A study of changes in the psychrotrophic microflora of raw milk during refrigerated storage

Psychrotrophs were isolated from raw milk after 1, 3 and 5 days storage at 6 degreesC. Using impedance the lag phase duration (LPD) and maximum growth rates (MGR) were calculated at 21 and 6 degreesC. At 21 degreesC there was no significant (p > 0.05) difference between either the LPD or MGR values for day 1, 3 or 5 isolates. At 6 degreesC, however, although there was no s ignificant (p > 0.05) difference in MGR values there was a significant diff erence (p < 0.05) in LPD values with day 1 > day 3 > day 5. In respect of p roteinase and lipase activity there was also a significant difference (p < 0.05) in the production of both enzymes with day 1 > day 3 > day 5. This st udy indicates that as raw milk is stored there is a trend in the psychrotro phic microflora towards those with a shorter lag phase and less enzyme prod uction. It also demonstrates the lack of a causal link between hydrolytic a ctivity and selective growth advantage amongst raw milk psychrotrophs with the genetic competence to produce extracellular proteinases and lipases.