Agricultural Microbiology

Ethanol Fermentation by Saccharomyces cerevisiae:

• Statement (I) is CORRECT: Brewer's/baker's yeast (Saccharomyces cerevisiae) has been utilized for millennia and remains the dominant industrial workhorse organism for large-scale bioethanol fuel and alcoholic beverage production owing to its high ethanol tolerance ($\gt 12\%\text{ v/v}$) and rapid fermentation kinetics.
• Statement (II) is CORRECT: Under anaerobic/fermentative conditions, S. cerevisiae metabolizes hexose sugars (glucose, fructose) to pyruvate via the classical Embden-Meyerhof-Parnas (EMP) glycolytic pathway. Pyruvate is subsequently decarboxylated to acetaldehyde by pyruvate decarboxylase and reduced to ethanol by alcohol dehydrogenase with regeneration of $\text{NAD}^+$.

Statement (I) is true: Biogas is a combustible fuel gas mixture produced from biomass, typically consisting of 50-70% methane ($CH_4$), 30-45% carbon dioxide ($CO_2$), and trace amounts of hydrogen ($H_2$), nitrogen, and hydrogen sulfide ($H_2S$).
Statement (II) is false: Biogas is generated through strictly ANAEROBIC digestion (in the complete absence of molecular oxygen) by a consortium of hydrolytic bacteria, acidogens, acetogens, and obligate anaerobic methanogenic archaea (such as Methanobacterium and Methanococcus). Aerobic digestion produces compost, $CO_2$, and water, NOT biogas.