Abstract

GARROTE, R. L.; SILVA, E. R.  and  BERTONE, R. A.. Kinetic parameters for thermal inactivation of cut green beans lipoxigenase using unsteady-state methods. Lat. Am. appl. res. [online]. 2003, vol.33, n.2, pp.87-90. ISSN 0327-0793.

Most raw vegetables can be stored for only a short time even at -20ºC. Blanching is the primary pre-freezing means of inactivating undesirable enzymes present in the vegetable. Lipoxygenase is widely distributed in vegetables and evidence is mounting to support its involvement in off-flavour development and colour loss. In order to optimize the blanching process of vegetables, it is essential to establish a kinetic model of the inactivation of the indicator enzyme. Isothermal and dynamic thermal approaches have been used to determine kinetic parameters. The unsteady-state procedure is more flexible and can be applied to uniform and non-uniform heating situations and generally a food medium, rather than an aqueous buffer solution, is always used for determining kinetic parameters. Kinetic parameters describing lipoxygenase inactivation during heating of cut green beans were determined using two unsteady-state procedures. The model used an analytical solution for heat conduction in a finite cylinder to predict time-temperature profiles, and a trial and error and a nonlinear regression of experimental lipoxygenase retentions to estimate kinetic parameters, rate constant, k and activations energy, E_a. Thermal diffusivity, a , and convective heat transfer coefficient, h, were experimentally determined, but thermal conductivity, l , was estimated. Mean values obtained, with standard deviations between parenthesis, were k_76ºC = 27.2 (9.4) s^-1, k_82ºC = 92.9 (7.5) s^-1; k_88ºC = 212.1 (52.7) s^-1; k_94ºC = 407.8 (56.7) s^-1; E_a = 160.7 (8.1) KJ/mol using the trial and error procedure; k_85ºC = 150 (26.3) s^-1 and E_a = 164 (4.7) KJ/mol using the nonlinear regression method. Predicted and observed lipoxygenase retentions showed good agreement.

Keywords : Blanching Index; Heating Treatment and Enzymatic Activity.

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