Why are industrial baker's yeast (Saccharomyces cerevisiae) cultivations typically run in fed-batch reactors rather than simple batch? Explain the role of glucose level in shifting yeast between respiration and ethanol-forming fermentation.

Introduction / Context:
Fed-batch is the workhorse mode for producing baker’s yeast and many yeast-derived products. The core reason relates to how extracellular glucose concentration controls the metabolic state of Crabtree-positive yeasts such as Saccharomyces cerevisiae, toggling between respiration (biomass-oriented) and ethanol-forming fermentation that can inhibit growth at high sugar levels.

Given Data / Assumptions:

• Saccharomyces cerevisiae shows the Crabtree effect: at high glucose it ferments even with oxygen present.
• Ethanol accumulation reduces specific growth rate and yield on substrate.
• Glucose setpoint can be controlled by feeding in fed-batch (low but non-zero).

Concept / Approach:
Keeping extracellular glucose low biases metabolism toward respiratory growth with high biomass yields and minimal ethanol. Batch charges often overshoot glucose, provoking aerobic fermentation and ethanol accumulation. Fed-batch solves this by continuously or intermittently adding concentrated feed while oxygen is supplied to meet demand.

Step-by-Step Solution:
1) If initial glucose is high (simple batch), S. cerevisiae triggers overflow metabolism → ethanol.2) Ethanol competes as a carbon sink and inhibits cells, depressing biomass yield.3) In fed-batch, glucose is dosed to a low setpoint (e.g., near zero), forcing respiration and maximizing biomass formation.4) Outcome: higher cell density, better oxygen utilization, and less inhibitory by-product.

Verification / Alternative check:
Oxygen uptake rate and CO2 evolution profiles in glucose-limited fed-batch confirm respiratory metabolism (low ethanol), whereas batch with excess glucose shows ethanol spikes and diauxic shifts upon ethanol re-consumption.

Why Other Options Are Wrong:
Option A and B invert the condition; fermentation and ethanol formation are associated with high, not low, glucose in Crabtree-positive yeast. Option D cannot be correct because A and B are false. Option E is wrong because C states the established rationale.

Common Pitfalls:
Confusing Pasteur and Crabtree effects; also assuming any feed rate works—overfeeding in fed-batch recreates batch-like glucose excess.

Final Answer:
Yeast cells respire at low glucose concentrations, so feed keeps glucose low to avoid ethanol inhibition