Stereospecific lactic acid production — organism yielding only L(+)-lactic acid without racemization Which microorganism is known to produce exclusively L(+)-lactic acid (optically pure) with minimal racemization during fermentation?

Introduction / Context:
Optically pure lactic acid is essential for producing stereoregular polylactic acid (PLA) polymers with desirable mechanical and thermal properties. Organism choice dictates the L/D ratio because many bacteria possess lactate racemases or dual dehydrogenases that yield DL mixtures.

Given Data / Assumptions:

• We seek an organism that produces L(+)-lactic acid with negligible racemization.
• Some Lactobacillus strains produce predominantly L or D but may racemize under certain conditions.
• Filamentous fungi can exhibit strong stereoselectivity.

Concept / Approach:
Rhizopus oryzae (formerly R. arrhizus) is a well-known fungal producer of optically pure L(+)-lactic acid via the L-lactate dehydrogenase pathway, with minimal racemization because of limited or absent lactate racemase activity. This makes it attractive for polymer-grade L-lactic acid production. In contrast, Leuconostoc mesenteroides and various lactobacilli often generate DL mixtures or require stringent control to maintain enantioenrichment.

Step-by-Step Solution:
Identify organisms associated with optically pure L-lactate.Match industrial experience to R. oryzae for high L-enantiomer purity.Select “Rhizopus oryzae.”

Verification / Alternative check:
Bioprocess literature reports L-lactic acid optical purities exceeding 95–99% using R. oryzae under optimized conditions.

Why Other Options Are Wrong:
Leuconostoc mesenteroides and some Lactobacillus spp. may produce D- or DL-lactate; L. delbrueckii and L. bulgaricus can favor L but are not as consistently racemization-free across conditions.

Common Pitfalls:
Assuming any “homofermentative” lactic acid bacterium guarantees single-enantiomer output; stereochemistry depends on enzymes expressed and culture conditions.

Final Answer:
Rhizopus oryzae.