ABE fermentation by Clostridium spp.: Riboflavin (vitamin B2) is known as a by-product of which classic Clostridial industrial process?

Introduction / Context:
Clostridial ABE (acetone–butanol–ethanol) fermentation is a hallmark of industrial microbiology. Besides solvents, minor metabolites and vitamins can be formed as by-products that may have analytical or nutritional significance.

Given Data / Assumptions:

• Clostridium spp. drive ABE fermentation.
• Question targets the process associated with riboflavin by-production.

Concept / Approach:
Among the listed options, only ABE fermentation is a Clostridial solvent process. Citric acid is produced mainly by Aspergillus niger; streptomycin by Streptomyces griseus; fuel ethanol chiefly by yeasts (though some bacteria can produce ethanol). Riboflavin formation at low levels has been documented in Clostridial cultures during ABE operations, whereas the other processes are not Clostridial and thus unlikely answers.

Step-by-Step Solution:
Map organism–process pairs: Clostridia → ABE solvents. Recognize vitamin by-products can accompany solventogenesis. Select “acetone butanol.”

Verification / Alternative check:
Process surveys and compositional analyses of ABE broths report trace B-vitamins including riboflavin.

Why Other Options Are Wrong:

• Citric acid: fungal, not Clostridial.
• Streptomycin: actinomycete antibiotic.
• Ethanol: typically yeast-based in industry; not the classic Clostridial platform for B2 mention.

Common Pitfalls:
Associating riboflavin solely with Ashbya gossypii production and overlooking minor vitamin formation in other fermentations.

Final Answer:
acetone butanol