Which process allows real ale to mature in the cask?

The process that allows real ale to mature in the cask is secondary fermentation. In the context of traditional cask-conditioned ales, after the primary fermentation has completed, a small amount of yeast and sugar is often added to the cask when the beer is transferred. This initiates a secondary fermentation, where the yeast consumes the added sugar, producing carbon dioxide, which naturally carbonates the beer within the cask. Additionally, during this maturation phase, the beer also develops more complex flavors and aromas as it conditions in the cask. This process is essential for achieving the desired taste profile and mouthfeel characteristic of real ale, making it an integral part of the brewing and serving tradition for these styles.

Primary fermentation refers to the initial stage of fermentation where the bulk of sugars are converted into alcohol and carbon dioxide, but it doesn't include the maturation aspect specific to cask conditioning. Filtration is a process used to remove yeast and other particulates from beer, which would hinder the natural conditioning of real ale in the cask. Pasteurization involves heating to kill bacteria and yeast, which would prevent any further fermentation or maturation within the cask, countering the goals of real ale presentation and flavor development.