Effect of Glycine Betaine on Chilling Injury in Eggplant Peel Revealed by FTIR Spectroscopy

Eggplant (Solanum melongena L.) is highly susceptible to chilling injury (CI) during cold storage, with severity being strongly influenced by fruit maturity stage. At the tissue level, the peel acts as the primary site of cold-stress metabolic responses. This study evaluated the effect of pre-storage glycine betaine treatment (GB, 10 mM) on CI in purple eggplant at baby (BB, low sensitivity) and commercial (CC, high sensitivity) maturity stages stored at 4 °C for 20 days, integrating the use of ATR-FTIR spectroscopy as a rapid, non-destructive tool to monitor quality in the fruit peel. Physiological traits including chilling injury index (CII) and fruit rigidity were integrated with peel-specific ATR-FTIR spectroscopy combined with chemometric analysis to describe structural and metabolic behavior. BB fruit showed higher tolerance to CI, reaching a CII 23% lower than CC after 20 d, along with greater rigidity retention. GB treatment was significantly effective in reducing CI in both maturity stages by decreasing CII by 23% for BB and 32% for CC fruit, and delaying symptom onset. FTIR analysis revealed that the main peel spectral changes during storage occurred in the amide–phenolic (1653–1515 cm−1) and polysaccharide (~1017 cm−1) regions. Control fruit showed progressive shifts in these regions indicating structural disorganization, while GB-treated fruit delayed and attenuated spectral changes. Chemometric analysis (OPLS-DA) clearly discriminated samples according to maturity stage, treatment, and storage time. Overall, the results demonstrate that chilling susceptibility is determined by maturity stage, that the GB treatment enhanced CI tolerance—especially in sensitive CC fruit—and that ATR-FTIR coupled with chemometrics provides an effective approach for rapid non-destructive monitoring of postharvest quality changes in eggplant during cold storage.