Diversity and spoilage potential of Pseudomonas spp. from Spanish milk and dairy products: Impact on fresh cheese and milk quality

Abstract

Psychrotrophic Pseudomonas spp. contaminate milk and dairy products, producing heat-resistant enzymes and pigments that reduce their shelf life. Identifying spoiling strains is essential for tracing contamination and improving preservation. This study examines 208 Pseudomonas spp. isolates from Spanish dairy products, assessing their genetic diversity and spoilage potential in vitro and under simulated storage. Pulsed-field gel electrophoresis (PFGE) identified 108 distinct strains, clustered into 70 groups (≥ 80 % similarity), showing high diversity. Gene sequencing (ileS or rpoD) classified the strains into 20 species, with P. fluorescens (19 %), P. fragi (16 %), P. lundensis (12 %), and P. shahriarae (6 %) being predominant. P. shahriarae, P. atacamensis, P. salmasensis, P. solani, and P. canadensis were isolated from milk or dairy products for the first time. In fresh cheese, 89 % of Pseudomonas spp. caused discoloration (fluorescent yellow, creamy, orange, and blue) after 7 days of cold storage. In refrigerated milk, 48 % of strains showed significant proteolysis after 5 days, with OPA (o-Phthaldialdehyde-based method) values > 0.274, especially some P. fluorescens, P. gessardii, P. fulva, P. shahriarae, Pseudomonas spp., and P. koreensis strains. After simulated UHT treatment and accelerated storage, 75 % of strains retained thermostable proteolytic activity, especially P. fluorescens, P. proteolytica, P. shahriarae, and P. fulva strains. The aprX gene, coding for a thermostable protease, was present in 57 % of strains, suggesting other proteases may also be produced. Overall, the isolated Pseudomonas spp. led to different spoilage patterns during milk and fresh cheese storage, emphasizing the need for specific strain identification to improve preservation strategies.