Potential applications of high pressure homogenisation in processing of liquid milk

Studies of the potential of high pressure homogenisation (HPH) for the combined pasteurisation/homogenisation of raw bovine milk were undertaken. Raw milk was preheated to 45 degreesC and HPH-treated at 150, 200 or 250 MPa; milk outlet temperature at these pressures were 67, 76.8 and 83.6 degreesC, respectively, with a holding time of similar to20 s. Raw and commercially pasteurized and homogenized (CPH) milk samples were analysed as controls. Fat globules in HPH samples were approximately half the size of those in CPH samples, although differences were not significant (P>0.05). P-Lactoglobulin was denatured at pressures greater than or equal to150MPa, although little denaturation of alpha-lactalbumin was observed. Numbers of psychrotrophic bacteria in raw milk were reduced by 2.73 log cycles by HPH at 150 MPa and were uncountable following HPH at 200 or 250 MPa. Mesophilic bacterial counts were reduced by 1.30, 1.83 and 3.06 log cycles by HPH at 150, 200 or 250 MPa, respectively. No viable Staphylococcus aureus nor coliform cells remained in any HPH milk samples. HPH did not affect the colour of milk and HPH samples did not cream during refrigerated storage. The activities of plasmin, alkaline phosphatase and lactoperoxidase in milk were all greatly reduced by HPH. Pseudomonas fluorescens, inoculated into milk (similar to10(6) cfu/ml), was reduced to undetectable levels by HPH at 200 MPa (milk inlet temperature, similar to10 degreesC); however, Ps. fluorescens proteinase was quite resistant to HPH under such conditions. Overall, owing to the significant increase in temperature and the possibility of varying the holding time, there may be potential applications for HPH as a novel liquid milk processing technique, combining many advantages of conventional homogenization and pasteurization of milk in a single process.