Which condition encourages ester formation in fermentation?

The formation of esters during fermentation is primarily influenced by the yeast's metabolic processes. When yeast is under pitched, meaning that fewer yeast cells are present than optimal for the fermentation volume, the yeast often undergoes a stress response. This stress can lead to increased production of esters as the yeast attempts to compensate for the low cell count and creates more byproducts through its metabolism.

Ester compounds are formed during fermentation when fatty acids react with alcohols produced by yeast. Under pitching yeast can cause the yeast to be more active as they work harder to ferment the sugars available. This situation can elevate the production of these desirable aromatic compounds, which are often responsible for the fruity flavors in beers.

In contrast, high aeration generally leads to increased production of other byproducts, and low fermentation temperatures tend to decrease ester production because they slow down yeast metabolism. Adding more hops relates primarily to the bitterness and aroma of the beer rather than ester formation during fermentation. Therefore, under pitching yeast is a key condition that effectively encourages ester formation in the fermentation process.