What term refers to microorganisms that convert grape sugars into alcohol during fermentation, which can be naturally occurring or added?

• A. Bacteria
• B. Mold
• C. Yeast
• D. Algae

Answer: (C)

Yeast is the microorganism that turns grape sugars into alcohol during fermentation. Yeast are single-celled fungi that metabolize the sugars in grape juice (glucose and fructose) anaerobically, producing ethanol and carbon dioxide. In winemaking, they can come from the grapes and vineyard environment (natural or wild fermentation) or be added as cultured yeasts, giving winemakers control over fermentation traits.

Bacteria can influence wine later through processes like malolactic fermentation, but they’re not the primary agents that convert sugars to alcohol. Mold isn’t involved in producing alcohol in wine and typically signals spoilage. Algae are not part of the fermentation process at all. That’s why yeast is the correct term.