Based on the initial concentrations proposed in Section 2.1 (which were determined according to the reading range of the reflectometric assay in Section 2.2), the specific volumetric costs of the indicators would be, approximately, 40 US$/L for POD, 2200 US$/L for LPO and 0.25 US$/L for ALP. Therefore, the application of indicator LPO would be unjustified; especially considering the large dispersion seen in the parity chart in Fig. 2. Moreover, the low value of z1 of indicator
LPO ( Table 1) makes it too sensitive to temperature variations to be used as a good TTI. Indicators GDC-0980 price POD and ALP were submitted to a slow discontinuous thermal treatment for validating the adjusted kinetic model, as described in Section 2.5. Fig. 4 presents the time-temperature curves obtained for indicator ALP, which are similar to the curves obtained for indicator POD. Fig. 5 compares the residual activity predicted by Eq. (6) with the experimental values obtained from the validation tests for indicator ALP. Since the distribution of the points in the parity chart is similar to the distribution seen in
Fig. 2 and no large discrepancies can be found in the residuals plot (predicted – measured), it is possible to validate the kinetic model adjusted for ALP. Indicator POD could not be validated, as can be seen in Fig. 6a. The experimental residual activity was much larger than the predicted activity. A new enzyme Y-27632 2HCl batch for prepared Forskolin cell line and all validation tests were repeated in order to confirm the discrepant behavior, and the same results were obtained (Fig. 6b). The thermal inactivation of POD showed distinct kinetics for fast heating and for slow heating conditions. This suggests that during slow heating, the POD enzyme suffers some sort of activation period. Authors have also detected activation of POD and other enzymes in thermal treatment (Martin et al., 2005 and Polakovič and Vrábel, 1996). Due to this behavior, the indicator POD is not suitable for practical use. From the three tested TTIs,
only indicator ALP showed potential to be actually used for the evaluation of continuous thermal processes under pasteurization conditions (70–80 °C). Its main advantages for practical use are: 1) the residual activity of the enzyme can be determined with an easily available commercial kit; 2) the specific cost of the TTI is low (0.25 US$/L); 3) the z-values of the thermostable and thermolabile fractions are very close to those of some microorganisms in liquid foods; and 4) this TTI can be considered as a multiple-response TTI because the thermostable and thermolabile fractions are inactivated in different conditions (different D-values). To improve the accuracy of the assessment, replicate measurements will be required.