Reduction: sulfur compounds and struck-match aromas

Reduction became far more visible as a topic when screwcaps became widespread, since a low-transmission closure leaves the wine less opportunity to resolve it in bottle.

The vocabulary is unhelpfully overloaded: "reductive winemaking" is a deliberate low-oxygen technique used to preserve aromatics, while "a reduced wine" is one showing an off-aroma. The two are related but not the same.

Hydrogen sulfide is produced by yeast, particularly when nitrogen nutrition is inadequate. It has an extremely low sensory threshold and smells of rotten eggs.

Hydrogen sulfide can react further to form mercaptans (thiols such as methanethiol and ethanethiol) and disulfides. These have their own characters — cooked cabbage, drains, rubber — and are considerably harder to remove.

The distinction matters practically: hydrogen sulfide often blows off with aeration, while the heavier compounds frequently do not, which is why decanting sometimes fixes the problem and sometimes does not.

At very low concentrations, some sulfur compounds contribute characters many tasters value — the flinty, struck-match note associated with some fine white Burgundy and its imitators. Whether a given example is a fault or a feature is a judgement about concentration and about style.

Copper binds sulfides and has long been used to treat them, though it carries its own consequences for the wine’s later development.

For a drinker, the practical upshot is worth knowing: a wine that smells of struck match or drains on opening may well clear with vigorous decanting, and it is worth trying before writing the bottle off.

It also explains why some producers deliberately choose closures with slightly higher oxygen transmission for wines they know run reductive.