Correct answer: (1) Ethanol and acetone-butanol

In microbial biotechnology, “anaerobic fermentations” are processes that occur without molecular oxygen as the terminal electron acceptor and generate ATP primarily by substrate-level phosphorylation.

Option analysis:

• (1) Ethanol and acetone-butanol.

  • Ethanol fermentation (by yeasts such as Saccharomyces) is a classic anaerobic process where sugars are converted to ethanol and CO₂ in the absence of oxygen.

  • Acetone–butanol (ABE) fermentation by Clostridium species is also strictly anaerobic, producing solvents like acetone and butanol from sugars.
    Both are textbook examples of anaerobic industrial fermentations, so this option is correct.

• (2) Citric acid and propionic acid.

  • Industrial citric acid production (e.g., by Aspergillus niger) is carried out under highly aerated, aerobic conditions.

  • Propionic acid fermentation by Propionibacterium is anaerobic, but since citric acid is not, the pair is not “both anaerobic”; thus this option is incorrect.

• (3) Penicillin and vitamin B12.

  • Penicillin is produced by Penicillium species in strongly aerobic submerged fermentations.

  • Vitamin B₁₂ is typically produced by bacteria in processes that require controlled aeration or microaerophilic conditions, not strictly classic anaerobic fermentation for both; hence this pair is not “anaerobic fermentations” in the classical sense.

• (4) Streptomycin and rifampicin.

  • Both are antibiotics produced by actinomycetes (Streptomyces species) in aerobic fermenters with active aeration and agitation.
    These are aerobic secondary metabolite productions, not anaerobic fermentations, so this option is incorrect.