Screwcaps and Sulphur Taint
Recommended: Read about the Formation of Volatile Sulphur Compounds first.
The reduction of disulphides to the more offensive and persistent mercaptans by reaction with sulphites is of growing interest to winemakers, especially when it happens after bottling. At wine’s pH, conditions are not acidic enough for these transformations to occur rapidly, but they will eventually occur. This is of particular concern for winemakers who choose to seal their wines with screwcaps. Screwcaps are easy to use and they reduce the risk of cork taint, but they are also particularly good at keeping oxygen out — oxygen which might otherwise help reverse and arrest the process of mercaptan formation. The problem of “screwcap reduction” is being called the new cork taint, and comparative tastings suggest that the proportion of wines sealed with screw caps which are affected by sulphur taint is as high as the proportion of wines sealed with cork which are affected by cork taint.
Some winemakers believe this is essentially a winemaking issue, and to some extent they are right. The precursors necessary for “screwcap reduction” to develop are formed before the wine is bottled, and better winemaking can help to minimise their formation, for instance by providing optimum nutrient conditions for the yeast during fermentation. The current fashion for overzealous anaerobic winemaking, in which oxygen is avoided at all costs and ample use of SO2 is made, only contributes to the problem. But the production of volatile sulphur compounds prior to bottling cannot be eliminated completely. It would be impossible to prevent the fermentation from producing any complex sulphides at all.
- Kinetics of the ethanethiol and diethyl disulphide interconversion in wine-like solutions, R. A. Bobet, A. C. Noble and R. B. Boulton, Journal of Agricultural and Food Chemistry, 1990.