Wine In Wood And Wood In Wine: Micro-oxygenation Part 1

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WINE IN WOOD AND WOOD IN WINE
Micro-oxygenation Part 1
by Charl Theron, guest lecturer in Oenology, Tim Rypstra, professor in Wood Science and Jan Swart, director: Wood and Fibre Institute, Stellenbosch University
The replacement of wooden barrels with alternative wood products, regardless of which kind, all have the same shortcoming, namely that unlike barrels, they do not make provision for slow oxygen uptake and consequently only enable the extraction of wood character. Micro-oxygenation addresses this shortcoming and will be discussed in a series of articles.
The following three aspects relevant to micro-oxygenation were studied in a research project in New Zealand:

The chemical and sensorial changes that take place during micro-oxygenation of wine at various dosages.
The influence of sulphur dioxide on tannin changes during micro-oxygenation of wine.
A better understanding of the micro-oxygenation process with regard to the influence of lees on oxygen uptake of wine and the relationship between oxygen particle size and the effectiveness of oxygenation.
Micro-oxygenation may be defined as the deliberate continuous, measured oxygen dosage of wine at various stages after alcohol fermentation until bottling of the wine. It was formally developed in France in 1995. The primary purpose of micro-oxygenation is the improvement of the sensorial qualities in a particular wine. Suppliers of the technology contend that it is able to soften the taste of the wine, improve the wine’s colour, reduce the so-called green vegetative character of wine, remove reductive character from a wine and improve the mouthfeel of white wines that are left on the lees. Practice has shown that the sensorial development of wine goes through various stages during micro-oxygenation. The first stage is usually before the addition of sulphur dioxide and lasts from 3 days to 6 weeks with oxygen additions of 20 - 90 mg oxygen/litre wine/month. It is characterised by an improvement in the tannin structure of the wine. It usually goes hand in hand with a decrease in the yeast and cultivar aromas of the wine and the impression is created that the wine’s quality has in fact deteriorated. The second stage is where the tannin structure of the wine reaches a peak and softens, while the cultivar aromas get stronger. The wine structure eventually decreases to a point where micro-oxygenation has to be stopped. Dosage during this phase which may last 3 weeks to 6 months, depending on the wine, is 1 - 10 mg oxygen/litre wine/month. The third phase, which should in fact be prevented, represents excessive treatment of the wine, which may result in the so-called dry tannins and oxidative character. Winemakers should take care to prevent this stage.
For the successful application of micro-oxygenation it is important that the tempo and dosage of oxygen application do not allow the accumulated oxygen concentration in the wine to increase. Should this be the case, it may be detrimental to the quality and long term potential of the wine. Depending on the type of wine and stage of treatment it may range from 1 - 90 mg/litre wine/month (refer to the original article for a table with detailed particulars).
It is generally accepted that wine polyphenols are the most important substrate for oxidation. Current research focuses on the nature of the tannin-anthocyanin interactions and the influence on the sensorial qualities of the wine.
Micro-oxygenation differs from normal aeration practices such as racking or aeration in that the dosage may be measured and controlled. Two methods are currently being used commercially for micro-oxygenation. Most common is the use of a micro-porous diffuser placed near the bottom in the middle of the tank. Oxygen bubbles moving upwards from below are dissolved in the wine. The dissolution of the oxygen is determined by the bubble size and density of the wine surface through which it moves. The use of high density polymer membranes as an alternative method has the advantage that bubbles do not have to be formed for the oxygen to dissolve in the wine. Large containers are consequently not necessary for dosage and wine flavourants cannot become volatile due to oxygen bubbles.
The next article looks at the influence of micro-oxygenation on wine composition and the influence of sulphur dioxide on micro-oxygenation.
Reference
Dykes, Stuart & Kilmartin, Paul. 2007. Micro-oxygenation – optimising the maturation process. Wine Industry Journal 22(5): 31 - 44.
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