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

The primary difference between ales and lagers lies in the type of yeast used during fermentation. Ales are fermented with top-fermenting yeast, typically Saccharomyces cerevisiae, which operates at warmer temperatures and ferments more quickly. This yeast tends to produce fruity and complex flavors, which are characteristic of many ales.

On the other hand, lagers are made using bottom-fermenting yeast, often Saccharomyces pastorianus, which ferments at cooler temperatures over a longer period. This process results in cleaner, crisper flavors and a smoother finish, distinguishing lagers from ales.

Understanding the fermentation process and the yeast used is crucial for differentiating these two major categories of beer. While barley, hops, and water are important ingredients in brewing, they do not define the fundamental differences between ales and lagers as the type of yeast used does.