Substrate selection for fuel ethanol: Why can’t a standard Saccharomyces cerevisiae strain be used directly on starch-rich feedstocks (for example, corn starch) without prior processing?

Introduction / Context:
Fuel ethanol plants often process starchy grains. However, yeast primarily ferments simple sugars like glucose and cannot directly degrade native starch polymers.

Given Data / Assumptions:

• Typical S. cerevisiae strains lack significant extracellular amylase activity.
• Starch requires hydrolysis (liquefaction and saccharification) to glucose or maltose.
• Industrial processes add enzymes (alpha-amylase, glucoamylase) or use engineered microbes.

Concept / Approach:
Polysaccharides must be enzymatically broken into fermentable sugars. Without amylases, yeast cannot access starch; therefore, pretreatment is essential before fermentation.

Step-by-Step Solution:

Verification / Alternative check:
Commercial ethanol processes universally include liquefaction/saccharification steps prior to yeast fermentation.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing yeast’s ability to ferment maltose/glucose with the ability to hydrolyse raw starch.

Final Answer:
It does not possess amylase enzymes needed to hydrolyse starch into fermentable sugars.