## Results and Discussion
### Fourier Transform Infrared Spectroscopy  
Responding to the main objective of the research, to evaluate the use of mid-infrared spectroscopy to study the impact of abiotic stresses, samples were also analyzed using FT-IR. The spectra of Ctr_8 and Ctr_24 are shown in Figure 2. The band presented at 3600-3000 cm\({}^{-1}\), with a maximum value close to 3300 cm\({}^{-1}\) was associated with the stretching vibration of O-H groups (3600-3200 cm\({}^{-1}\)), at 3400-3300 cm\({}^{-1}\) with N-H stretching, and at 3100-3000 cm\({}^{-1}\) with =C-H stretching [33; 34]. The peak at 1719 cm\({}^{-1}\) was associated with carbonyl group C=O stretching. Since the samples were lyophilized, the O-H stretching band should have been mainly associated with compounds such as carboxylic acids, alcohols, and phenols [34]. In fact, according to Table 1, the TPC values of the shell control samples, which had more intense bands in this region, were higher (6.4%). On the other hand, the band 3000-2800 cm\({}^{-1}\), with a maximum near 2930 cm\({}^{-1}\), corresponded to the C-H stretching of CH\({}_{2}\) groups. This band is usually associated with carbohydrates and fats [35]. The bands in the 1800-1500 cm\({}^{-1}\) region corresponded to C=C and C=O stretching. In the pineapple sample, these bonds may have been related to proteins, amide I (1700-1600 cm\({}^{-1}\)) and amide II (1565-1520 cm\({}^{-1}\)), and fats (1745-1725 cm\({}^{-1}\)) [34]. Several marker bands were identified in the spectral range of 1400-800 cm\({}^{-1}\). These bands may have been associated with the stretching and bending of carbohydrates. The bands in the region of 1150-900 cm\({}^{-1}\) were assigned to C-O and C-C stretch modes, while those in the 1400-1200 cm\({}^{-1}\) region were due to O-C-H, C-C-H, and C-O-H bending vibrational modes of the carbohydrates [36]. In addition to sugars, bands characteristic of organic acids (malic, citric, and ascorbic acids) might also have been observed in the absorption region 1500-900 cm\({}^{-1}\)[37]. The bands between 900-600 cm\({}^{-1}\) corresponded to a fingerprint region with vibrational C-H or CH\({}_{2}\) group deformation. The absorption region at 750-500 cm\({}^{-1}\) may have been associated with =C-H bending or C-C bending vibrations that occur at low frequencies (below 500 cm\({}^{-1}\)). The absorption region at 700-720 cm\({}^{-1}\) was related to colour pigments (\(\beta\)-carotene) [34,38]. The score plots of the principal component analysis of the PCA model established with all the FT-IR spectra of the shell and core samples are shown in Figures 3 and 4, respectively. In Figure 3b, the loadings plot of the first principal component (PC1) of the shell samples is also presented. As shown, this plot presented the most significant peaks at around 3300 cm\({}^{-1}\), which related to the combination bands of O-H bonds, and at 1000 cm\({}^{-1}\), which were assigned to the O-C-H, C-C-H, and C-O-H vibrational modes of the carbohydrates. This result confirmed that PC1 captured the change of the TPC of the samples. The loadings plot with respect to core samples was similar to the one obtained for the shell samples, with two important peaks at around 3300 cm\({}^{-1}\) and 1000 cm\({}^{-1}\) (data not shown). As shown in Figures 3 and 4, PC1 captured 99.08% and 98.75% of the spectra variance for shell and core samples, respectively. In shell samples (Figure 3a), it was observed that the model separated the samples with higher contents of phenolic compounds (Ctr_8, Ctr_24, T30_24, and T40_8), from the samples with lower contents (T30_8, T40_24, T50_8, and T50_24). Figure 3 also indicates that the FT-IR allowed detection of the differences between the samples in terms of the TPC and antioxidant capacity, and for results consistent with the conventional analyses to be obtained. The loadings plot of the model PC1, presented in Figure 3b, permitted the identification of the wavenumbers that were the most important to describe PC1.