How is wine made?

Harvest and crushing

Winemaking starts in the vineyard with the choice of harvest date. Growers track sugar levels, measured in degrees Brix, along with titratable acidity and pH, and pick when the balance suits the style they want. Riper grapes give more potential alcohol; earlier picking keeps freshness. Grapes are picked by hand or by machine and brought to the winery quickly. For red wine, bunches are usually destemmed and lightly crushed so the juice can mix with skins. For white wine, grapes generally go straight to the press, sometimes as whole bunches, so the juice separates from the skins before fermentation. In cool regions where grapes struggle to ripen, local rules may allow chaptalization, adding sugar to the must; about 18 grams of sugar per litre raise the final alcohol by about 1%.

Alcoholic fermentation

Fermentation is the step that turns grape juice, called must, into wine. Yeast, mostly Saccharomyces cerevisiae, consumes sugar and produces ethanol and carbon dioxide; each gram of sugar yields roughly half a gram of alcohol. Some producers rely on the wild yeasts already present on grape skins and in the cellar, while others add cultured strains for a predictable result. Temperature is controlled closely. White wines usually ferment cool, around 15 to 18 °C, to keep fruity aromas, often in stainless steel tanks. Red wines ferment warmer, typically 22 to 25 °C, which helps extract colour and tannin from the skins. Fermentation lasts from several days to a few weeks. If it is stopped before all the sugar is consumed, by chilling or by adding spirit, the wine keeps residual sweetness.

Red, white and rosé: the role of skins

Almost all grape juice is clear, so colour comes from the skins. In red winemaking the juice ferments together with skins and seeds, a process called maceration. Carbon dioxide pushes the skins up into a cap, which winemakers punch down or pump juice over to keep extraction going. After fermentation the wine is drained and the remaining skins are pressed. Grapes such as Cabernet Sauvignon or Nebbiolo give deeply tannic wines this way. Rosé wine is made from red grapes whose skins stay in contact with the juice only long enough to tint it, usually a few hours, before fermentation continues like a white. Another technique, saignée, bleeds off pink juice from a red wine tank. In Beaujolais, carbonic maceration ferments whole uncrushed bunches of Gamay under carbon dioxide for a soft, fruity style.

Malolactic fermentation

After the alcoholic fermentation, many wines go through a second, bacterial conversion. In malolactic fermentation, lactic acid bacteria, chiefly Oenococcus oeni, turn sharp malic acid, the acid of green apples, into softer lactic acid, releasing carbon dioxide. Titratable acidity typically drops by 1 to 3 grams per litre and pH rises by about 0.3. A by-product, diacetyl, can give the buttery note found in some Chardonnay. Nearly all red wines undergo malolactic fermentation. For whites it is a stylistic choice: winemakers often block it in aromatic varieties such as Riesling and Gewurztraminer to preserve crisp acidity and fruit.

Ageing, blending and bottling

Once fermentation ends, the wine is matured, a stage the French call élevage. Stainless steel or concrete keeps fruit flavours fresh, while oak barrels, often 225-litre barriques, allow slow oxygen exchange and can add vanilla, spice and toasty notes, more so when the wood is new. Some white wines rest on their lees, the spent yeast, and are stirred to gain texture. Many wines are blends: Bordeaux châteaux assemble Merlot, Cabernet Sauvignon and Cabernet Franc from separate lots. Before bottling, wine may be fined with agents that remove haze-forming compounds, then filtered. A final dose of sulfites protects it from oxidation and microbes. Bottles are sealed with natural cork, synthetic stoppers or screw caps, which have become increasingly popular.

• Harvest: pick when sugar, acidity and pH are in balance
• Crushing and destemming (reds) or direct pressing (whites)
• Alcoholic fermentation: about 15 to 18 °C for whites, 22 to 25 °C for reds
• Maceration on skins for reds; short skin contact for rosé
• Malolactic fermentation: nearly all reds, some whites
• Ageing in oak, steel or concrete; blending
• Fining, filtration, sulfite addition and bottling

Sparkling and fortified wines in brief

Sparkling wine needs a second fermentation that traps carbon dioxide. In the traditional method used for Champagne, Cava and Franciacorta, a still base wine is bottled with sugar and yeast, ferments again in the bottle, ages on its lees, is riddled so sediment collects in the neck, then disgorged and topped up with a dosage that sets the sweetness. The tank method, patented by Federico Martinotti in 1895 and known as Charmat, runs the second fermentation in pressurised steel tanks and is used for Prosecco and Asti. Fortified wine has distilled spirit, usually grape brandy, added. Adding it during fermentation, as for Port, kills the yeast and leaves sweetness; adding it after fermentation, as for Sherry, gives a dry base. EU rules set fortified wines at 15 to 22% ABV. See Champagne vs Prosecco vs Cava.