MICROBIOLOGICAL AND CHEMICAL-CHANGES IN HIGH-PRESSURE-TREATED MILK DURING REFRIGERATED STORAGE

The microbiological and biochemical changes during storage of high-pre ssure-treated (400 MPa at 25 degrees C, for 30 min) whole (3.5% fat) a nd skim (0.3% fat) milk at refrigeration temperatures (7 degrees C) we re studied. From a microbiological point of view, high-pressure treatm ent of milk led to an increase in the shelf life because, after 45 day s of refrigerated storage, the psychrotrophic and pseudomonad counts o f the pressurized milk were lower than those of the unpressurized milk after 15 days. Capillary electrophoresis of the casein fraction showe d that proteolysis by bacterial proteinases was not relevant in high-p ressure-treated milk, as evidenced by a negligible degradation of K-ca sein. However, since the pressure conditions assayed did not lead to p lasmin inactivation, considerable beta-, alpha(S2)- and alpha(S1)-case in hydrolysis took place during refrigerated storage, which can be res ponsible for flavor defects. No significant differences were found bet ween skim and whole high-pressure-treated milks.