What is the primary reason for keeping beer away from oxygen after fermentation?

The primary reason for keeping beer away from oxygen after fermentation is to prevent spoilage from Acetobacter. Acetobacter is a type of bacteria that can convert ethanol into acetic acid in the presence of oxygen, leading to off-flavors that can ruin the beer's intended profile. By minimizing oxygen exposure, brewers can maintain the beer's freshness and desired characteristics, effectively inhibiting the growth of this spoilage organism and preserving the overall quality of the beer.

While carbonation and flavor can also be affected by oxygen levels, those aspects are secondary to the risk of spoilage, which directly impacts the safety and palatability of the beer. Increasing alcohol content is not a relevant concern in the context of post-fermentation handling of beer, as oxygen exposure would neither directly contribute to nor enhance alcohol production.