procedures, filtration and bottling of the wine. During the bottling process oxygen ingress may occur in the wine bin of the filling machine, or from the air in the fill space, or it may derive from the bottle closure. This article focuses in particular on the role of the bottle closure during the closing process of the bottle and afterwards. If a vacuum can be created in the fill space before the bottle is closed, the air may be expected to move into the fill space after the closing process; if the closure does not expand rapidly enough, or with compression of the closure, oxygen will move from it in various directions and especially to the lower pressure in the fill space.
After bottling and during storage of wine the most important source of exposure to oxygen occurs as a result of air moving through the bottle closure to the fill space. The Oxygen Transmission Rate (OTR) depends on the inherent oxygen permeability of the matter that the closure is made of, on the contact surface and length of the closure, as well as on the difference in oxygen concentration inside and outside the bottle. The OTR of long corks will obviously be lower than that of short corks as a result of the distance through which the oxygen has to diffuse, provided the cork is in close contact with the inner surface of the bottle over its entire length. The OTR values of bottle closures are usually expressed as ml/annum or mg/annum, but should actually be expressed as ml/bottles/annum and the closure and bottle neck details should also be specified. If OTR values are expressed per litre, for example, the value must be adjusted for different bottle sizes. For example, a value of 2ml/closure/annum is 2.7ml/l/annum for a 750ml bottle and 5.3ml/l/annum for a 375ml bottle.
The question arises how the OTR value of a bottle closure, if known, may be used. According to previous research oxidative characters will develop in a white wine if the free sulphur dioxide quality of the wine drops below 10mg/l. Theoretically, 1mg oxygen is able to react with 4mg sulphur dioxide. If one presupposes that the main reaction of the dissolved oxygen in the wine is with sulphur dioxide and that the free and total sulphur dioxide in a wine are not in equilibrium with each other, the following calculation may be made and conclusion reached:
Suppose a white wine with a low phenol content contains 30mg/l free sulphur dioxide after bottling with a bottle closure with an OTR value of 2mg/annum/closure, then:
Up to now it has been very complicated to determine the OTR value of bottle closures. A chemical dye has been developed, however, that will indicate the extent to which oxygen penetrates the bottle by means of colour changes inside the bottle. This method has considerable potential for application in the wine industry (Skouroumounis & Waters, 2007).
Reference
George Skouroumounis & Liz Waters. 2007. Oxygen ingress into bottled wines. AWRI Technical Review no 170: 13 - 19.
The authors may be contacted via the following e-mail addresses:
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