Microbiological and chemical changes in high-pressure-treated milk during refrigerated storage

The microbiological and biochemical changes during storage of high-pressure-treated (400 MPa at 25 degrees C, for 30 min) whole (3.5% fat) and skim (0.3% fat) milk at refrigeration temperatures (7 degrees C) were studied. From a microbiological point of view, high-pressure treatment of milk led to an increase in the shelf life because, after 45 days of refrigerated storage, the psychrotrophic and pseudomonad counts of the pressurized milk were lower than those of the unpressurized milk after 15 days. Capillary electrophoresis of the casein fraction showed that proteolysis by bacterial proteinases was not relevant in high-pressure-treated milk, as evidenced by a negligible degradation of K-casein. However, since the pressure conditions assayed did not lead to plasmin inactivation, considerable beta-, alpha(S2)- and alpha(S1)-casein hydrolysis took place during refrigerated storage, which can be responsible for flavor defects. No significant differences were found between skim and whole high-pressure-treated milks.