Discipline: contamination; Keywords: Pseudomonas fluorescens , psychrotrophic bacteria, refrigeration temperature, unhygienic surfaces, biofilms, milk flocculation, UHT milk.                 

Pseudomonas is a highly competitive bacterial genus that is generally found in the environment.  They also act as opportunistic pathogens of both animals and humans. In the dairy environment it can cause a damaging form of mastitis in dairy cows and is often resistant to a wide spectrum of antibiotics which makes it difficult to treat. Ps aeruginosa, is not a factor in causing spoilage of refrigerated milk due to its high optimum growth temperature. Another species, namely Ps fluorescens, forms part of the psychrotrophic group of bacteria that can grow at refrigeration temperatures and is notorious for causing spoilage defects in milk.  Most studies have found that Ps. fluorescens is the most damaging member of the psychrotrophic bacterial group.

Psychrotrophic bacteria are not part of the normal udder microflora, so the numbers present in raw milk are related to unhygienic surfaces of pipelines and other milk contact surfaces. The presence of psychrotrophs have an economic impact on the global dairy industry, causing significantly negative effects on milk yield and limiting the shelf-life of milk and dairy products. They are also notorious for being able to form biofilms on milk contact surfaces.

Ps fluorescens produces heat-stable proteolytic enzymes that attack and destabilise the casein protein in milk causing the milk to flocculate/precipitate with the alizarol platform test.  It had previously been determined that milk flocculating with this test, would possibly result in protein precipitating on heated surfaces in the production of Ultra High Temperature (UHT) milk.  This would cause fouling of the expensive heating equipment involved.  Even if the milk does not flocculate, excessive levels of proteolytic enzymes in the milk are able to survive UHT temperatures.  In a long-life product such as UHT milk, these enzymes can, over time at room temperature, cause a defect in the UHT milk known as gelation. This results in custard-like thickening of the milk or precipitation of milk when added to hot beverages such as coffee or tea.

A study at the UFS showed that as the psychrotrophic and especially the Pseudomonas count increased, the time that the milk could be stored at 7oC before flocculation dramatically decreased.  Ps. fluorescens, was found to be the prevalent organisms at the time of flocculation. Therefore, to ensure good shelf-life of the raw milk, it is essential that the psychrotrophic count and especially the Pseudomonas count in the raw milk be kept as low as possible, preferably below 5000 cfu per ml.


P.J. Jooste, 2020. Pseudomonas, a continuing thorn in the flesh of the dairy industry. Article prepared for The Dairy Mail, 12 November 2020.