What is refermentation in bottled wine?

Quick answer

Refermentation in bottled wine is a second alcoholic fermentation that takes place after the wine’s first fermentation, sometimes inside a sealed bottle.

During fermentation, yeast consumes fermentable sugar and produces alcohol and carbon dioxide. In traditional-method sparkling wine, this second fermentation is intentional. Sugar and yeast are added to a bottled base wine, the bottle is sealed, and the carbon dioxide dissolves into the wine, creating its bubbles.

In a still wine, fermentation restarting after bottling is generally unintended. It can produce unexpected spritz, extra pressure, haze, sediment, leakage or changes in aroma and flavour. These signs are only clues, however. Bubbles alone do not prove that a bottle has refermented.

When refermentation is intentional

Traditional-method sparkling wine undergoes its second fermentation in the bottle. This is sometimes called secondary fermentation or bottle fermentation.

The wine begins as a still base wine. The winemaker adds fermentable sugar and yeast before sealing it in a bottle. The yeast then produces more alcohol and carbon dioxide. Because the bottle is sealed, much of that carbon dioxide remains dissolved in the wine rather than escaping.

This is one reason sparkling wine can have both bubbles and yeast sediment. In traditional-method production, time spent in contact with yeast lees—the settled yeast cells left after fermentation—is an intended part of the process.

Not every sparkling wine is made this way. In tank-method sparkling wine, the second fermentation occurs in a sealed tank before bottling. Carbonated wine has carbon dioxide added directly and has not undergone refermentation. Pét-nat is also different: it is bottled before fermentation has completely finished, allowing the remaining fermentation to create its bubbles.

When it is unintended

A still wine can sometimes begin fermenting again after bottling if fermentable sugar and viable yeast are both present. The risk depends on several conditions, including the wine’s composition, yeast activity and how the wine was stabilised or filtered.

Possible signs include:

• a light fizz or spritz in a wine intended to be still;
• unusual pressure when the closure is disturbed;
• a pushed-up cork or leakage;
• cloudiness or haze;
• yeast sediment or another deposit;
• changed aromas, flavour or texture.

These observations are not a diagnosis. Sediment can have other causes, and some wines contain dissolved carbon dioxide without active refermentation. The best starting point is to consider the wine’s intended style, label information and production method.

What refermentation is not

Refermentation is not the same as carbonation. Carbonation adds carbon dioxide to wine; refermentation involves yeast converting fermentable sugar into alcohol and carbon dioxide.

It is also not malolactic fermentation. Malolactic fermentation is a separate process in which bacteria convert malic acid into lactic acid. It can affect a wine’s acidity and texture, but it is not the second alcoholic fermentation that creates sparkling-wine bubbles.

An Australian perspective

Australian producers use several methods to make sparkling wine. Traditional-method wines use deliberate bottle fermentation, while other sparkling wines may be made by tank fermentation or by adding carbon dioxide. Pét-nat follows a different approach again.

So, seeing terms such as “traditional method” or encountering bubbles does not automatically tell you that every stage happened in the bottle. The method and style matter.

If a bottle is visibly leaking, swollen, cracked or unusually pressurised, do not shake it or open it near people, faces, glassware or other breakable objects. Set it aside and seek advice from the retailer or producer rather than trying to diagnose it remotely.

Frequently asked questions

Is refermentation in a bottle always a fault?

No. It is an intentional and important step in traditional-method sparkling wine. It is generally considered unintended when it starts again in a wine that was meant to be still.

Do bubbles prove that a still wine has refermented?

No. Bubbles may be intentional, may come from another production method, or may simply be dissolved carbon dioxide. Bubbles are a clue, not proof.

Is refermentation the same as secondary fermentation?

Often, the terms are used for the same second alcoholic fermentation. Context matters: winemakers may use “secondary fermentation” for a planned sparkling-wine process, while drinkers may use it to describe unexpected renewed fermentation in a still wine.

What should I do with a pressurised bottle?

Do not shake or open a visibly damaged, leaking or unusually pressurised bottle near people or breakable objects. Seek advice from the retailer or producer.