How is carbonation naturally achieved in cask ales?

The carbonation in cask ales is naturally achieved through the process of fermentation, in which yeast consumes residual sugars present in the beer. As the yeast metabolizes the sugars, it produces alcohol and carbon dioxide as byproducts. In a cask-conditioned ale, the beer is usually unfiltered and unpasteurized and is allowed to undergo secondary fermentation in the cask. This occurs in a sealed environment, which traps the carbon dioxide produced by the yeast, leading to a natural carbonation of the beer.

In contrast, methods such as forced carbonation and the external addition of CO2 are not natural processes; they involve manipulating the gas levels using outside sources. Rapid chilling does not contribute to carbonation; instead, it can impact the solubility of gas in the liquid but does not create carbonation by itself. Therefore, the process of yeast consuming sugar is the hallmark of achieving natural carbonation in traditional cask ales.