Pseudomonas fluorescens i melk: Vekst og metabolisme under lagring ved ulike temperaturer, med spesielt fokus på proteolytisk aktivitet. | Pseudomonas fluorescens in milk: growth and metabolism during storage at different temperatures, with a focus on proteolytic activity

M-MAT

English. Milk is a biological and nutritious product particularly susceptible to microbial degradation ifnot handled properly. One type of psychrotrophic bacteria often associated with microbialspoilage and impaired quality in milk and other dairy products, is Pseudomonas fluorescens.The specie is omnipresent, has the ability to multiply at low temperatures, and many of themmay produce heat stable extracellular enzymes such as proteinases and lipases. These attackand degrade proteins and fats respectively, which can lead to undesirable changes in milk, forexample in physical properties, appearance, taste and smell.Trends in the dairy industry in recent years, such as efficiency and less frequent retrieval,have resulted in prolonged cold storage of untreated milk. This means that these bacteria haveincreased opportunities for growth and enzyme production, where especially the latter cancause problems, despite pasteurization. In addition, contamination of the milk afterpasteurization can be problematic. In this study, it was desirable to investigate growth,metabolic activity and proteolytic activity in milk during storage at three differenttemperatures, by two different strains of P. fluorescens. If possible, this was supposed to beseen in relation to the practice in the dairy industry.In this study both the two P. fluorescens strains ATCC 13525 and ATCC 17400 showed goodgrowth in UHT milk, both at 4, 7 and 22 °C. The latter strain also showed strong proteolysis,mainly of casein protein by incubation at 22 °C. The proteinases were fairly heat stable. Inmilk with growth of strain B at 22 °C it was also possible to observe the biggest changes inthe content of organic acids, free amino acids and volatiles. It became evident that bothtemperature and strain were important for the pattern that evolved in the milk. This points outthe importance to strive to reduce the degree of contamination and keep a low temperature,and that good hygiene is crucial at every stage where milk is produced and processed.