Which of the following is a by-product of yeast metabolism that can affect beer flavor?

Phenols are indeed by-products of yeast metabolism that can significantly influence the flavor profile of beer. During fermentation, yeast not only converts sugars into alcohol (ethanol) but also produces various compounds, including phenolic compounds. These compounds can impart distinct flavors and aromas to the beer, often described as spicy, medicinal, or clove-like. They are particularly important in certain beer styles, such as Belgian ales and wheat beers, where their presence can enhance the overall complexity and character of the beverage.

The other options do have roles in beer production but do not have the same direct impact on flavor as phenols. Ethanol is the primary alcoholic component and contributes to the drink's strength rather than its specific flavor profile. Carbon dioxide is primarily responsible for the beer's carbonation and mouthfeel but is not a flavor compound in itself. Acetaldehyde, while it can be a flavor compound, is typically considered an off-flavor in higher concentrations and is usually not a desirable by-product in most styles of beer. It presents a green apple character, which is often looked upon unfavorably. Thus, phenols stand out as key flavor influencers resulting from yeast metabolism.