## 3 Results and Discussion
### Residual Activity of Amylase and Interase during Mango Drying  
For a better understanding of the activity degradation of amylase and invertase along with the changes in water activity during mango drying, see Figure 3a-e. The results reveal that at an air velocity of 1.0 m s\({}^{-1}\), drying at 40 \({}^{\circ}\)C, 60 \({}^{\circ}\)C, and 80 \({}^{\circ}\)C exhibited destruction rates (k\({}_{\rm d}\)) for amylase of 0.0013, 0.0020, and 0.0029 min\({}^{-1}\), and for invertase 0.0008, 0.0017, and 0.0023 min\({}^{-1}\), respectively (Figure 3a,c). The maximum degradation rate (k\({}_{\rm d}\)), both for amylase and invertase, was found at 80 \({}^{\circ}\)C compared to 60 \({}^{\circ}\)C and 40 \({}^{\circ}\)C. The related results on heat inactivation of different enzymes were also found in previous studies [21; 38; 39]. In addition, at temperatures 40 \({}^{\circ}\)C, 60 \({}^{\circ}\)C, and 80 \({}^{\circ}\)C, an increase in the air velocity to 1.4 m s\({}^{-1}\) increased the destruction rate (k\({}_{\rm d}\)) values of amylase (7.57-13.13%) and invertase (7.24-14.08%). [PERSON] et al. [12] reported that the heat diffusion capacity of a high air velocity is more pronounced than that of a lower one. Therefore, it was believed that drying at 1.4 m s\({}^{-1}\) air velocity causes a larger destruction of enzymes. Related information was also discussed in our previous study [2] where higher product surface temperature and internal temperature were observed at an air velocity of 1.4 m s\({}^{-1}\) compared to 1.0 m s\({}^{-1}\). Furthermore, this finding can be explained well by comparing the energy of deactivation (E\({}_{\rm d}\)) values for both air velocities, 1.0 m s\({}^{-1}\) and 1.4 m s\({}^{-1}\), that was measured from the slope of Arrhenius plot, ln k\({}_{\rm d}\) versus reciprocal of temperature, 1/T (K) (Figure 3b,d). It was found that the air velocity 1.4 m s\({}^{-1}\) exhibited a higher E\({}_{\rm d}\) value for amylase (0.32%) and invertase (6.94%) compared to an air velocity of 1.0 m s\({}^{-1}\), confirming that more energy is involved to denature the enzyme at a higher air velocity. Hence, a larger enzyme destruction was observed at air velocity of 1.4 m s\({}^{-1}\). Despite all these facts, the residual enzyme activity is also directly related to the drying time or heat exposure time [40]. The mango slabs dried at 80 \({}^{\circ}\)C and air velocity of 1.4 m s\({}^{-1}\) needed less time 240 min to reach the safe water activity level \(\leq\) 0.6, as it required 1080 min at 40 \({}^{\circ}\)C and air velocity of 1.0 m s\({}^{-1}\) (Figure 3e). Many studies have shown that prolonged drying time causes cell lysis or disruption, and could modify the biochemical reactions inside the fruits by raising their exposure to heat, light, and oxidation [41; 42], which may negatively influence the activity retention of amylase and invertase in dried mango. Therefore, the mango slabs dried at 40 \({}^{\circ}\)C and air velocity of 1.0 m s\({}^{-1}\) could eventually be retaining a minimum of enzymes although the destruction rate was lower. Moreover, it is also well-known that the optimum temperature and high water activity stimulate the enzyme activity inside the fruit. During drying at a low temperature of 40 \({}^{\circ}\)C, the amylase and invertase retained near their optimal temperature and a good water activity level for a longer duration, where they can perform their maximum activity, and may involve in self-denaturation due to accumulation of end products or unavailability of the substrate which could be verified further by measuring their end products.