Raw milk microbiota is dominated by LAB (Lactococcus, Lactobacillus, Leuconostoc) originating from the udder, milking equipment, and environment. However, the most significant spoilage microorganisms in refrigerated raw milk are psychrotrophic Gram-negative rods, particularly Pseudomonas fluorescens and Pseudomonas fragi. These organisms grow during refrigerated storage and produce heat-stable extracellular lipases and proteases that survive pasteurization and ultra-high temperature (UHT) processing. Post-processing enzymatic activity results in age gelation, bitterness, and rancidity in UHT milk and cream during storage.
Souring of milk is caused by LAB that ferment lactose to lactic acid, reducing pH and causing casein coagulation. This process is desirable for fermented dairy products but constitutes spoilage when uncontrolled in fluid milk. The typical souring sequence in raw milk at ambient temperatures involves Streptococcus spp. initially, followed by more acid-tolerant Lactobacillus spp. In pasteurized milk, post-contamination by psychrotrophic Gram-negative rods (Pseudomonas, Enterobacter) is the primary spoilage route, producing proteolysis and off-flavors before sufficient acidification occurs for coagulation.
Cheese spoilage manifests in several forms depending on the cheese type and storage conditions. Proteolysis by contaminating bacteria or excessive residual coagulant can lead to bitterness and textural breakdown. Lipolysis by lipases from contaminating molds and psychrotrophic bacteria produces free fatty acids that cause rancidity (butyric, caproic, caprylic, capric acids). Surface spoilage of hard cheeses by molds (Penicillium, Aspergillus, Mucor) is controlled by cleaning, waxing, or vacuum packaging. Soft-rot spoilage of high-moisture cheeses is often caused by Gram-negative rods (Pseudomonas, Flavobacterium) that degrade proteins.
Yeast spoilage (Debaryomyces hansenii, Candida, Kluyveromyces marxianus) is common in fermented dairy products such as yogurt and sour cream, where low pH inhibits bacteria but selects for yeasts. Gas production leads to swollen containers and yeasty off-flavors. Shelf-life extension of dairy products relies on good hygienic practices throughout production, rapid cooling, cold chain maintenance, and the use of protective cultures (LAB producing bacteriocins) or carbon dioxide addition. Predictive shelf-life models based on initial psychrotrophic counts and storage temperature are used in the dairy industry for quality management. Dairy spoilage is often caused by psychrotrophic organisms that produce heat-stable enzymes. Proper pasteurization eliminates pathogens but not all spoilage organisms. The microbiology of fermented dairy involves controlled fermentation to prevent spoilage.
