Psychrotrophic microorganisms are capable of growth at refrigeration temperatures (typically 0-7°C), although their optimal growth temperatures are higher (20-30°C). They are the primary spoilage agents of refrigerated foods with extended shelf life. The most significant psychrotrophic spoilage genus is Pseudomonas, encompassing P. fluorescens, P. fragi, and P. lundensis. These are Gram-negative, aerobic, motile rods that produce extracellular proteases and lipases that degrade food components, leading to off-odors, off-flavors, and textural defects.
Pseudomonas species dominate the spoilage microbiota of aerobically stored fresh meat, poultry, fish, and milk at refrigeration temperatures due to their short generation times at low temperatures and their ability to utilize a wide range of low-molecular-weight substrates. Sensory changes include the production of fruity esters (ethyl esters), sulfurous compounds (H₂S, methanethiol), and volatile amines. The characteristic “sweet, fruity” off-odor of spoiled raw milk is attributable to P. fragi, while P. fluorescens is known for its heat-stable lipases that cause rancidity in pasteurized dairy products.
Shewanella putrefaciens is a Gram-negative, motile rod that reduces trimethylamine N-oxide (TMAO) to trimethylamine (TMA), the compound responsible for the characteristic “fishy” odor of spoiled seafood. It is also a significant spoilage organism of aerobically stored chilled chicken and beef. Brochothrix thermosphacta is a Gram-positive, facultatively anaerobic rod that spoils vacuum-packaged meats and modified-atmosphere-packaged products. It produces a range of metabolic end products including lactic acid, acetoin, diacetyl, and 2-methylbutanol, contributing to sour, cheesy, or dairy-like off-flavors.
Shelf-life testing relies on enumeration of specific spoilage organisms (SSO) on selective media: Pseudomonas on cetrimide-fucidin-cephaloridine (CFC) agar, B. thermosphacta on streptomycin sulfate thallous acetate actidione (STAA) agar, and S. putrefaciens on iron agar for H₂S production. Modified atmosphere packaging (MAP) with elevated CO₂ (20-50%) inhibits Pseudomonas growth and shifts the spoilage microbiota toward facultative anaerobes such as LAB and B. thermosphacta, thereby extending shelf life. Predictive microbiology models (e.g., the square root model for temperature dependence) are used to estimate remaining shelf life under dynamic temperature conditions. Psychrotrophic spoilage is a major concern for refrigerated foods, affecting dairy products and meat and seafood. Modified atmosphere packaging can inhibit these organisms and extend shelf-life.
