This term is used to describe wine faults caused by molecular compounds which contain sulphur. The term “reductive” is often used to describe wines whose aroma has been compromised by sulphur compounds, although this is somewhat of a misnomer: reductive conditions encourage the formation of sulphur compounds, but sulphur compounds can develop in non-reductive conditions, and reduction is not in itself a fault.
There are nearly 100 volatile sulphur compounds that have been identified in wine, but only seven of them are commonly associated with aroma defects. These are hydrogen sulphide, ethane thiol (also known as ethyl mercaptan), methane thiol (also known as methyl mercaptan), diethyl disulphide, dimethyl disulphide, diethyl sulphide, and dimethyl sulphide. These molecules are pungent chemical compounds with aromas reminiscent of struck matches, burnt rubber, rotten cabbage, garlic, onion, rotten egg, skunk, and other more putrid and offensive smells. They are also “chemical chameleons”1 that can morph from one compound into another depending on the chemical environment they are in (e.g. the pH of a wine or its reductive conditions). Thus, a volatile sulphur compound that was below the detection limit in one condition may suddenly become noticeable in another condition as another less pungent volatile sulphur compound morphs into the more pungent form.
These transformations from one sulphur compound to another can occur as the wine is being made, or as it is maturing in bottle. During the winemaking process, volatile sulphur compounds are like time bombs: off aromas arise and then seem to disappear, only to return later as chemical conditions change. The more oxidised forms of these compounds are less aromatic or even non-aromatic, but they can easily morph back into the more aromatic reduced form if conditions in the wine become more reductive.2 In bottle, their emergence can be catalysed by the UV radiation which is present in both artificial and natural light, leading to a sensory fault that is usually categorised on its own as “light strike” but which is nothing more than sulphur taint which has been brought about or exacerbated by light.
At very low concentrations, some volatile sulphur compounds such as dimethyl sulphide can have a positive effect on wine aroma, and recent research has shown that they actually contribute to varietal character in certain wines. Terms commonly used to described the more positive aromas associated with these volatile sulphur compounds include asparagus, cooked corn, cabbage, and molasses. Unfortunately, thiols and disulphides are generally always perceived as detrimental to wine aroma, and the formation of these compounds is considered to be a chronic problem for the wine industry worldwide.2 Other volatile sulphur compounds such as methyl thioacetate and ethyl thioacetate, which have unpleasant aromas, are imperceptible in most wines. Like all esters, however, these thioacetates hydrolyse over time, leading to an acetate and a foul-smelling thiol. Hence, they are considered to act as a reservoir for aroma defects.
- Wine Chemistry and Biochemistry, M. V. Moreno-Arribas and C. Polo, Springer-Verlag New York, 2009, Chapter 8D.
- Evolution of Volatile Sulfur Compounds during Wine Fermentation, M. I. Kinzurik, M. Herbst-Johnstone, R. C. Gardner and B. Fedrizzi, Journal of Agricultural and Food Chemistry, 2015.