High-pressure homogenisation of raw bovine milk. Effects on fat globule size distribution and microbial inactivation

Whole raw milk was processed using a 15 L h(-1) homogeniser with a high-pressure (HP) valve immediately followed by a cooling heat exchanger. The influence of homogenisation pressure (100-300 MPa) and milk inlet temperature T-in (4degreesC, 14degreesC or 24degreesC) on milk temperature T, at the HP valve outlet, on fat globule size distribution and on the reduction of the endogenous flora were investigated. The T-in values of 4-24degreesC led to milk temperatures of 14-33degreesC before the HP valve, mainly because of compression heating. High T-in and/or homogenisation pressure decreased the fat globule size. At 200 MPa, the d(4.3) diameter of fat globules decreased from 3.8 +/- 0.2 (control milk) to 0.80 +/- 0.08 mum, 0.65 +/- 0.10 or 0.37 +/- 0.07 mum at T-in = 4, 14degreesC or 24degreesC, respectively. A second homogenisation pass at 200 MPa (T-in = 4degreesC, 14degreesC or 24degreesC) further decreased d(4.3) diameters to about 0.2 mum and narrowed the size distribution. At all T tested, an homogenisation pressure of 300 MPa induced clusters of fat globules, easily dissociated with SDS, and probably formed by sharing protein constituents adsorbed at the fat globule surface. The total endogenous flora of raw milk was reduced by more than I log cycle, provided homogenisation pressure was greater than or equal to 200 MPa at T-in = 24degreesC (T(2)similar to60degreesC) 250 MPa at T-in = 14degreesC (T(2)similar to62degreesC), or 300 MPa at T-in = 4degreesC (T(2)similar to65degreesC). At all T-in tested, a second pass through the HP valve (200 MPa) doubled the inactivation ratio of the total flora. Microbial patterns of raw milk were also affected; Gram-negative bacteria were less resistant than Gram-positive bacteria. (C) 2003 Elsevier Science Ltd. All rights reserved.