## 3 Results and Discussion
### Enzymatic Activity
#### 3.2.1 Pineapple Shell
PAL's enzymatic activity is related to the synthesis of TPC, but this behaviour was not observed in all studied samples. The whole pineapple shell sample with storage time 24 h (169.31 \(\pm\) 2.99 \(\upmu\)mol t-cinnamic\(\cdot\)h\({}^{-1}\) g\({}^{-1}\) dry matter) showed an increase of 22% in the enzymatic activity of PAL, and consequently an increase of 9 % in the TPC values (104.08 \(\pm\) 0.74 mg CAE\(\cdot\)g\({}^{-1}\) dry matter), compared to the raw material (PAL: 138.77 \(\pm\) 0.97 \(\upmu\)mol t-cinnamic\(\cdot\)h\({}^{-1}\) g\({}^{-1}\) dry matter; TPC: 95.89 \(\pm\) 0.84 mg CAE\(\cdot\)g\({}^{-1}\) dry matter). The sample with heat treatment of 30 \({}^{\circ}\)C and storage time of 48 h (137.34 \(\pm\) 2.46 \(\upmu\)mol t-cinnamic\(\cdot\)h\({}^{-1}\) g\({}^{-1}\) dry matter) showed non-significant differences concerning the raw material (138.77 \(\pm\) 0.97 \(\upmu\)mol t-cinnamic\(\cdot\)h\({}^{-1}\) g\({}^{-1}\) dry matter) in the enzymatic activity of PAL, although it showed a significant (\(p\) < 0.05) increase of 14% in the TPC values (109.40 \(\pm\) 0.70 mg CAE\(\cdot\)g\({}^{-1}\) dry matter), comparatively with the raw material (95.89 \(\pm\) 0.84 mg CAE\(\cdot\)g\({}^{-1}\) dry matter). Figure 5: Effect of heat treatment, on the enzymatic activity of pineapple shell: (**a**) bromelain activity; (**b**) phenylalanine ammonia-lyase activity (PAL); and (**c**) polyphenol oxidase activity (PPO). Error bars stand for \(\pm\) standard deviation (\(n=9\)). Different letters express significant differences between samples. The biosynthesis of several secondary products derived from phenylpropanoids is triggered by the activity of PAL as a first step in the metabolism of phenylpropanoids to catalyze the deamination of L-phenylalanine in trans-cinnamic acid [54]. The accumulation of secondary metabolites is crucial in the defense mechanisms of plants, such as flavonoids, lignins, coumarins, and other phenolic compounds [22]. A kinetic model was developed to describe the transformations that occur in phenolic compounds during the storage of fresh products through the reactions responsible for the de novo biosynthesis of phenols by the enzymatic activity of PAL and oxidation catalyzed by the enzyme PPO. The model explains the development of phenolic content as a function of time and described the effect of variables on phenolic synthesis and oxidation. An increase of 5 \({}^{\circ}\)C in the storage temperature in fresh-cut purslane samples did not show modifications in the metabolism of phenylpropanoids, although it did show a significant increase in the oxidation rate. The apples and broccoli samples indicated differences in the initial phenolic synthesis and content [55]. The stress caused by temperature did not alter the accumulation of phenolic compounds significantly compared to the control in purple-flesh potatoes, although the damage promoted an increase in the enzymatic activity of PAL and the accumulation of phenolic compounds [35]. Contrary to the previous study, the temperature applied to the pineapple shell samples did not affect the enzymatic activity of PAL, but it did affect the synthesis of phenolic compounds. The de novo biosynthesis of phenols in pineapple peel samples by the enzymatic activity of PAL and oxidation catalyzed by PPO may have been limited by packaging conditions (vacuum). The enzymatic activity of PAL increased with increasing storage temperature is defined as the ideal temperature for enzymatic activity at 37 \({}^{\circ}\)C [56]. Storage conditions can limit PAL activity since the refrigeration temperature (5 \({}^{\circ}\)C) has been carried out. This reason may explain the fact that the enzyme activity is reduced and the synthesis of phenolic content also. In the pineapple shell, the storage time (48 h) and the moderate temperature had no effect on the development of phenolic content and antioxidant activity.