Date

2013

Author

Güzel, Burçin Hülya

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The inactivation of Listeria monocytogenes and Escherichia coli suspended in UHT treated apple and orange juices by ultrasound under pressure at nonlethal (Manosonication-MS) and lethal temperatures (Manothermosonication-MTS) was evaluated. Significant differences were found among the MS resistance (35 °C, 110 μm, 200 kPa) of five strains of L. monocytogenes and three of E. coli in pH 3.5 buffer, being L. monocytogenes STCC 5672 and E. coli O157:H7 the most resistant strains. Regarding the interspecific differences, L. monocytogenes showed higher MS resistance than E. coli. Although pH and treatment medium composition did not significantly change the bacterial MS resistance, the effectiveness of ultrasound increased by both raising the amplitude of ultrasonic waves and the pressure. The energy transmitted to the fruit juices by ultrasonic waves at different combinations of amplitudes (46.5, 90, 110, and 130.5 μm) and pressures (0, 100, and 200 kPa) was also studied, obtaining an exponential relationship between DMS values and power input: an increase of 116 W increased the inactivation rate approximately 10-fold in both juices. The MS resistance of both species decreased when heat was applied jointly with ultrasound (MTS), which was more effective on inactivating L. monocytogenes and E. coli than the sum of MS and heat acting simultaneously but independently. Therefore, MTS showed a synergistic lethal effect in acidic juices, whose magnitude was dependent on the treatment conditions. The inactivation of polyphenoloxidase (PPO) in freshly squeezed apple juice and pectinmethlyesterase (PME) in freshly squeezed orange juice by ultrasound under pressure at nonlethal (MS) and lethal temperatures (MTS) was evaluated. The temperature profiles of enzyme inactivation and the energy transmission in the fruit juices by ultrasonic waves at different combinations of amplitudes (90, 110, 130.5 μm) and pressures (0, 100, 200 kPa) were also studied. Amplitude and pressure had significant effects (p ≤ 0.05) on the final temperatures of acidic fruit juices. An exponential relationship between DMS values and power input was obtained and an increase of 113 W in the energy transferred into the freshly squeezed apple and orange juices by US will make the inactivation rate of PPO and PME increase by 10 times in both juices. Both increasing the amplitude and the pressure caused an increase in the lethality of MS treatment in both freshly squeezed fruit juices. The comparison of the inactivation by thermal treatment (TT) with the inactivation by MS treatment showed that the combined process (MTS) was more efficient on reduction of enzyme activity than TT acting alone. MS was less effective on the inactivation of PPO enzyme than heat and MTS. Thermal inactivation was more effective on the inactivation of PPO in apple juice during MTS; contrary to MS and MTS process, which were more effective on the inactivation of PME in orange juice than on the inactivation of heat, because of the high thermo-stability of PME. Overall, the ultrasonic resistance of PME in orange juice was much higher than the PPO in apple juice. According to treatment conditions, MTS treatment showed a synergistic lethal effect in the acidic fruit juices. The magnitude of the synergistic effect had almost the same values for both enzymes.

Subject Keywords

Pathogenic microorganisms., Escherichia coli., Apple juice., Orange juice.

URI

http://etd.lib.metu.edu.tr/upload/12616507/index.pdf
https://hdl.handle.net/11511/22870

Collections

Graduate School of Natural and Applied Sciences, Thesis