How sparkling wine gets its bubbles

Bubbles are trapped carbon dioxide

When yeast ferments sugar it makes alcohol and carbon dioxide. In a still wine the gas escapes into the air. In a sparkling wine the winemaker traps it, so it dissolves into the wine under pressure and comes out as bubbles when the bottle is opened.

The usual route is a second fermentation. The winemaker makes a dry base wine first, then adds a measured dose of sugar and yeast and ferments it again in a closed vessel. A useful rule of thumb is that about 4 grams of sugar per litre produces roughly one atmosphere of pressure. A dose of around 24 grams per litre gives the five to six atmospheres typical of Champagne. Our guide to how wine is made covers the first fermentation.

The traditional method

In the traditional method, the one used for Champagne, the second fermentation happens inside the bottle you eventually buy. The base wine is bottled with sugar and yeast, sealed with a crown cap and laid down in a cool cellar.

After fermentation the dead yeast, called lees, stays in the bottle. The wine rests on it for months or years and picks up bready, biscuity flavours. Champagne rules require non-vintage wines to spend at least 15 months in bottle before release, at least 12 of them on the lees. Vintage Champagne needs at least three years.

The yeast then has to come out. Bottles are slowly turned and tilted neck down so the sediment collects in the neck, a step called riddling. The neck is frozen, the cap is removed and the pressure pushes out a plug of frozen yeast. This is disgorging. A small top-up of wine and sugar, the dosage, sets the final sweetness before the cork goes in.

The tank method

Prosecco and many other fruity sparkling wines are made by the tank method, sometimes called the Charmat method. The second fermentation happens in a large sealed pressure tank. The wine is then filtered and bottled under pressure.

It is faster and cheaper than the traditional method. It also keeps fresh fruit and floral aromas to the fore because there is little or no time on the lees. Neither method is better. They make different wines for different prices and occasions.

Other routes

• Transfer method. Fermented in bottle like the traditional method, then emptied into a tank to remove the yeast and rebottled. Common for large formats.
• Ancestral method. The wine is bottled before the first fermentation finishes, so it carries on in the bottle. Often sold as pet-nat and often slightly cloudy.
• Carbonation. Carbon dioxide is injected directly, as with soft drinks. Quick and cheap, with larger bubbles that fade faster.

Reading sweetness on the label

Sweetness in sparkling wine is set by the dosage and named with standard terms. Brut nature has little or no added sugar. Extra brut is very dry. Brut, the most common, has less than 12 grams of sugar per litre. Confusingly, extra dry is slightly sweeter than brut. Sec and demi-sec are sweeter again.

These terms matter to anyone writing product copy or handling complaints. A customer who expected a dry wine and bought extra dry may well have a point.

Serving and the bubble

Serve sparkling wine well chilled, around 6 to 10 °C. Cold wine holds its carbon dioxide better, so it pours with less foam and keeps its fizz longer. Open it by holding the cork and turning the bottle, letting the gas sigh out rather than pop. A flying cork under that pressure can injure someone, which matters at any office celebration.

Glass shape changes how the bubbles behave. Narrow flutes keep the stream of bubbles going and look festive. Many winemakers now prefer a tulip-shaped wine glass, which keeps the bubbles but gives more room for aroma. Traditional method wines tend to form finer, more persistent bubbles than carbonated ones, which is one way tasters tell them apart.

Sparkling wine in India

Indian wineries in Maharashtra and Karnataka make sparkling wine, some by the traditional method and some in tanks. Our guide to Indian wine regions describes where they grow.

Heat is the main practical risk. Pressure inside a bottle rises as it warms. Warm storage ages the wine quickly. A case of sparkling wine left in a hot vehicle or a sunny shop window loses its freshness well before the date anyone planned for. Sparkling wine belongs in the cool part of the cold chain, alongside beer, not with spirits on an open shelf.

Common questions

Where do the bubbles in sparkling wine come from?

From carbon dioxide made by yeast during a second fermentation in a sealed bottle or tank. The gas dissolves under pressure and is released when the bottle is opened.

What is the difference between Champagne and Prosecco methods?

Champagne ferments a second time in the bottle and rests on the yeast. Prosecco usually ferments a second time in a pressure tank, which keeps it fruitier and fresher.

Is extra dry sparkling wine drier than brut?

No. Extra dry is slightly sweeter than brut. Brut nature and extra brut are the driest terms.