Which organisms are primarily responsible for spoilage in refrigerated milk?

• A. Psychrotrophs and enzymes.
• B. Lactic acid bacteria only.
• C. Salmonella.
• D. Enterobacteriaceae.

Answer: (A)

Milk stored in the fridge spoils mainly because psychrotrophic bacteria can grow at low temperatures and produce enzymes that attack milk components. These organisms, like Pseudomonas species, secrete proteases and lipases that break down casein and fats. The enzymes are heat-stable, so even after pasteurization they can remain active and cause spoilage during cold storage—leading to off-flavors, curdling, and rancidity.

Salmonella is a pathogen, not a typical spoilage agent in refrigerated milk. Lactic acid bacteria can cause souring but are not the primary spoilage culprits in refrigeration, and the Enterobacteriaceae family includes a range of microbes with varying roles, not the main heat-tolerant spoilage agents.