Pseudomonas fluorescens i melk: Vekst og metabolisme under lagring ved ulike temperaturer, med spesielt fokus på proteolytisk aktivitet.

Abstract

Milk is a biological and nutritious product particularly susceptible to microbial degradation if not handled properly. One type of psychrotrophic bacteria often associated with microbial spoilage 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 them may produce heat stable extracellular enzymes such as proteinases and lipases. These attack and degrade proteins and fats respectively, which can lead to undesirable changes in milk, for example 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 have increased opportunities for growth and enzyme production, where especially the latter can cause problems, despite pasteurization. In addition, contamination of the milk after pasteurization can be problematic. In this study, it was desirable to investigate growth, metabolic activity and proteolytic activity in milk during storage at three different temperatures, by two different strains of P. fluorescens. If possible, this was supposed to be seen in relation to the practice in the dairy industry. In this study both the two P. fluorescens strains ATCC 13525 and ATCC 17400 showed good growth 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. In milk with growth of strain B at 22 °C it was also possible to observe the biggest changes in the content of organic acids, free amino acids and volatiles. It became evident that both temperature and strain were important for the pattern that evolved in the milk. This points out the 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.