Changes in Physicochemical Characteristics and Bioactive Potential of Corfu ‘Nagami’ Kumquat (Fortunella margarita Swingle) During the Harvesting Period

‘Nagami’ kumquat (Fortunella margarita Swingle), a citrus fruit cultivated in Corfu, Greece, contains bioactive constituents whose levels may vary during fruit maturation. This study investigated changes in the physicochemical composition and bioactive potential of Corfu ‘Nagami’ kumquat during the commercial harvesting period (December–March) and optimized ultrasound-assisted extraction conditions for standardized comparisons among harvest stages. Fruits collected during the study period were analyzed for moisture, soluble sugars, proteins, lipids, total phenolic content, and free-radical scavenging capacity. Response surface methodology was used to optimize methanol concentration, extraction temperature, and extraction time, while principal component analysis (PCA) was applied to evaluate patterns associated with fruit maturation. During maturation, moisture and protein contents decreased, whereas lipid content increased. The optimized extraction conditions were 14% methanol, 45 min, and 54 °C. PCA showed clear separation of early- and late-harvest-stage samples, with the first two principal components explaining 97.9% of the total variability. The observed differentiation was primarily associated with DPPH scavenging, moisture, proteins, lipids, soluble sugars, and phenolic content. Late-harvest-stage fruits exhibited the highest free-radical scavenging capacity and phenolic content, with harvest stage related to variations in the bioactive potential of Corfu ‘Nagami’ kumquat.