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A Technical Guide for Wine Producers
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Reductive vinification
It goes without saying that oxygen, being one of the most important natural atmospheric gases, plays a very significant role in vinification. This role could be either beneficial or detrimental, depending on the stage or process of vinification. The winemaker decides whether an oxidative or reductive approach is to be followed. The oxidative approach allows normal exposure to oxygen, while the reductive approach tries to prevent or eliminate oxygen contact. However, it is also possible to adopt a golden mean or compromise using a different strategy at various stages. The decisions taken by the winemaker, whatever they may be, will have a marked influence on the style and quality of the wine.
The trend to produce fruity wines induced winemakers to adopt extreme approaches in various respects. Grapes must be picked at a higher sugar content, inter alia, obviously resulting in a higher alcohol content. Despite their preference for fruitiness, consumers may object to the higher alcohol content. The higher alcohol content of wines could also cause problems with alcoholic or malolactic fermentation. Lower fermentation temperatures will result in the retention of more flavour in wine, but on the other hand complexity will be enhanced by higher fermentation temperatures. The use of different skin contact practices or different wood products will likewise result in different flavour profiles. The required balance between the different extremes will therefore have to be maintained in order to obtain the desired results. From one point of view, reductive vinification is an extreme approach.
Traditionally the vinification process has always been oxidative. The arrival of stainless steel tanks, the availability of inert gas, cooling, dry ice and other resources enabled winemakers to implement reductive vinification. The extent to which reductive vinification is applied, may be determined quantitatively by measuring the redox potential of the juice or wine. Although most wineries do not use redox potential measurements, it is common knowledge that practices such as racking, topping up, filtration and bottling cause the uptake of oxygen to a certain extent, which will increase the redox potential of the product. On the other hand the use of anti-oxidants such as sulphur dioxide, ascorbic acid and inert gases may reduce the redox potential of the product. Under controlled conditions oxygen can be beneficial, however. Macro-oxygenation is especially effective during alcoholic fermentation and also improves the colour stability and clarity of the wine. The tannins of red wines are also softened by controlled contact with oxygen.
While reductive vinification is a novelty in the wine industry, reductive processes have traditionally been noticeable in various wines. The bread or yeasty character of bottle fermented sparkling wine and the butteriness of Chardonnay that has been exposed to lees contact are typical examples, but in the present context reductive vinification is also linked to the pursuit of wines with more fruitiness. The presence of oxygen is limited, as far as possible, from the vineyard to the bottle; the result being fruit driven white and red wines. The use of cool cultivation areas, inert gas, low fermentation temperatures and specially selected yeast strains in particular has resulted in excessively fruity wines from New Zealand and Australia.
As with other extreme practices mentioned above, reductive vinification can also have detrimental consequences. A lack of oxygen in wine may result in the development of reduced sulphur compounds. Hydrogen sulphide (rotten egg smell), mercaptans or tiols (cabbage or burnt rubber smells) and disulphides (boiled wheat or canned tomato flavours) are the best known sulphide compounds. A consumer who expects a spicy Gewürztraminer or grassy Sauvignon blanc will definitely not be impressed by the above-mentioned sulphur compounds when opening the bottle. During the vinification process sulphides are usually first observed during alcoholic fermentation. This is generally the result of a nitrogen deficit in the juice which may be prevented by the addition of yeast nutrients such as diammonium phosphate, yeast cell walls and ammonium salts. The use of sulphur powder in vineyards is another possible source of hydrogen sulphide in wines. It is common practice in wineries to aerate wine if hydrogen sulphide is detected, but this is merely an apparent solution seeing that the mercaptans causing the smell are converted to disulphides, that are later split by free sulphur dioxide only to be detected once again as an off-odour. The best solution if hydrogen sulphide is detected, is to treat it immediately with copper instead, thereby removing the sulphur compounds. Preventative practices such as aeration during fermentation, the addition of sufficient yeast nutrients and regular sensorial evaluation of the lees are still the best option.
One suspects reductively vinified wines that are bottled using metal closures to be more prone to the potential formation of undesirable sulphur compounds. It is speculated that the free sulphur dioxide content in such bottles is likely to remain higher than in bottles that have been sealed with synthetic or natural corks. The free sulphur dioxide may then break up the disulphides occurring in the wine, releasing them as off-odours.
Winemakers should decide therefore to which extent reductive vinification is to be applied, while being aware of the possible consequences of their decision(s).
Reference:
Cutler, L. 2006. Achieving Balance in Reductive Winemaking. Wine Business Monthly. October 2006: 24 - 27.
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