How does real ale achieve carbonation during fermentation?

Real ale achieves carbonation during fermentation through a natural process where the yeast consumes sugars and produces carbon dioxide (CO2). This fermentation occurs in the cask, a traditional container used for storing and serving real ale, allowing the beer to undergo a secondary fermentation. During this phase, the yeast is typically left in the cask, which enables it to continue fermenting any residual sugars that remain after the primary fermentation.

As the yeast consumes these sugars, it generates CO2 as a metabolic byproduct. This CO2 becomes trapped in the cask, leading to natural carbonation. This method of achieving carbonation is a hallmark of real ale, as it emphasizes the traditional brewing techniques and ingredients, resulting in a product that is often more complex and flavorful due to the ongoing yeast activity.

The other methods mentioned do not reflect the traditional and natural approach of carbonation associated with real ale brewing. Adding CO2 directly to the cask would introduce artificial carbonation, which goes against the real ale's fundamental character, whereas forced carbonation with nitrogen gas is not suitable for traditional ale styles. Mixing with other carbonated beverages would also dilute the ale's unique flavors and characteristics, thereby not contributing to the natural carbonation process that defines real ale.