When does secondary fermentation in real ales begin?

Secondary fermentation in real ales is a critical stage that occurs after the primary fermentation has completed and involves the conditioning of the beer in cask or bottle. This fermentation often starts when the beer is racked into a cask, allowing the yeast remaining in the beer to become active again. This can happen due to the presence of residual sugars that were not fully fermented during the primary fermentation, alongside any priming sugars added for carbonation.

The cask environment, with its specific conditions and the presence of yeast, creates the perfect opportunity for this process. The yeast consumes the sugars, producing carbon dioxide and contributing to both carbonation and flavor development. While bottling may also allow for secondary fermentation if priming sugar is added, the specific context of real ales focuses on the cask conditioning process.

The other options describe different processes or stages unrelated to when secondary fermentation specifically begins in real ales, making them less relevant in this context.