The relationship between numbers of Pseudomonas bacteria in milk used to manufacture UHT milk and the effect on product quality

The definition of “quality” raw milk for UHT milk manufacture is not well defined. The UHT process produces a commercially sterile product. However, some proteases produced by bacteria from the raw milk survive UHT treatment, limiting the shelf life of UHT milk. To provide some information on the microbiological quality limits for raw milk for the manufacture of UHT products, bacterial isolates (Pseudomonas) showing heat-stable proteolytic activity and commonly found in raw milk were grown to different numbers (4–7 log cfu mL−1) in UHT milk and then the milk was UHT treated again. The storage life of this product was monitored over different temperatures (20, 30 and 55 °C) and time periods (up to 9 months). With this information guidelines for the manufacture of UHT milk with a specific shelf life can be made based on the initial microbiological content of the raw milk.