Milk discoloration — brown color development: Brown color in stored milk can result from which of the following causes?

Introduction / Context:
Color defects in milk arise from microbial pigments, enzymatic reactions, or chemical browning. Correctly linking brown discoloration to likely microbial mechanisms assists in corrective actions such as improved CIP, raw milk screening, and temperature control.

Given Data / Assumptions:

• Observed defect: brown coloration of milk.
• Potential causes: Pseudomonas putrefaciens; enzymatic oxidation of tyrosine by Pseudomonas fluorescens.
• We need to determine if one or both causes are valid.

Concept / Approach:
P. putrefaciens and other psychrotrophs can form pigments or catalyze reactions leading to brownish hues. P. fluorescens produces extracellular enzymes that can oxidize aromatic amino acids such as tyrosine, generating brown pigments in milk during storage, especially at low temperatures where these psychrotrophs thrive.

Step-by-Step Solution:
Associate brown discoloration with proteolysis and amino acid oxidation pathways.Confirm that both cited pseudomonads can be implicated in brown pigments through enzyme activity.Conclude that both listed mechanisms are valid causes.

Verification / Alternative check:
Dairy defect references frequently list psychrotrophic pseudomonads as sources of brown, yellow, and blue discolorations via enzymatic oxidation of milk constituents.

Why Other Options Are Wrong:

• S. marcescens: Characteristically produces a red/pink pigment (prodigiosin).
• Propionibacterium freudenreichii: Primarily contributes to eye formation and flavor in cheese; not a typical cause of brown milk.

Common Pitfalls:
Confusing Maillard browning (heat/age related) with microbial enzymatic browning; both can occur but have different controls.

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