Which yeast is used for the commercial production of ethanol?

The answer is B, Saccharomyces cerevisiae, the baker's and brewer's yeast. It ferments sugars to ethanol and carbon dioxide under anaerobic conditions and tolerates high ethanol concentrations, which is why it runs the ethanol industry, from fuel ethanol in Brazil and the US to alcoholic beverages and industrial spirits.

Why the others are wrong:

• A) Candida albicans is an opportunistic human pathogen that causes candidiasis. It is a medical microbe, not an industrial fermenter.
• C) Candida sloofi is a less common yeast species. It is not the organism used for commercial ethanol production.
• D) Leucosporidium frigidum is a psychrophilic yeast isolated from cold environments. It is studied for cold-active enzymes, not bulk ethanol.

The core concept is the role of microbes in industrial biotechnology and fermentation, which links to Biotechnology in Science and Technology and to Mains answers on biofuels and the ethanol blending programme.

Trap: Candida is often remembered as a fermenting yeast because of its name and common occurrence in food contexts. Aspirants pick A or C on that basis. Remember that Saccharomyces cerevisiae is the standard industrial ethanol yeast.