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A Technical Guide for Wine Producers
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Off-odours in Shiraz
Sulphur compounds with a low molecular weight are responsible for many undesirable flavours such as rotten egg and a burnt character, often referred to as reductive flavours. These are usually formed by yeasts and even in trace quantities they have a considerable influence on wine flavours. The reductive flavours occurring in wines are summarised in Table 1.
These unpleasant compounds and their precursors are formed mainly by yeasts during the alcoholic fermentation. Correct fermentation management is therefore extremely important to prevent such reductive flavours. The reductive characters in wines are also said to become significant after the alcoholic fermentation if sulphur compounds with a high flavour threshold value can be converted to ones with a lower flavour threshold.
In order to prevent or remove the flavour problems, it is important to be able to analyse the flavour compounds and their precursors. The methods that are most commonly used to remove the off-odours are fining with copper sulphate, the addition of ascorbic acid, sulphur dioxide and copper sulphate, or the sparging of wine with nitrogen. The addition of copper sulphate is the most common, but may unfortunately also remove thioles, which are associated with cultivar characteristics. High copper values in wines may also result in copper turbidity in the wines. Since copper sulphate does not remove all layers of molecular weight sulphur compounds, repeated occurrence of off-odours is possible after treating the wine with copper sulphate. Bottle closures with a low oxygen permeability will also promote the occurrence of reductive flavours in wines (O’Brien and Colby, 2008).
Shiraz wines often display reductive off-flavours and it is therefore important to pay attention to the oxygen management of such wines. Grape flavour compounds consist mainly of two groups, i.e. free flavourants such as terpenes, phenols, aldehydes, norisprenoids, esters, higher alcohols, pyrazine or flavour precursors that may represent a broad range of flavours. Shiraz has few free flavour compounds, but contains several flavour precursors. Shiraz is associated with black pepper flavours, among others. On the whole, however, reductive flavours are associated with the low molecular sulphur compounds. One that often occurs in Shiraz wines is dimethyl sulphide. Its precursor, S-methyl methyonin, has also been shown to occur in Shiraz juice. In high concentrations dimethyl sulphide may impart an undesirable cabbage flavour, while lower concentrations may contribute to the flavour complexity of Shiraz.
Oxygen plays a basic role in vinification and any process that occurs simultaneously will influence the oxygen quality of the product, which will then influence the flavour, colour and taste of the wine. Provision has to be made for the requisite oxygen during the various vinification phases to ensure optimal expression of the Shiraz character. Although flavour precursors constitute the biggest part of the Shiraz flavour pool, oxygen management thereof will be important, but unfortunately too little is known about the extent to which such flavour precursor concentrations are influenced by different oxygen concentrations in the wine. As Shiraz wines tend to form reductive sulphur compounds, these may be influenced by oxygen. Sulphur compounds are sensitive to oxygen, which implies that if wine is exposed to oxygen, the concentration of the sulphur compounds will be reduced. It is therefore important for Shiraz wines to be exposed to a moderate supply of oxygen on an ongoing basis. It is important to remember, however, that the interaction between tannins and oxygen may result in softer tannins and increased bitterness. It goes without saying, therefore, that the exposure of Shiraz wines to oxygen should be managed in such a way to prevent any negative consequences.
Shiraz wines may even develop reductive characteristics after bottling. Hermetic bottle closures should therefore not be used (Vidal and Aagaard, 2008).
References
O’Brien, Vince & Colby, Chris. 2008. Wine faults caused by reductive characters: Low molecular weight sulfur compounds.
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