What is the difference between top-fermenting and bottom-fermenting yeast?

• A. Ale yeast (S. cerevisiae) ferments warm at top; lager yeast ferments cooler at bottom.
• B. Ale yeast ferments cooler at bottom; lager at top.
• C. Both ferment at same temp.
• D. Lager ferments warm at top; ale at bottom.

Answer: (A)

Fermentation behavior and temperature preference distinguish ale from lager yeasts. Ale yeasts, which are top-fermenting (Saccharomyces cerevisiae), work best at warmer temperatures and tend to rise to the surface during fermentation, forming krausen at the top. Lager yeasts, which are bottom-fermenting (Saccharomyces pastorianus), prefer cooler temperatures and settle toward the bottom as they ferment, producing a cleaner, crisper finish. So the description that ale yeast ferments warm at the top and lager yeast ferments cooler at the bottom matches how these yeasts behave in the fermenter. Other options mix up where the yeast activity occurs or the temperatures, which isn’t accurate for typical ale versus lager fermentation.