Identifying the Microorganism in Refrigerated Milk Spoilage

The primary microorganism responsible for the spoilage of refrigerated milk is typically Pseudomonas spp.. These bacteria are known as psychrotrophs, meaning they can grow effectively even at low temperatures found in refrigeration (typically below 7°C).

Why Pseudomonas spp. Cause Refrigerated Milk Spoilage

• Psychrotrophic Growth: Pseudomonas spp. are well-adapted to cold environments. Unlike most mesophilic bacteria that are inhibited by refrigeration, Pseudomonas actively grow, multiply, and produce enzymes.
• Enzyme Production: These enzymes (like lipases and proteases) break down milk fats and proteins, leading to undesirable changes in flavor (e.g., rancidity, bitterness), texture, and odor, even if the bacterial count isn't excessively high initially.
• Common Contamination Source: They often originate from the farm environment (soil, water, animal feces) and can proliferate in milking equipment and bulk tanks if not properly cleaned, contaminating the raw milk.

Other Microorganisms and Refrigerated Milk

While other microorganisms can be present in milk, they are generally less significant for spoilage specifically under refrigeration:

• Bacillus subtilis: More associated with spoilage at warmer temperatures or specific food matrices; generally inhibited by refrigeration.
• Aspergillus niger: A mold, less common as a primary cause of liquid milk spoilage compared to bacteria, especially in refrigerated conditions.
• Saccharomyces cerevisiae: A yeast, primarily involved in fermentation, not the typical primary spoilage agent for refrigerated fluid milk.

Therefore, the ability of Pseudomonas spp. to grow and produce spoilage enzymes at low temperatures makes them the key culprits in refrigerated milk spoilage.