What is the primary reason for the production of phenols during brewing?

Phenols are produced primarily due to yeast metabolism during the fermentation process in brewing. As yeast consumes sugars, it also produces various byproducts, including phenolic compounds. These phenolic compounds can impart distinct flavors and aromas to the beer, ranging from spicy and clove-like to medicinal, depending on the yeast strain used. Certain yeast strains are known for producing higher levels of phenols, which can significantly influence the final sensory characteristics of the beer.

The other options, while they may have some relevance in different contexts, do not directly relate to the primary mechanism of phenol production. For instance, while oxygen exposure can lead to oxidation and off-flavors in beer, it is not a direct contributor to the formation of phenolic compounds attributed to yeast metabolism. High temperatures during boiling can serve to volatilize certain compounds but are not specific to the production of phenols by yeast. Contamination by bacteria may introduce different flavors or sour notes but is not a route for the natural production of phenols through normal yeast activity in controlled brewing environments.