Why must cask ale be consumed within a few days?

Cask ale must be consumed within a few days primarily to prevent oxidation and souring. Cask ale is unfiltered and unpasteurized, which means it is a living product that continues to evolve after being casked. Once opened, the exposure to oxygen can lead to rapid changes in flavor and aroma, often resulting in undesirable off-flavors. Over time, cask ale can also develop sour notes as wild yeast and bacteria may begin to ferment residual sugars. Therefore, to enjoy the fresh qualities of cask ale and ensure it remains at its best, it should ideally be consumed shortly after tapping.

The other options, while they may relate to aspects of beer preservation and enjoyment, do not directly address the primary reason for the short shelf life of cask ale. For instance, maintaining carbonation is less of an issue in cask ale since it is traditionally served still or with very low carbonation. Keeping it cold is also not a primary concern for cask ale, as it is often served at cellar temperature. Lastly, while some flavors may improve over a short period, the risk of oxidation and souring overrides any potential for flavor enhancement in cask ale after being opened.