Whey valorization — lactose utilization during lactic acid production During lactic acid fermentation of whey-based media (lactose-rich), which microorganism among the following is well-adapted to utilize lactose as the primary carbon source?

Introduction / Context:
Dairy byproduct streams such as whey are rich in lactose and can be converted to lactic acid for food, chemical, and bioplastic applications. Selecting a microbe with robust beta-galactosidase activity and efficient lactose uptake is critical for high space–time yields.

Given Data / Assumptions:

• Whey contains lactose as the dominant sugar.
• Organisms differ in their ability to hydrolyze and metabolize lactose.
• Lactobacillus bulgaricus (L. delbrueckii subsp. bulgaricus) is a classic dairy organism with strong lactose utilization.

Concept / Approach:
L. bulgaricus expresses beta-galactosidase, splitting lactose to glucose and galactose which enter the Embden–Meyerhof pathway. It is adapted to dairy environments (yogurt manufacture with Streptococcus thermophilus) and can efficiently ferment lactose in whey-based media. In contrast, L. pentosus specializes in pentoses; Leuconostoc mesenteroides commonly lacks efficient lactose metabolism; some L. delbrueckii strains outside the bulgaricus subspecies have variable lactose use.

Step-by-Step Solution:
Identify lactose-rich substrate (whey).Match to dairy-adapted lactobacilli with proven lactose utilization.Select L. bulgaricus as the robust lactose user among the options.

Verification / Alternative check:
Starter culture literature and whey fermentation studies routinely employ L. bulgaricus or engineered strains with high beta-galactosidase for lactose conversion.

Why Other Options Are Wrong:
L. pentosus targets pentoses; Leuconostoc mesenteroides is heterofermentative with limited lactose use; non-bulgaricus L. delbrueckii strains may be weak on lactose; Zymomonas mobilis ferments glucose/fructose via Entner–Doudoroff and does not metabolize lactose.

Common Pitfalls:
Ignoring the need for sufficient beta-galactosidase; not supplementing minerals or pH control (neutralization) during high-rate lactic acid production.

Final Answer:
Lactobacillus bulgaricus.