Dairy microbiology — alkaline reaction in milk: Which organisms are classic “alkali formers” that cause an alkaline shift in stored raw milk due to protein breakdown and ammonia release?

Introduction / Context:
In dairy microbiology, psychrotrophic Gram-negative rods are frequent contaminants of raw and refrigerated milk. Certain species produce proteases and deaminases that liberate ammonia and amines, driving the pH upward and creating an alkaline reaction. Recognizing the organisms behind this change helps troubleshoot quality issues and select appropriate sanitation and cold-chain controls.

Given Data / Assumptions:

• Milk shows an alkaline reaction rather than the more common acidification.
• Organisms listed include Pseudomonas fluorescens and A. viscolactis (historically Achromobacter/Alcaligenes).
• Question asks which organisms are alkali formers in milk.

Concept / Approach:
Alkali formers hydrolyze proteins (caseins, whey proteins) and deaminate amino acids. The reaction amino acid → keto acid + NH3 raises pH. Pseudomonads (especially Pseudomonas fluorescens) secrete heat-stable proteases and are well known in refrigerated milk spoilage. Alcaligenes/ Achromobacter (including A. viscolactis) are classic “alkali milk” bacteria documented to cause ropiness/viscosity changes and alkaline shifts.

Step-by-Step Solution:
Identify traits of alkali formers: intense proteolysis and deamination with ammonia release.Match traits to listed taxa: Pseudomonas fluorescens (psychrotroph, protease producer) fits.A. viscolactis has been historically cited for alkaline milk and ropiness defects.Therefore, both organisms are correct sources of alkaline reaction in milk.

Verification / Alternative check:
Creamery texts and dairy QC manuals consistently group Pseudomonas fluorescens and Alcaligenes/ Achromobacter among alkali milk bacteria, especially in refrigerated raw milk where psychrotrophs proliferate.

Why Other Options Are Wrong:

• None of these: Incorrect because two listed organisms are classic alkali formers.
• Micrococcus luteus: May grow in milk but is not a typical cause of pronounced alkaline reaction.

Common Pitfalls:
Confusing alkaline defects with lactic acid fermentation; also overlooking that heat-stable proteases from psychrotrophs survive pasteurization and cause age gelation later.

Final Answer:
Both (a) and (b).