How Is Wine Made? From Grape to Bottle

Nearly all wine comes from one species, Vitis vinifera, in thousands of varieties: Cabernet Sauvignon, Merlot, Pinot Noir, Chardonnay, Riesling and the rest are cultivars of it, propagated by cuttings for centuries so that every vine of a variety is genetically the same plant. A wine grape is small, thick-skinned, seedy and much sweeter than a table grape, with about a quarter of its weight as sugar at harvest, and the timing of the harvest is the first decision: picked early the grapes have more acid and less sugar, picked late the reverse, and the sugar sets the alcohol, since yeast turns roughly half the weight of sugar into alcohol.

Where the vine grows matters as much as what it is. Soil, slope, climate and the weather of the particular year, the French idea of terroir, change the balance of sugar, acid, tannin and flavour in the fruit, and a vintage is simply the year the grapes were picked, good or bad according to that summer's weather.

The juice of almost every grape, red or white, is clear. Colour, and much of the flavour and structure of red wine, comes from the skins, so the main fork in winemaking is whether the skins stay in the juice while it ferments:

• •White wine: the grapes are pressed at once and only the juice is fermented, giving a wine without colour or tannin; white grapes are usual but red grapes pressed gently make white wine too, as in champagne
• •Red wine: the grapes are crushed and fermented with their skins for one to three weeks, and the alcohol draws out the pigments, the tannins that give the drying grip on the tongue, and much of the flavour
• •Rosé: red grapes left on their skins for a few hours only, enough to tint the juice, then pressed and fermented as a white
• •Orange wine: white grapes fermented on their skins like a red, an ancient method from Georgia revived in the last twenty years

Yeast, either the wild strains on the skins or a cultured strain added by the winemaker, consumes the sugar and produces alcohol, carbon dioxide and heat, along with hundreds of minor compounds that give wine its aromas. Red wine ferments warm, at up to thirty degrees, to extract the skins; white is kept cool, around fifteen, to preserve fruit aromas. The yeast stops when the sugar runs out, which gives a dry wine, or when the alcohol reaches about fifteen percent and kills it, or when the winemaker chills the tank to leave some sugar for a sweet wine. Most reds and some whites then go through a second, bacterial fermentation that converts sharp malic acid to softer lactic acid, which is why a buttery Chardonnay tastes as it does.

Sparkling wine is made by bottling a finished wine with a little extra sugar and yeast and sealing it, so that the second fermentation happens inside the bottle and the carbon dioxide has nowhere to go. That is the champagne method, and the pressure in the bottle is two to three times that in a car tyre.

After fermentation the wine is separated from its sediment and left to settle and mature, in steel tanks for wines meant to be fresh and fruity, or in oak barrels for wines meant to be complex. Oak lets a little oxygen in through the wood, which softens tannins, and it adds its own flavours, vanilla and spice and toast, especially from new barrels, which is why an expensive red tastes of a forest and a cheap one can be given the same effect with oak chips. The wine is then filtered, perhaps blended from several tanks or grapes, and bottled, with a small dose of sulphur dioxide to stop it spoiling; the sulphites warning on the label refers to that.

Most wine is made to be drunk within a year or two, and does not improve. A few, with high tannin, acid or sugar to preserve them, change in the bottle over decades as the tannins link up and soften, the fruit flavours give way to leather, earth and dried fruit, and the colour of a red shifts from purple to brick. The cork, which lets in a trace of oxygen, has been part of that process since the seventeenth century, and the argument over whether a screw cap does it as well is not settled.

Pottery jars with wine residue from Georgia are eight thousand years old, and the Egyptians, Greeks and Romans each ran an industry with labelled vintages, vineyard rankings and shipping across the whole Mediterranean. Monasteries kept the craft through the Middle Ages, the Cistercians of Burgundy walling off the plots that still carry their names. The nineteenth century brought disaster, when an American root aphid, phylloxera, destroyed nearly every vineyard in Europe, and the cure, grafting European vines onto resistant American roots, means almost every vine in France today stands on American feet. The world now makes about 25 billion litres a year, and the largest producer, since 2000, is Italy or France depending on the harvest, with Spain close behind and the Americas and Australia making the wines that changed how the old countries sold theirs.

Port and sherry are wines to which grape spirit has been added, a practice that grew up partly to help wine survive long sea voyages. The timing of the spirit decides the style. For port, made in the Douro valley of Portugal, the spirit goes in partway through fermentation, raising the alcohol high enough to kill the yeast while plenty of grape sugar remains, so the wine is both strong and sweet. For sherry, from around Jerez in southern Spain, the wine is usually fermented dry first and fortified afterwards, and any sweetness is added later.

Some sweet wines get their sugar in the vineyard instead. In Sauternes and Tokaj, a fungus called noble rot pierces the skins of ripe grapes on damp autumn mornings and lets water evaporate in the dry afternoons, shrivelling the berries and concentrating their sugar and acid. Ice wine, from Germany and Canada, is made from grapes left on the vine until they freeze; pressed while still frozen, they give up only a little intensely sweet juice, because much of the water stays behind as ice. Both methods yield very little wine per vine, which explains the price.

Swirl wine in a glass and drops called legs or tears run down the inside. They are often taken as a sign of quality, but they are physics. Alcohol evaporates faster than water, so the thin film of wine on the glass loses alcohol and ends up with a higher surface tension than the wine below. That difference pulls more liquid up the glass until it gathers into drops that slide back down, an example of what physicists call the Marangoni effect. Strong legs tell you about alcohol and, to some extent, sugar, not whether the wine is any good.

The second myth is that sulphites cause wine headaches. A small number of people, many of them with asthma, are genuinely sensitive to sulphites, but for most drinkers they are an unlikely culprit. Dried apricots and other dried fruit typically contain much more sulphite than a glass of wine, and white wines often carry more than reds, yet red wine is the one most blamed for headaches. Researchers have looked at histamine, tannins and other compounds in red wine, as well as the simple effects of alcohol, and the cause is still not settled.

Young wine is often hazy with proteins, yeast cells and harsh tannins, and as well as filtering it, winemakers clear it by fining: adding a substance that binds to the unwanted particles and sinks with them, after which the clear wine is drawn off. The traditional agents are surprising. Whipped egg whites are used to soften the tannins of fine reds, and isinglass, made from fish swim bladders, milk protein and gelatin have all been widely used, alongside a clay called bentonite and plant proteins. Hardly any of the agent remains in the bottle, but because animal products touched the wine, some wines are not vegan.

A different deposit alarms people more. Small, hard, glittering crystals sometimes appear in a bottle or on the underside of a cork, especially in white wine kept in a cold fridge, and they are often mistaken for broken glass. They are potassium bitartrate, a harmless natural salt of the tartaric acid in grapes and the same substance sold for baking as cream of tartar. It dissolves less well in cold liquid, so it can come out of solution after bottling. Many producers prevent this by chilling the wine close to freezing before bottling, so the crystals form in the tank instead.

Winemakers judge ripeness by measuring sugar, often with a refractometer, which reads how strongly a drop of juice bends light. Suppose the juice holds about 240 grams of sugar per litre, typical for ripe wine grapes. Yeast turns a little over half the sugar's weight into alcohol, so complete fermentation could give about 122 grams of ethanol per litre. Ethanol weighs about 0.79 grams per millilitre, making roughly 155 millilitres, or 15.5 percent by volume. In practice some sugar feeds the yeast's growth and by-products, so winemakers use a rule of thumb of about 17 grams per litre for each percentage point, which predicts about 14 percent.

During fermentation the winemaker follows progress with a hydrometer, a weighted float that rides higher in denser liquid. Sugary juice has a specific gravity around 1.09 or 1.10, so the float sits high. As the yeast eats the sugar the density falls, and because alcohol is lighter than water, a fully dry wine ends slightly below 1.000, often around 0.990 to 0.995. A reading that stops falling while still well above that means the fermentation has stuck, perhaps because the tank got too hot or too cold or the yeast ran short of nutrients, and the winemaker must act before spoilage microbes exploit the leftover sugar.

Wine is grape juice fermented by yeast into alcohol, and its character is set by the grape variety, the place and year it grew, and the winemaker's choices: fermenting with the skins for red or without for white, warm or cool, dry or sweet, in steel for freshness or oak for complexity. A second fermentation in the bottle makes it sparkle, sulphur keeps it sound, and a few wines built on tannin and acid go on changing in the bottle for decades.