Beer styles and yeast behavior — what yeast type defines lager fermentation? Lagers are produced by fermentation led primarily by which yeast behavior/class under cool conditions?

Introduction / Context:
Beer styles are differentiated by yeast species/strains, fermentation temperature, and cell sedimentation behavior. Lagers and ales are the two major families, each with characteristic yeast performance and flavor outcomes.

Given Data / Assumptions:

• Lagers ferment at cooler temperatures (approximately 7–13°C).
• Yeast flocculation and sedimentation differ between lager and ale strains.
• Flavor profile for lagers is clean and crisp with low fruity esters.

Concept / Approach:

Lager fermentations use bottom-fermenting yeast, classically Saccharomyces pastorianus (formerly S. carlsbergensis). These strains work well at low temperatures and settle toward the bottom as fermentation completes, aiding clarification and enabling extended cold storage (lagering).

Step-by-Step Solution:

Verification / Alternative check:

Brewing texts and yeast suppliers categorize S. pastorianus as bottom-fermenting with optimal cool ranges, distinguishing lagers from ales.

Why Other Options Are Wrong:

Top yeast (option b) refers to ale fermentations (S. cerevisiae). “Middle yeast” (option c) is nonstandard terminology. Brettanomyces (option d) is used in specialty sour/funky beers, not standard lagers. Acetic acid bacteria (option e) are spoilage organisms in beer.

Common Pitfalls:

Equating bottom yeast with poor flocculation; in practice, settling patterns reflect strain traits and conditions, not quality.

Final Answer:

Bottom yeast