V. Ferragut

High-pressure treatment of milk: effects on casein micelle structure and on enzymic coagulation.

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 beta-lactoglobulin, were progressively denatured as increasing pressure was applied, and the denatured beta-lactoglobulin was incorporated into the rennet gels. Pressure-treated micelles were coagulated rapidly by rennet, but the presence of denatured beta-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.

Application of high pressure treatment for cheese production

Abstract High hydrostatic-pressure treatment offers the food industry a new technology for food preservation. Interest in high pressure application on milk has recently increased. Pressures between 300 and 600 MPa can inactivate microorganisms including most infectious food-borne pathogens. In addition to microbial destruction, it has been reported that high pressure improves rennet or acid coagulation of milk and increases cheese yield. A lot of work has been published on microorganism inactivation, denaturation of whey proteins, changes in the mineral distribution and coagulating properties on model or real milk systems. However, practical applications of high pressure treatment in the dairy industry have received little attention. This paper examines recent work in this area and summarizes parts of our ongoing work toward the development of high pressure applications for the cheese industry.

Effect of High Hydrostatic Pressure on Listeria innocua 910 CECT Inoculated into Ewe's Milk

Ewes milk standardized to 6% fat was inoculated with Listeria innocua 910 CECT at a concentration of 10(7)CFU/ml and treated by high hydrostatic pressure. Treatments consisted of combinations of pressure (200, 300, 350, 400, 450, and 500 MPa), temperature (2, 10, 25, and 50 degrees C), and time (5, 10, and 15 min). To determine numbers of L. innocua, listeria selective agar base with listeria selective supplement and plate count agar was used. Low-temperature (2 degrees C) pressurizations produced higher L. innocua inactivation than treatments at room temperatures (25 degrees C). Pressures between 450 and 500 MPa for 10 to 15 min were needed to achieve reductions of 7 to 8 log units. The kinetics of destruction of L. innocua were first order with D-values of 3.12 min at 2 degrees C and 400 MPa and 4 min at 25 degrees C and 400 MPa. A baroprotective effect of ewes milk (6% fat) on L. innocua was observed in comparison with other studies using different media and similar pressurization conditions.

Influence of Pressurization on Goat Milk and Cheese Composition and Yield

Fresh goat cheese was made from pasteurized (72 °C, 15 s) or HP- treated milk (500 MPa, 15 min, 20 °C). Microbiological quality (i.e. mesophilic aerobic bacteria and enterobacteria) of pressurized milk was comparable to pasteurized milk. Pasteurized and HP-treated milks had different pHs and non-casein nitrogen. During the curd formation, the pH fell faster in pasteurized milk than in HP-treated milk and there was a slightly larger amount of milk fat separated from the curd whey in HP- treated milk. HP treatment reduced the level of whey expelled from the curd making the whey removal process more difficult, and so the time involved in the cheesemaking was increased. The cheese made from HP-treated milk had higher levels of moisture and salt, but a slightly lower fat content than pasteurized milk. Additional retention of whey protein and, especially, the greater moisture led to improvements in cheese yield from HP-treated milk.

Ripening control of Manchego type cheese salted by brine vacuum impregnation

Abstract A new salting procedure was tested in Manchego type cheeses in order to shorten this processing step by decreasing the initial salt gradient in the cheese. This procedure was based on a fast mass transfer mechanism (the hydrodynamic mechanism) which acts when pressure differences are applied to porous products immersed in a liquid phase. The effective porosity of the studied cheese was 0.049 as established by the hydrodynamic mechanism. This value represents the introduction of 1.03 g NaCl 100 g −1 of cheese in the cheese pores, when a 19% ( w w ) brine was used, at 37mbar (absolute) of vacuum pressure. This quantity agreed with the salt content in this experiment. The duration of the new salting process step was 2 h. The hydrodynamic mechanism is based on the supposition that salt ions penetrate the internal part of the cheese through pores. Water loss during ripening was controlled by the external drying rate for the cheese. Vacuum-impregnated cheeses always had a higher water content as a consequence of lower drying rates during the brine immersion period. Nevertheless, the differences in water content neither affected the change in pH nor the ripening process during the maturation of the cheese.

Effects of ultra-high-pressure homogenization treatment on the lipolysis and lipid oxidation of milk during refrigerated storage.

Free fatty acid (FFA) release and quantification and lipid oxidation extent of ultra-high-pressure homogenized (UHPH) milk samples were evaluated to assess the effect of UHPH on the susceptibility of milk lipids to lipolysis and oxidation. Milk was UHPH-treated at 200 and 300 MPa with inlet temperatures of 30 and 40 degrees C. UHPH-treated samples were compared to high-pasteurized milk (PA; 90 degrees C, 15 s). Results showed that all FFA increased significantly during storage only in 200 MPa samples. Lipid oxidation was measured as an accumulation of lipid hydroperoxides as the primary oxidation product and malondialdehyde and hexanal as the secondary oxidation products. Samples treated at 300 MPa presented higher malondialdehyde and hexanal content compared to 200 MPa treated-samples and to PA milk.

Changes in microstructural, textural and colour characteristics during ripening of Manchego-type cheese salted by brine vacuum impregnation

A new salting procedure which was published in a previous paper on Manchego-type cheese is now studied in order to evaluate how it affects physical properties of cheeses during ripening. Textural characteristics were analysed within the ripening period in two cheese areas (internal and medium) by means of the uniaxial compression and the stress relaxation test. Cheese microstructure was assessed by confocal laser scanning microscopy. Colour values were also evaluated. Cheeses salted by brine vacuum impregnation were less fracturable and more elastic than conventional brine salted cheeses. This is a result of microstructural changes induced by the pressure gradients imposed on the system. Vacuum impregnated cheeses showed a compact and homogeneous protein matrix with small fat globules well dispersed. Conventional brine immersed cheeses showed a more irregular protein network, with larger pores and relatively large fat aggregates.

Comparing the effects of ultra-high-pressure homogenization and conventional thermal treatments on the microbiological, physical, and chemical quality of almond beverages.

UNLABELLED The effects of ultra-high-pressure homogenization (UHPH) at 200 and 300 MPa, in combination with different inlet temperatures (55, 65, and 75 °C) on almond beverages with lecithin (AML) and without lecithin (AM), were studied. UHPH-treated samples were compared with the base product (untreated), pasteurized (90 °C, 90 s), and ultra-high-temperature (UHT, 142 °C, 6 s) samples. Microbiological analysis, physical (dispersion stability, particle size distribution, and hydrophobicity), and chemical (hydroperoxide index) parameters of special relevance in almond beverages were studied. Microbiological results showed that pressure and inlet temperature combination had a significant impact on the lethal effect of UHPH treatment. While most UHPH treatments applied produced a higher quality of almond beverage than the pasteurized samples, the combination of 300 MPa and 65 and/or 75 °C corresponded to a maximum temperature after high pressure valve of 127.7 ± 9.7 and 129.3 ± 12.6 °C, respectively. This temperature acted during less than 0.7 s and produced no bacterial growth in almond beverages after incubation at 30 °C for 20 d. UHPH treatments of AML samples caused a significant decrease in particle size, resulting in a high physical stability of products compared to conventional heat treatments. UHPH treatment produced higher values of hydroperoxide index at day 1 of production than heat-treated almond beverage. Hydrophobicity increased in AML-UHPH-treated samples compared to AM and conventional treatments. PRACTICAL APPLICATION  Ultra-high-pressure-homogenization (UHPH) is an emerging technology, a potential alternative to conventional heat treatments. It is a simple process consisting of single step. When liquid food (almond beverage in this study) passes through the high-pressure valve, a very good stability and reduction of microorganisms is achieved, both effects due to the particle breakdown. Specific UHPH conditions could produce commercial sterilization of almond beverage.

Ultra-high pressure homogenization-induced changes in skim milk: impact on acid coagulation properties.

The effects of ultra-high pressure homogenization (UHPH) on skim milk yogurt making properties were investigated. UHPH-treated milk was compared with conventionally homogenised (15 MPa) heat-treated skim milk (90 degrees C for 90 s), and to skim milk treated under the same thermal conditions but fortified with 3% skim milk powder. Results of the present study showed that UHPH is capable of reducing skim milk particle size which leads to the formation of finer dispersions than those obtained by conventional homogenisation combined with heat treatment. In addition, results involving coagulation properties and yogurt characteristics reflected that, when increasing UHPH pressure conditions some parameters such as density of the gel, aggregation rate and water retention are improved.

Proteolysis of yogurts made from ultra-high-pressure homogenized milk during cold storage.

Proteolysis was investigated in yogurts made from milk that was ultra-high-pressure homogenized at 200 or 300 MPa and at 30 or 40 degrees C and compared with those produced from heat-treated milk containing 3% skim milk powder. To evaluate changes in the protein fraction, samples were analyzed at d 1, 7, 14, 21, and 28 of storage for residual caseins, peptides, and total free amino acids. Results showed that yogurts from heat-treated milk and 300 MPa-treated milk presented similar levels of residual caseins, as well as similar profiles of soluble peptides and total free amino acids. On the contrary, greater amounts of hydrophobic peptides were detected in yogurts made from 200 MPa-treated milk at both 30 and 40 degrees C, especially at the end of storage. In all treatments studied, caseins were hydrolyzed and hydrophobic peptides were increased during storage, as reflected by the increase in soluble nitrogen at the end of the storage.