Today I’m going to share 4 faults in wine that fall under the “reductive” category. In essence, this is the opposite effect of oxidation and can produce stale smelling wine. If extreme, reduction can cause some pretty foul odors – just-boiled eggs, anybody?
1. Sulphur (SO2). The smell of gassy hot springs is not exactly what you want in your glass of wine. Excess sulphur’s not always easy to detect (and it often blows off with a little time), but in some cases it can lead to heachaches or migraines. Maximum doses are set by law in each wine-producing country. Sulphur gets a bad rap, but keep in mind that it does some important winemaking related things: it inhibits the development of wild yeasts and undesirable micro-organisms, it helps the winemaker control fermentation, it cleanses the wine (and the vessels that hold the wine) and it assists aging wine by helping to prevent refermentation in the bottle and by helping to prevent discoloration through oxidation, especially in white wines. Winemakers have to be sensitive to pH levels, temperature and storage conditions to strike the right sulphur balance. I touched on the use of sulphites in an earlier post.
2. Rotten egg (H2S). When an egg rots, its proteins start to give off hydrogen suphide (H2S). The very same gas can form in wine when surplus sulphur dioxide is reduced by yeasts during alcoholic fermentation. This aroma can arise due to adding too much SO2 to the wine, to certain insecticides or fungicides used to treat the vine or to the grape itself – sulphuric amino acids are especially common in Riesling, Chardonnay and Syrah.
3. Onion (ethanethiol or mercaptan). Thiols can be good or bad. Good ones smell of grapefruit or blackcurrant. Bad ones smell like an angry skunk (3-methylbutan-1-thiol), stagnant water or halitosis (methanethiol). When too much H2S reacts with the ethanol (alcohol) in the wine, it can give an “eau de onion”, often with a hint of rubber. The appearance of thiols is related to the wine being handled in a reductive manner. Racking the wine (tranferring it to another container and exposing it to air) can often eliminate these odors.
4. Cauliflower (dimethyl sulphide). The cauliflower aroma will appear at the most advanced stage of reduction and just a little of dimethyl sulphide goes a long way – it takes just 0.33 mg/l for tasters to detect it. Its precursors are the onion smell and the stagnant water smell and these mercaptans may react among themselves to form disulphurs or trisulphurs, becoming more pungent and gross. The aroma could also arise from wine that was badly racked.
Check back for phenols and fungi.