Bottle refermentation: How beer creates its bubbles

1. The theory: What happens inside the bottle?

Bottle refermentation is a true micro‑fermentation that takes place in a closed environment. Its principle is based on a simple yet magical biological reaction:

• Reactivating the yeast: Once primary fermentation is complete, the yeast has consumed all the sugars in the wort and has gone dormant. By adding a precise amount of sugar at bottling time, you give it new fuel.
• CO₂ trapping: The yeast becomes active again and consumes this sugar. As in the first fermentation, it converts it into alcohol and carbon dioxide (CO₂). However, this time the gas is trapped by the bottle cap, cannot escape, and gradually dissolves into the liquid to give the beer its bubbles.
• Impact on alcohol: This mini‑fermentation also produces a very small additional amount of alcohol. As a general rule, bottle priming adds around 0.3% to 0.5% ABV to your final beer.

2. Preparing the batch and the importance of the transfer

To ensure this stage goes smoothly, the transition between the fermenter and the bottle requires care:

• Ensuring fermentation is finished: This is the absolute rule. Before considering any priming, you must be certain that primary fermentation is fully complete by checking that your gravity is stable over several days.
• The priming bucket: Never add sugar directly into your fermenter, as this will stir up all the sediment. Use a meticulously cleaned and sanitised priming bucket (or bottling bucket). Transfer your beer gently into it to separate it from the yeast cake left at the bottom of the main fermenter.
• Bottle hygiene: Just like your bucket, your bottles must be impeccable. Thorough cleaning followed by rigorous sanitisation is essential to prevent bacteria or wild yeast from spoiling your work.

3. Sugar dosage: Finding the right balance for each style

The level of carbonation (the amount of bubbles) depends directly on the amount of sugar you add. This dosage is calculated in grams per litre (g/L) and must be adapted to the beer style:

• Standard dosages (6 to 7 g/L): The ideal average for most French, English, or American styles (Ales, IPAs, Stouts). It provides balanced carbonation without overwhelming the palate.
• Low‑carbonation styles (4 to 5 g/L): Suitable for certain traditional English beers (Cask Ales or Porters) where a very fine, discreet carbonation is desired.
• Highly carbonated styles (7.5 to 9 g/L): This higher level is perfect for Belgian‑style beers (Tripel, Saison) or wheat beers (Weizen), which require generous foam and lively effervescence.

4. Beware of “Gush” and over‑carbonation: Warnings to follow

Poor control of refermentation can turn your tasting moment into a cleaning session — or even a safety hazard:

The “Gush” phenomenon (beer geyser)

“Gush” refers to beer that erupts uncontrollably from the bottle as soon as it is opened. This happens when the CO₂ level is far too high. The main causes are an excessive sugar dose during priming or poor homogenisation of the sugar syrup, which results in some bottles receiving much more sugar than others.

The danger of over‑carbonation and exploding bottles

If you bottle while primary fermentation is not fully complete, the yeast will consume both the priming sugar AND the residual wort sugars. The pressure will quickly exceed the resistance of the glass. In the best case, you’ll get an undrinkable beer full of foam; in the worst case, you risk literal bottle explosions in your storage area.