Changes in Phenolic Compounds, Antioxidant Capacity, Color, and Microstructure of Hass Avocado (Persea americana) Peel During Ripening

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Abstract
Avocado peel is a high-value by-product rich in bioactive compounds; however, its structural and chemical integrity undergoes critical transitions during postharvest ripening. This study evaluated the influence of ripening stage (unripe, ripe, and overripe) on the chemical composition, color attributes, antioxidant capacity, and microstructural evolution of Hass avocado peel. Significant differences (p < 0.05) were observed in proximate composition and color coordinates among ripening stages. Overripe peel exhibited the highest concentration of total phenolic compounds (168.02 ± 3.28 mg GAE/g) and antioxidant capacity, determined by ABTS•+ (410.25 ± 1.01 µM TE/g) and FRAP (782.84 ± 2.62 µM TE/g). Environmental scanning electron microscopy (ESEM) and confocal laser scanning microscopy (CLSM) revealed progressive degradation of cuticular layers and chlorophyll-containing cells during ripening. Surface fractal dimension texture (FDT) significantly decreased from 1.54 ± 0.02 in unripe peel to 1.47 ± 0.01 in overripe peel, indicating a reduction in surface structural complexity. Principal component analysis (PCA) explained 99.0% of the total variance and revealed a clear association between FDT, chlorophyll fluorescence (λex = 450 nm), and color parameters, while total phenolic content and antioxidant capacity were associated with advanced ripening stages. These findings demonstrate that FDT is a promising indicator of peel senescence and suggest that structural disassembly during ripening is closely associated with enhanced phenolic extractability and antioxidant potential. This work provides a comprehensive framework for monitoring postharvest quality and supports the valorization of avocado peel as a source of bioactive compounds for food applications.

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