Butter defects — Pseudomonas nigrifaciens in lightly salted butter: Which characteristic defect does Pseudomonas nigrifaciens cause in mildly salted butter?

Introduction / Context:
Spoilage of butter and other high-fat dairy products often involves pigment-producing environmental bacteria. Correctly linking a defect appearance to its likely microbial cause speeds root-cause analysis and targeted hygienic interventions in creameries and butter plants.

Given Data / Assumptions:

• Product: mildly salted butter.
• Organism implicated: Pseudomonas nigrifaciens.
• Task: select the characteristic visual defect.

Concept / Approach:
Pseudomonas nigrifaciens can form dark pigments and has been classically associated with black surface discolorations on salted butter. While several pseudomonads generate green, blue, or yellow pigments, the term “black smudge” is historically tied to P. nigrifaciens, particularly when salt levels are insufficient to suppress growth during storage.

Step-by-Step Solution:
Identify defect library associations: “black smudge” aligns with P. nigrifaciens.Exclude colorations typical of other species (e.g., greenish tints with fluorescent pseudomonads, pink with Serratia).Select the defect most specific to the named organism.

Verification / Alternative check:
Traditional dairy spoilage references list P. nigrifaciens among pigmenting pseudomonads that produce blackish discolorations on fat surfaces under suboptimal sanitation or storage.

Why Other Options Are Wrong:

• Greenish areas: More typical of fluorescent pseudomonads (pyoverdine, pyocyanin mixtures).
• Pink color: Classically associated with Serratia marcescens, not P. nigrifaciens.
• None of these/Greasy sheen: Do not reflect the diagnostic “black smudge.”

Common Pitfalls:
Relying on salt alone as a preservative; psychrotrophs can still grow at refrigeration temperatures and low to moderate salt levels.

Final Answer:
Black smudge.