High-pressure homogenisation of raw bovine milk. Effects on fat globule size distribution and microbial inactivation
Résumé
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 Tin (4°C, 14°C or 24°C) on milk temperature T2 at the HP valve outlet, on fat globule size distribution and on the reduction of the endogenous flora were investigated. The Tin values of 4–24°C led to milk temperatures of 14–33°C before the HP valve, mainly because of compression heating. High Tin and/or homogenisation pressure decreased the fat globule size. At 200 MPa, the d4.3 diameter of fat globules decreased from 3.8±0.2 (control milk) to 0.80±0.08 μm, 0.65±0.10 or 0.37±0.07 μm at Tin=4, 14°C or 24°C, respectively. A second homogenisation pass at 200 MPa (Tin=4°C, 14°C or 24°C) further decreased d4.3 diameters to about 0.2 μm and narrowed the size distribution. At all Tin 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 1 log cycle, provided homogenisation pressure was ⩾200 MPa at Tin=24°C (T2∼60°C), 250 MPa at Tin=14°C (T2∼62°C), or 300 MPa at Tin=4°C (T2∼65°C). At all Tin 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.