The effect of ultra-high pressure homogenization (UHPH) on microbial and physicochemical shelf life of milk during storage at 4 degrees C was studied and compared with a conventional heat preservation technology used in industry. Milk was standardized at 3.5% fat and was processed using a Stansted high-pressure homogenizer. High-pressure treatments applied were 100, 200, and 300 MPa (single stage) with a milk inlet temperature of 40 degrees C, and 200 and 300 MPa (single stage) with a milk inlet temperature of 30 degrees C. The UHPH-treated milks were compared with high-pasteurized milk (PA; 90 degrees C for 15 s). The microbiological quality was studied by enumerating total counts, psychrotropic bacteria, lactococci, lactobacilli, enterococci, coliforms, spores, and Pseudomonas. Physicochemical parameters assessed in milks were viscosity, color, pH, acidity, rate of creaming, particle size, and residual peroxidase and phosphatase activities. Immediately after treatment, UHPH was as efficient (99.99%) in reducing psychrotrophic, lactococci, and total bacteria as was the PA treatment, reaching reductions of 3.5 log cfu/mL. Coliforms, lactobacilli, and enterococci were eliminated. Microbial results of treated milks during storage at 4 degrees C showed that UHPH treatment produced milk with a microbial shelf life between 14 and 18 d, similar to that achieved for PA milk. The UHPH treatments reduced the L* value of treated milks and induced a reduction in viscosity values of milks treated at 200 MPa compared with PA milks; however, these differences would not be appreciated by consumers. In spite of the fat aggregates detected in milks treated at 300 MPa, no creaming was observed in any UHPH-treated milk. Hence, alternative methods such as UHPH may give new opportunities to develop fluid milk with an equivalent shelf life to that of PA milk in terms of microbial and physicochemical characteristics.

High isostatic pressures up to 600 MPa were applied to samples of skim milk before addition of rennet and preparation of cheese curds. Electron microscopy revealed the structure of rennet gels produced from pressure-treated milks. These contained dense networks of fine strands, which were continuous over much bigger distances than in gels produced from untreated milk, where the strands were coarser with large interstitial spaces. Alterations in gel network structure gave rise to differences in rheology with much higher values for the storage moduli in the pressure-treated milk gels. The rate of gel formation and the water retention within the gel matrix were also affected by the processing of the milk. Casein micelles were disrupted by pressure and disruption appeared to be complete at treatments of 400 MPa and above. Whey proteins, particularly β-lactoglobulin, were progressively denatured as increasing pressure was applied, and the denatured β-lactoglobulin was incorporated into the rennet gels. Pressure-treated micelles were coagulated rapidly by rennet, but the presence of denatured β-lactoglobulin interfered with the secondary aggregation phase and reduced the overall rate of coagulation. Syneresis from the curds was significantly reduced following treatment of the milk at 600 MPa, probably owing to the effects of a finer gel network and increased inclusion of whey protein. Levels of syneresis were more similar to control samples when the milk was treated at 400 MPa or less.

The effects of single- or 2-stage ultra-high pressure homogenization (UHPH; 100 to 330 MPa) at an inlet temperature of 30 degrees C on the cheese-making properties of bovine milk were investigated. Effects were compared with those from raw, heat-pasteurized (72 degrees C for 15 s), and conventional homogenized-pasteurized (15 + 3 MPa, 72 degrees C for 15 s) treatments. Rennet coagulation time, rate of curd firming, curd firmness, wet yield, and moisture content of curds were assessed. Results of particle size and distribution of milk, whey composition, and gel microstructure observed by confocal laser scanning microscopy were analyzed to understand the effect of UHPH. Single-stage UHPH at 200 and 300 MPa enhanced rennet coagulation properties. However, these properties were negatively affected by the use of the UHPH secondary stage. Increasing the pressure led to higher yields and moisture content of curds. The improvement in the cheese-making properties of milk by UHPH could be explained by changes to the protein-fat structures due to the combined effect of heat and homogenization.