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Reconstituted micellar casein concentrates and milk protein concentrates of 2.5 and 10% (wt/vol) protein concentration were subjected to high-pressure processing at pressures from 150 to 450 MPa, for 15 min, at ambient temperature. The structural changes induced in milk proteins by high-pressure processing were investigated using a range of physical, physicochemical, and chemical methods, including dynamic light scattering, rheology, mid-infrared spectroscopy, scanning electron microscopy, proteomics, and soluble mineral analyses. The experimental data clearly indicate pressure-induced changes of casein micelles, as well as denaturation of serum proteins. Calcium-binding α- and α-casein levels increased in the soluble phase after all pressure treatments. Pressurization up to 350 MPa also increased levels of soluble calcium and phosphorus, in all samples and concentrations, whereas treatment at 450 MPa reduced the levels of soluble Ca and P. Experimental data suggest dissociation of calcium phosphate and subsequent casein micelle destabilization as a result of pressure treatment. Treatment of 10% micellar casein concentrate and 10% milk protein concentrate samples at 450 MPa resulted in weak, physical gels, which featured aggregates of uniformly distributed, casein substructures of 15 to 20 nm in diameter. Serum proteins were significantly denatured by pressures above 250 MPa. These results provide information on pressure-induced changes in high-concentration protein systems, and may inform the development on new milk protein-based foods with novel textures and potentially high nutritional quality, of particular interest being the soft gel structures formed at high pressure levels.

The effect of thermosonication (TS) and pulsed electric fields (PEF) on inactivation of Staphylococcus aureus (SST 2.4) and selected quality aspects in orange juice was investigated. Conventional pasteurization (HTST, 94°C for 26 s) was used as a control. TS (10 min at 55°C) applied in combination with PEF (40 kV/cm for 150 μs) resulted in a comparable inactivation of S. aureus to that achieved by conventional HTST. TS/PEF did not affect the pH, conductivity, or°Brix and had a milder impact on the juice color than thermal treatment. Furthermore, the nonenzymatic browning index was significantly affected by HTST (P<0.05) but not by TS and PEF. Ascorbic acid retention was almost complete after TS and PEF (96.0%), but it was substantially lower (P < 0.05) after HTST (80.5%). Residual activity of pectin methyl esterase (PME) decreased as PEF field strength and treatment time increased; however, applying TS and PEF in combination left a greater residual PME activity than HTST (12.9 vs 5.0%, respectively).

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Moderate heat in combination with pulsed electric fields (PEF) was investigated as a potential alternative to thermal pasteurization of a tropical fruit smoothie based on pineapple, banana, and coconut milk, inoculated with Escherichia coli K12. The smoothie was heated from 25 degrees C to either 45 or 55 degrees C over 60 s and subsequently cooled to 10 degrees C. PEF was applied at electric field strengths of 24 and 34 kV/cm with specific energy inputs of 350, 500, and 650 kJ/L. Both processing technologies were combined using heat (45 or 55 degrees C) and the most effective set of PEF conditions. Bacterial inactivation was estimated on standard and NaCl-supplemented tryptone soy agar (TSA) to enumerate sublethally injured cells. By increasing the temperature from 45 to 55 degrees C, a higher reduction in E. coli numbers (1 compared with 1.7 log(10) colony forming units {CFU} per milliliter, P < 0.05) was achieved. Similarly, as the field strength was increased during stand-alone PEF treatment from 24 to 34 kV/cm, a greater number of E. coli cells were inactivated (2.8 compared with 4.2 log(10) CFU/mL, P < 0.05). An increase in heating temperature from 45 to 55 degrees C during a combined heat/PEF hurdle approach induced a higher inactivation (5.1 compared with 6.9 log(10) CFU/mL, respectively [P < 0.05]) with the latter value comparable to the bacterial reduction of 6.3 log(10) CFU/mL (P> or = 0.05) achieved by thermal pasteurization (72 degrees C, 15 s). A reversed hurdle processing sequence did not affect bacterial inactivation (P> or = 0.05). No differences were observed (P> or = 0.05) between the bacterial counts estimated on nonselective and selective TSA, suggesting that sublethal cell injury did not occur during single PEF treatments or combined heat/PEF treatments.

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