Which aspect does fermentation primarily reduce concerning alpha acids?

Fermentation primarily reduces the perception of bitterness related to alpha acids, which are primarily responsible for the bitter flavor in beer. During the fermentation process, yeast converts sugars into alcohol and carbon dioxide, but it can also affect the overall taste profile of the beer. The interaction between yeast and hops can lead to the attenuation of bitter flavors.

When yeast is active, it can bind to certain compounds and alter their evaporative qualities, which can dampen the intensity of bitterness. This is particularly noticeable in certain styles of beer where hop character is emphasized, and while alpha acids contribute to bitterness when hops are added, the fermentation phase can modify how these hops express themselves in the final product.

The other aspects mentioned in the options are less directly influenced by fermentation. While fermentation does contribute to the overall complexity of aroma and flavor, it does not inherently reduce aroma or color in the same way as it does bitterness. Alcohol content is generated during fermentation rather than reduced, making the primary effect on alpha acids one of bitterness reduction.