Application to Juices

PEF research had been mostly focused on the inactivation of microorganisms suspended in foods, including semisolid and liquid foods such as pea soup, milk, liquid eggs, and juices, particularly orange and apple juices. Sitzmann [67] obtained a 3 log10 reduction of native microbiota for freshly squeezed orange juice using a continuous PEF process with an electric field of 15kV/cm. There was no significant change in quality. Zhang et al. [68] found that total aerobic counts of reconstituted orange juice were reduced 3- to 4-log10 cycles when treated with an integrated PEF pilot plant system operating at less than 32 kV/ cm. Raso et al. [69] investigated PEF inactivation of ascospores and vegetative cells of Zygosaccharomyces bailii suspended in apple, orange, pineapple, cranberry, and grape juices. Two pulses of 32 to 36.5kV/cm decreased the population of vegetative cells or ascospores 3.5 to 5log10 cycles for each fruit juice studied. Evrendilek et al. [70] treated fresh apple juice inoculated with E. coli O157:H7 and E. coli 8739 using bipolar PEF. A 5log10 reduction was obtained for each culture when the treatment temperature was below 35°C. The lethality for fresh apple cider inoculated with E. coli O157:H7 was also reported for PEF treatment with instant charge reversal pulses [71]. In orange juice, McDonald et al. [72] inactivated Leuconostoc mesenteroides, E. coli, and L. innocua by as much as 5log10 cycles at 30kV/cm and 50°C. A maximum of 2.5log10 cycle reduction was achieved for Saccharomyces cerevisiae ascospores at 50 kV/cm and 50° C.

The synergy of PEF, pH, water activity, ionic strength, temperature, antimicrobial agents (e.g., nisin, lysozyme), and other combinations of hurdle technology (ozone treatment or high hydrostatic pressure) can increase microorganism inactivation [73-75]. Using the hurdle approach, Hodgins et al. [76] studied the effect of temperature, acidity, and number of pulses on microbial inactivation in orange juice. A 6log10 reduction in the natural microbiota was obtained under optimal conditions consisting of 20 pulses of an electric field of 80 kV/cm, at pH 3.5, and a temperature of 44°C with a dose of 100 U nisin/ml. The process was most influenced by a change in temperature (p < 0.0001). There was a 97.5% retention of vitamin C along with a 92.7% reduction in pectinmethylesterase activity after PEF treatment. The shelf life of the orange juice was at least 28 days when stored at 4°C without aseptic packaging. Gas chromatography revealed no significant differences in aroma compounds before and after pulsing.

Liang et al. [77] applied PEF to pasteurized and freshly squeezed orange juices (with and without pulp) and determined the reduction of Salmonella Typhimurium at moderately high temperatures (<60°C). The effect of antimicrobial compounds (nisin and lysozyme) was examined. PEF treatment (90 kV/cm and 20 pulses) did not have a notable effect on cell viability or injury until the temperature reached 46° C or above. Presence of nisin, lysozyme, or a mixture of nisin and lysozyme increased cell viability loss by an additional 0.04 to 2.75log10 cycles for PEF treatment. The combination of nisin and lysozyme had a more pronounced bactericidal effect than did either nisin or lysozyme alone.

The inactivation of enzymes also accompanies pasteurization of juices using PEF technology. In general, higher electric field strengths and longer total treatment times are more effective for the purpose of enzyme inactiva-tion [78]. Decrease of polyphenoloxidase (PPO) activity in peach juices was reported to follow an exponential decay kinetic model [79]. For those orange juices that had a similar shelf life (196 days at 4°C) after thermally processing (90°C and 90 seconds) and PEF processing (40kV/cm and 97 milliseconds), ascorbic acid, flavor, and color of PEF-treated juice were found to be superior to that of thermally processed juice (P < 0.05) [78]. This is also true for tomato juices processed either by PEF at 40 kV/cm (57 milliseconds) or thermally processed (92° C and 90 seconds) and stored at 4°C for 112 days [80]. In both cases, sensory evaluations indicated that the flavor of PEF-processed juices was preferred to that of thermally processed juices (P < 0.01) [78,80].

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