Brewing yeasts — Which organism is the classic bottom-fermenting, cold-tolerant lager beer yeast used for “low” fermentation?

Introduction / Context:
Modern brewing distinguishes top-fermenting ale yeasts from bottom-fermenting lager yeasts. The latter perform well at lower temperatures and flocculate differently, yielding the crisp profile associated with lagers and pilsners. Historically, the lager yeast was called Saccharomyces carlsbergensis (now commonly S. pastorianus).

Given Data / Assumptions:

• Ales: warmer fermentation, top-cropping (S. cerevisiae).
• Lagers: cooler fermentation, bottom-cropping (S. carlsbergensis/S. pastorianus).
• Temperature influences flavor (esters, sulfur compounds) and fermentation kinetics.

Concept / Approach:
The bottom yeast (S. carlsbergensis) is adapted to cool fermentation (typically 8–13°C), settling at the bottom and producing a clean flavor profile. The distractors include the ale yeast (S. cerevisiae) and a dairy bacterium (Streptococcus thermophilus, misspelled here), neither of which is the lager yeast.

Step-by-Step Solution:

Verification / Alternative check:
Brewing references credit the Carlsberg Laboratory with characterizing this yeast, leading to consistent lager production worldwide.

Why Other Options Are Wrong:

• S. cerevisiae: ale yeast; top fermenter.
• S. thermophillus: not a brewing yeast; a lactic starter organism.
• None of these: incorrect as the correct lager yeast is listed.

Common Pitfalls:
Equating flocculation behavior solely with species; strain differences and fermentation conditions also influence sedimentation.

Final Answer:
S. carlsbergensis