73C-4 |
Thermodynamic parameters for fungal inactivation during thermal and thermo-ultrasonication treatments |
A. LOPEZ-MALO1, M. Jimenez-Fernandez, E. Palou, and L. G. Rios-Casas. (1) Ingenieria Quimica y Alimentos, Universidad de las Americas-Puebla, Cholula, Puebla, 72820, Mexico
Although a combination of decimal reduction times (D values) and their dependence on temperature (z value) reveals microbial inactivation kinetics, it does not fully define heat resistance of microorganisms, especially when selected physical treatments, such as heat and ultrasound, need to be compared. Enthalpy (DH**), entropy (DS**) and Gibb’s free energy (DG**) calculations can be useful to explain such resistance differences. Inactivation kinetics of Aspergillus flavus spores suspended in laboratory media formulated at aw 0.99 or 0.95 and pH 5.5 or 3.0, during thermal treatments (52.5 - 60°C) combined with the application of 20 kHz ultrasound applied at selected amplitude values (60 — 120 microns) were evaluated and compared using D and z values, or with thermodynamic parameters DH**, DS**, and DG**. Inactivation kinetics for every condition tested were evaluated by surface platting treated samples taken at selected time intervals. Spore viability was recorded after incubation for 3-5 days at 27ºC. D values and inactivation rates were calculated from survival curves. DH**, DS**, and DG** for the inactivation of A. flavus were also calculated following activated complex (transition state) theory. For aw 0.99, increasing ultrasound amplitude and reducing pH resulted in reduced D values. At constant aw, D values decreased with pH reduction. At constant pH, D values were lower for aw 0.99 than for 0.95. Thermodynamic parameters diminished when increasing ultrasound amplitude and when decreasing aw and pH of the laboratory media. A small value of DS** implies a low generation of entropy, thus treatments that combined the effects of heat and ultrasound decreased the system irreversibility, which increased mold spore inactivation in comparison with heat treatments alone.Decreasing internal irreversibility, measured in terms of DS**, can be explained as the result of a decreasing heat resistance caused by the synergistic effects of heat and ultrasound, helping to elucidate microbial resistance differences.
Session 73C, Food Engineering: Transport Processes and Kinetics
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