Cap management

Left alone, a red fermentation separates into liquid below and a compacted raft of skins above. Nothing useful happens at that interface, and the exposed cap dries out and becomes a home for spoilage organisms.

So the cap is worked, several times a day, for the duration of fermentation. How vigorously and how often is one of the most consequential stylistic decisions in red winemaking, and it is entirely invisible from the label.

Working the cap serves four purposes at once: it extracts colour, tannin and flavour from the skins; it releases heat from the cap into the liquid; it introduces oxygen, which yeast need; and it keeps the cap wet, which limits spoilage.

Those purposes can conflict. More extraction may be wanted and more oxygen may not, which is why the technique is chosen rather than the frequency simply increased.

Extraction is not linear over time. Anthocyanins are extracted quickly and early, largely within the first few days; tannins extract more slowly and continue throughout and beyond fermentation. So the same amount of work early and late in a ferment produces different wines.

Rising alcohol also changes what is extracted, because ethanol is a better solvent for the larger phenolic compounds than water is. Late-fermentation working therefore favours tannin over colour.

It explains why two producers with fruit from the same vineyard reliably make different wines, and why a Burgundian approach applied to Bordeaux fruit would give something unlike either.