Substrate utilization for lactic fermentation — Which organism is used industrially to ferment pentose sugars from sulfite waste liquor to lactic acid?

Introduction / Context:
Using industrial by-products like sulfite waste liquor (rich in pentoses) improves sustainability of bioprocessing. The selection of lactic acid bacteria (LAB) depends on sugar utilization profiles, particularly the ability to metabolize pentoses efficiently.

Given Data / Assumptions:

• Sulfite waste liquor contains pentose sugars from wood pulping.
• Not all LAB ferment pentoses; some specialize in hexoses.
• The question asks for the organism that ferments pentoses to lactic acid.

Concept / Approach:
Lactobacillus pentosus (name hints at pentose utilization) is known for fermenting pentose sugars to lactic acid via heterofermentative pathways. Other LAB listed (L. bulgaricus, L. casei) primarily prefer hexoses; Streptococcus lactis is now Lactococcus lactis and is also more associated with lactose/hexoses in dairy fermentations.

Step-by-Step Solution:

Verification / Alternative check:
Process references for lactic acid production highlight L. pentosus and related species for hemicellulose hydrolysates rich in xylose and arabinose.

Why Other Options Are Wrong:

• Streptococcus (Lactococcus) lactis: dairy hexose/lactose fermentation.
• L. bulgaricus: yogurt cultures, primarily hexose fermenter.
• L. casei: broader substrate range but not the classic pentose specialist in this context.

Common Pitfalls:
Choosing based on industrial familiarity (yogurt/cheese strains) rather than matching sugar profile of the feedstock.

Final Answer:
Lactobacillus pentosus