## 3 Results and Discussion
### Total Phenolic Content and Antioxidant Activity
#### 3.1.1 Pineapple Shell
Figure 2: Effect of abiotic stresses, heat treatment and wounding, on antioxidant activity of pineapple shell: (**a**) DPPH (2,2-diphenyl-l-1-picrylhydrazyl); (**b**) FRAP (Ferric Reducing Antioxidant Power); and (**c**) ABTS (2,2’-azino-bis (3-ethylbenzothazoline-6-sulphonic acid)). Error bars stand for \(\pm\) standard deviation (\(n=9\)). Different letters express significant differences between samples. The content of phenolic compounds in whole strawberries without hull was 13% greater than in halved strawberry samples. Whole strawberries without hull and halved strawberries showed no differences in the content of phenolic compounds [34]. [PERSON] and [PERSON] (2003) [35] studied purple-fleshed potatoes (cv. All Blue) and also evaluated the effect of various abiotic stresses on the induction of phenolic compounds and antioxidant activity. The total phenolic compounds in the purple-fleshed potatoes increased by ~60%, as well as the antioxidant capacity increased by ~85%. Contrary to that obtained by other authors, wounds in pineapple shell by-products maintained or decreased the content of phenolic compounds. The synthesis of total phenolic compounds and the consequent antioxidant activity in pineapple by-products may have been limited by the storage temperature since the refrigeration temperature (5 \({}^{\circ}\)C) is not close to the optimum temperature of enzymatic activity. Not all studies have seen an increase in antioxidant activity after wounding. A study of several fruits such as pineapple, mango, canalaloupe, watermelon, strawberry, and kiwi fruit compared the effect of fresh cut with the whole fruit and concluded that the cut induced losses of antioxidant carotenoids (0-25%) during 9 days storage in air at 5 \({}^{\circ}\)C [36]. In the present study, the wound pineapple shell samples showed a decrease in antioxidant activity only by the DPPH method, although the storage time in present study (24 h or 48 h) was shorter than the storage time (9 days) studied by [PERSON], [PERSON], and [PERSON] (2006) [36]. Our results show that heat treatment, as well as storage time, influence the synthesis of bioactive compounds. A study carried out with mangoes observed an increase in polyphenols and carotenoids, as well as an increase in antioxidant capacity in samples treated with hot water (immersion at 50 \({}^{\circ}\)C for 60 min and successive storage at 5 \({}^{\circ}\)C and 20 \({}^{\circ}\)C) compared to untreated fruits. The determining factor in changes in polyphenic content and antioxidant capacity was the storage temperature and not the effect of the treatment with immersion in hot water [37]. The heat treatment (46 or 50 \({}^{\circ}\)C for 30 or 75 min and stored at 6 \({}^{\circ}\)C for 9 d) also prolonged the shelf-life and promoted an increase in bioactive compounds in another study developed with mangoes [38]. The total phenolic compounds and the antioxidant capacity in fresh-cut pineapple showed higher values under low O\({}_{2}\) (12% O\({}_{2}\) in combination with 1% CO\({}_{2}\)) than under high O\({}_{2}\) (38% O\({}_{2}\)) atmosphere [39]. In our work, the pineapple shell samples were packed with 85% of vacuum; therefore, the oxygen inside the package (about 3% O\({}_{2}\)) is less than the oxygen (21% O\({}_{2}\)) present in a package with atmospheric pressure. Postharvest stress can stimulate some enzymes and/or non-enzymatic antioxidant systems of fruits, maintaining fruit quality and improving antioxidant capacity [40]. On the other hand, postharvest techniques cause damage and increase the oxidative stress that is reduced or limited by atmospheres with low oxygen content [41]. Another possible factor that may have inhibited the synthesis and accumulation of phenolic compounds was the composition of the atmosphere (absence of oxygen) since samples of pineapple shell by-products were vacuum-packed and there was virtually no oxygen available for oxidation reactions.