Variation in which of the following ingredients is responsible for the difference between ales and lagers?

The distinction between ales and lagers is primarily attributed to the type of yeast used during fermentation. Ales are fermented with top-fermenting yeast, specifically Saccharomyces cerevisiae, which operates best at warmer temperatures (around 60-75°F or 15-24°C). This yeast ferments quickly and produces esters and phenols, contributing to the fruity and complex flavors characteristic of many ales.

On the other hand, lagers are fermented with bottom-fermenting yeast, primarily Saccharomyces pastorianus, which thrives at cooler temperatures (around 45-55°F or 7-13°C). This yeast ferments more slowly, resulting in cleaner, crisper flavors with less fruity esters and phenols. The cooler fermentation process also allows for a longer maturation period, which is often a defining feature of lagers.

While hops, malt, and water are essential components of beer that contribute to flavor, aroma, body, and overall character, they do not differentiate between ales and lagers in the same fundamental way that yeast does. Therefore, the primary factor responsible for the differences in fermentation characteristics, flavor profiles, and styles between ales and lagers is indeed the type of yeast used.