What happens to beer when oxygen is introduced after tapping a cask?

When oxygen is introduced to beer after tapping a cask, it can lead to several significant changes in the beer's quality and flavor profile. The presence of oxygen can cause oxidative reactions, which typically soften the beer's flavor, making it taste less vibrant and fresh. Over time, these reactions can result in the development of undesirable flavors, often described as cardboard-like or stale. Additionally, exposure to oxygen can lead to the growth of certain microorganisms that may cause the beer to sour, further altering its original taste.

In contrast, carbonation levels remain stable unless other processes (such as the addition of priming sugar or further fermentation) take place. In this context, introducing oxygen does not create a positive effect, as some might think it could improve flavors, but instead leads to less favorable outcomes in terms of the beer's character. This highlights the importance of minimizing oxygen exposure when handling and serving beer to preserve its intended flavor and aroma.