:  Bacteria in milk have the ability to adhere and aggregate on stainless steel surfaces, resulting in biofilm formation in milk storage tanks and milk process lines. Growth of biofilms in milk processing environments leads to increased opportunity for microbial contamination of the processed dairy products. These biofilms may contain spoilage and pathogenic microorganisms. Bacteria within biofilms are protected from sanitizers due to multispecies cooperation and the presence of extracellular polymeric substances, by which their survival and subsequent contamination of processed milk products is promoted. This paper reviews the most critical factors in biofilm formation, with special attention to pseudomonads, the predominant spoilage bacteria originating from raw milk. Biofilm interactions between pseudomonads and milk pathogens are also addressed, as emerging risks and future research perspectives, specifically related to the milk processing environment.

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The refrigerated storage of raw milk throughout the dairy chain prior to heat treatment creates selective conditions for growth of psychrotolerant bacteria. These bacteria, mainly belonging to the genus Pseudomonas, are capable of producing thermoresistant extracellular proteases and lipases, which can cause spoilage and structural defects in pasteurized and ultra-high-temperature-treated milk (products). To map the influence of refrigerated storage on the growth of these pseudomonads, milk samples were taken after the first milking turn and incubated laboratory scale at temperatures simulating optimal and suboptimal preprocessing storage conditions. The outgrowth of Pseudomonas members was monitored over time by means of cultivation-independent denaturing gradient gel electrophoresis (DGGE). Isolates were identified by a polyphasic approach. These incubations revealed that outgrowth of Pseudomonas members occurred from the beginning of the dairy chain (farm tank) under both optimal and suboptimal storage conditions. An even greater risk for outgrowth, as indicated by a vast increase of about 2 log CFU per ml raw milk, existed downstream in the chain, especially when raw milk was stored under suboptimal conditions. This difference in Pseudomonas outgrowth between optimal and suboptimal storage was already statistically significant within the farm tank. The predominant taxa were identified as Pseudomonas gessardii, Pseudomonas gessardii-like, Pseudomonas fluorescens-like, Pseudomonas lundensis, Pseudomonas fragi, and Pseudomonas fragi-like. Those taxa show an important spoilage potential as determined on elective media for proteolysis and lipolysis.

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