Wine microbiology — principal microorganism for grape must fermentation Which organism is classically used to ferment grape sugars to ethanol and CO2 during winemaking?

Introduction / Context:
Alcoholic fermentation of grape must converts sugars (glucose and fructose) into ethanol and carbon dioxide. The reliability, stress tolerance, and flavor contributions of the yeast strain determine fermentation performance and wine style.

Given Data / Assumptions:

• Substrate: grape must rich in fermentable sugars.
• Target product: wine with balanced ethanol, aroma, and mouthfeel.
• Process constraints: osmotic stress, low pH, rising alcohol, and limited oxygen.

Concept / Approach:

Saccharomyces cerevisiae is the workhorse yeast of winemaking. It exhibits high ethanol tolerance, rapid fermentation rates, and predictable kinetics. It also produces desirable secondary metabolites (esters, higher alcohols) that shape the wine’s aromatic profile. While non-Saccharomyces yeasts may contribute to complexity early, S. cerevisiae typically dominates as alcohol increases.

Step-by-Step Solution:

Verification / Alternative check:

Enology handbooks and commercial starter culture catalogs consistently center on S. cerevisiae strains for primary fermentations, corroborating its central role.

Why Other Options Are Wrong:

Lactobacillus vermiformis (option b) is lactic acid bacteria, important in malolactic fermentation, not primary alcoholic fermentation. Aspergillus oryzae (option c) is a mold used in sake and soy fermentation for saccharification. Rhizopus species (option d) are molds for certain Asian fermentations, not grape wine. Pichia pastoris (option e) is an expression system yeast, not standard for winemaking.

Common Pitfalls:

Confusing malolactic fermentation (lactic acid bacteria) with alcoholic fermentation (yeast). Also, assuming any “fermentation microbe” suits all beverages—each has domain-specific roles.

Final Answer:

Saccharomyces cerevisiae