Why should compressed air not be used in draft systems?

Using compressed air in draft systems is highly discouraged primarily because it can lead to the oxidation of beer. Oxidation occurs when oxygen comes into contact with beer, causing undesirable changes to its flavor, aroma, and overall quality. Oxygen can produce off-flavors, such as cardboard or sherry notes, which are not appealing and can turn a well-brewed beer into something unpalatable.

In draft systems, maintaining a controlled environment to keep oxygen exposure low is crucial. This is why CO2 is commonly used; it does not introduce oxygen and helps maintain carbonation while also keeping the beer fresh. Compressed air, on the other hand, contains a significant amount of oxygen, making it detrimental to the beer's integrity.

Other options refer to different issues related to beer handling, but these do not capture the primary concern of using compressed air. For instance, while excessive carbonation can be a problem, it is not specifically tied to the use of compressed air. Similarly, cooling beer too quickly or adding off-flavors are not principal issues directly linked to the introduction of oxygen through compressed air. Therefore, the significant risk of oxidation from using compressed air is the main reason it should not be utilized in draft systems.