Fermentation

Fermentation is the single process that separates wine from grape juice. Yeast consumes the sugar in the juice and produces ethanol and carbon dioxide, together with heat and several hundred other compounds. Everything else a winemaker does — pressing, ageing, blending, filtering — arranges the conditions around this one reaction or tidies up after it.

The conversion is close to a fixed ratio, which is why the sugar in the grapes at picking effectively decides the alcohol in the bottle. Roughly 17 grams of sugar per litre yields about 1% alcohol by volume. That single relationship explains a great deal: why warm regions produce stronger wine, why a hot vintage is an alcohol problem before it is anything else, and why the picking date is the most consequential decision of the year.

It is exothermic. A fermenting tank of red wine generates real heat, and left alone it will climb high enough to kill the yeast and stop itself. Temperature control — cooling jackets, cold rooms, or simply a cellar — is not a refinement but a requirement, and it is one of the main things that changed when refrigeration reached the wine industry in the mid-twentieth century.

Temperature is also the winemaker's most powerful stylistic lever. Cool fermentation, around 12–16 °C, preserves delicate fruit and floral aromatics and is standard for aromatic whites; the fresh, pungent character of commercial Sauvignon Blanc depends on it. Warmer fermentation, 25–30 °C, extracts more colour and tannin from red skins and produces a different and generally less overtly fruity set of aromas.

The yeast can be added or arrived with. Cultured yeast — usually a selected strain of Saccharomyces cerevisiae — ferments predictably to a known finish. Ambient or "wild" fermentation relies on the mixed population already on the fruit and in the cellar, starts more slowly, and is more likely to stick. Neither is better in principle; the trade is reliability against the chance of something more distinctive.

Red and white differ in one structural respect. White wine is normally pressed first and fermented as clear juice; red wine is fermented on its skins, because colour and tannin live in the skins and only alcohol will extract them properly. That is the whole reason red wine is red, and it is why a red wine made from the same grapes without skin contact comes out pale.

The vessel matters less than it is usually credited with, and it does matter. Stainless steel is neutral and easy to keep cold. Oak is porous, insulating, and contributes flavour and a slow trickle of oxygen. Concrete is thermally stable and neutral. Amphorae and qvevri are neutral and porous. These change texture and the pace of the ferment more than they change what the yeast is doing.

A fermentation that stops before the sugar is gone is called stuck, and it is a genuine problem: a wine with residual sugar and no protective alcohol or carbon dioxide is exposed to spoilage organisms. Common causes are excessive heat, a nutrient shortage in the juice, or very high initial sugar. Restarting one is difficult and rarely leaves the wine unmarked.

Stopping it deliberately is how most sweet wine is made. Chilling drops the yeast out of suspension, filtration removes it, and fortification — adding grape spirit — raises the alcohol past what the yeast can survive. Port, Sherry and the vins doux naturels all use the third method, which is why they are both sweet and strong.

Two further fermentations are worth keeping distinct from this one. Malolactic conversion is not a fermentation by yeast at all: bacteria convert sharp malic acid into softer lactic acid, lowering acidity and sometimes producing the buttery note associated with oaked Chardonnay. And the bubbles in traditional-method sparkling wine come from a second alcoholic fermentation, deliberately induced inside a sealed bottle so the carbon dioxide has nowhere to go.