Citrus fruits reach an annual worldwide production of more than 130 Mt, mainly constituting sweet orange (Citrus sinensis), lemon (Citrus limon) and grapefruit (Citrus paradisi), among others. Around 40 % of such production is lost during handling and processing activities, especially peel by-products, which contain a great amount of health promoting compounds that can be extracted and subsequently used in several industries. The main bioactive compounds identified in the citrus peels are flavonoids, specifically glycosylated flavanones. In this study, the total phenolic content (TPC) and the concentration of individual flavonoids extracted from the peels were quantified, and the color was measured by the CIELab* system. As significant differences were observed, the correlation between the color of orange, grapefruit and lemon peels and their TPC has been evaluated using principal component analysis (PCA).

The results showed up to ̴27 % higher TPC in grapefruit and lemon peels compared to orange. The main flavonoids quantified (poncirin, diosmin, dydimin, hesperidin, naringin and narirutin) are found in varying proportions in orange, lemon and grapefruit peels. Grapefruit had the highest content of naringin, poncirin and narirutin ( ̴680, ̴150 and ̴240 % higher than orange, and even more compared to lemon). Therefore, these compounds have a red-orange color. Lemon peel had a lower content of naringin, poncirin and narirutin, but up to eight-fold more diosmin than orange (and even more than grapefruit), so it correlates with smaller values of a* and b* (yellow color). Orange had the highest concentration of hesperidin and didymin ( ̴114 % and ̴20 % higher than grapefruit), so they are orange in color. In the valorization of citrus by-products, it is important to understand the differences in the phenolic profile of all species, and to be able to correlate them with simple measures such as color.