What is the purpose of a diacetyl rest during fermentation?

The purpose of a diacetyl rest during fermentation is to remove diacetyl, which may be present after the primary fermentation process. Diacetyl is a byproduct of yeast metabolism, specifically produced during the fermentation of certain styles, notably in lagers. It imparts a buttery flavor to the beer, which is undesirable in most styles.

During a diacetyl rest, the fermentation temperature is increased toward the end of primary fermentation. This temperature elevation encourages the yeast to become more active again, specifically facilitating the reabsorption of diacetyl and its conversion into less flavor-active compounds. This step is crucial in producing a clean-tasting beer, as it mitigates the risk of diacetyl affecting the final flavor profile.

Other options do not address the role of diacetyl specifically. Increasing alcohol concentration is not the purpose of a diacetyl rest, as that is achieved through the fermentation process itself. Enhancing hop aroma is related to late additions and dry hopping, not the diacetyl rest. Clarifying beer is usually achieved through the aging process or filtration rather than through a diacetyl rest.