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A Technical Guide for Wine Producers
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WHAT'S UP IN THE VINEYARD AND CELLAR
The practical management of hydrogen sulphide
The occurrence of any off-odour in wine is obviously unacceptable to winemakers and a source of concern. Mouldy character is probably the most prevalent off-odour and its causes are fairly well-known. Sulphur compounds or the association thereof with the so-called reductive off-odours that occur in wines, are probably the second most common problem.
Hydrogen sulphide (H2S) is widespread in fermentations in all cellars, regardless of vintage, type of wine or country, and although certain associations exist between the formation of H2S and certain vineyards as well as yeast strains, not enough is known about it to accurately predict its occurrence during fermentation. It may occur at any stage during fermentation, although it is mostly prevalent during the early phases, less so during the later stages and sometimes throughout fermentation. Wines with an H2S character are usually treated effectively with copper sulphate to remove the reductive off-odours. Although copper sulphate effectively removes H2S and mercaptans, it does not remove disulphides. This represents a latent form of sulphides in wine which may become noticeable again under highly reductive conditions or after the wine has been packaged. Disulphides may also be formed if aeration is used to reduce H2S. This applies when mercaptans occur in the wine in conjunction with H2S. A disadvantage of using copper sulphate is the reaction of the Cu ion with other wine compounds which may result in flavour losses or less pronounced cultivar character. The Cu ion, for example, is able to react with the fruity long chain polyfunctional thiols characteristic of Sauvignon blanc and related cultivars. The consequences of H2S during vinification are not always fully understood. Some winemakers are not overly phased by the early formation of H2S, as it is usually removed by the mobile carbon dioxide which occurs mainly in the early stages of fermentation. During the later phases of fermentation it will not occur to the same extent.
There are several causes for the formation of H2S and ways to prevent or treat it. Various agrochemicals contain elementary sulphur (S) which may be reduced to H2S under reductive conditions. If such products have to be used, the withholding period should be followed strictly and rackings of the lees should occur as quickly as possible. Metal ions such as zinc and copper may also occur in such products, which may result in the formation of H2S through chemical reactions or yeast metabolism. The late application of such products should therefore be avoided and copper finings should preferably not be conducted during fermentation. Metals occurring in equipment that is not made of stainless steel may also play a role in the formation of H2S. New stainless steel tanks have to be rinsed with citric acid or tartaric acid before using it for the first time, in order to remove the manganic sulphide which occurs on the surface. It is generally accepted that the yeast nutrition status of juice is able to play an important role in the formation of H2S. The exhaustion of YAN (Yeast Assimilable Nitrogen) is considered to be a common cause of H2S. The nitrogen demand is high during the yeast growth which occurs at the beginning of fermentation and if there is a YAN deficiency, yeasts have to conduct other metabolisms thereby resulting in the formation of H2S. In many instances this may be prevented by using diammonium phosphate (DAP) or specialist yeast nutrition products containing vitamins and inactivated yeast supplements. Sometimes the problem is not caused by a nitrogen deficiency, however, in which case DAP supplements will not solve the problem and the latter type of products must be used. Yeast strains also differ in respect of the extent and stage of H2S formation and of all the cultivars, Shiraz is the one with the most H2S problems. The role of wild yeasts in the formation of H2S is not clear. The traditional view was that such yeasts were an important cause thereof, but the increasing interest in the use of "natural fermentations" belie this belief. The management of H2S formation in such fermentations is consequently the same as for inoculated fermentations.
Current research suggests the stage during fermentation when H2S formation occurs, to be a very important determining factor with regard to the occurrence of residual H2S in the final wine. If formation ceases before the middle of fermentation, no H2S is observed in the final wine, but if it is formed by yeasts during the second half of fermentation, significant H2S is encountered in the final wine.
As yet there is no accurate prediction whether H2S will occur during fermentation. This is mainly because various factors come into play and different combinations of these may also occur. There are, however, various quick analytical methods to determine H2S, which facilitates monitoring during fermentation. At present YAN analyses are still considered the most reliable problem indicator and values above 250 mg/l are deemed safe for potential H2S formation. The choice of yeast strain remains one of the most important preventive tools for this problem.
Reference
Practical management of hydrogen sulphide during fermentation - an updated overview according to recent studies on red wine fermentation (AWRI publication #1121). Technical Review 180, June 2009.
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