Polymethoxyflavones, Synephrine and Volatile Constitution of Peels of Citrus Fruit Grown in Okinawa

Shiikuwasha (Citrus depressa Hayata) and other endemic citrus fruits have been cultivated for several hundred years in the Ryukyu Islands. Citrus fruits contain high levels of functional phytochemicals, including specific and appealing aroma compounds. We analyzed the volatile compounds in immature and mature peel of 10 citrus fruits grown on Okinawa by GC-MS analysis. In addition, we used HPLC to quantify synephrine and six polymethoxyflavones (PMFs: sinensetin, hexamethoxyflavone, heptamethoxyflavone, nobiletin, natsudaidain, and tangeretin). All local citrus cultivars showed unique aroma profiles. The volatile compositions of ‘Kabuchii’ (C. keraji hort. ex Tanaka var. kabuchii) had high contents of sesquiterpene hydrocarbons (3.90–5.17%), ‘Keraji’ (C. keraji hort. ex Tanaka) was high in esters (12.15–19.10%), and ‘Ogimikuganii’ (C. depressa Hayata) was high in γ-terpinene (21.17–29.60%) and p-cymene (6.49–9.84%). The highest levels of synephrine were found in immature peel of ‘Tokunibu’ (C. nobilis Lour.) (8.97 mg·gDW−1), ‘Izumibeni’ (C. tangerina hort. ex Tanaka) (7.03 mg·gDW−1), and ‘Ogimikuganii’ (5.17 mg·gDW−1). There were high levels of PMFs in immature peel of ‘Ogimikuganii’ (20.62 mg·gDW−1), ‘Kabuchii’ (20.66 mg·gDW−1), ‘Oto’ (C. oto hort. ex Yu. Tanaka) (12.52 mg·gDW−1), and ‘T-132’ (C. tankan Hayata) (18.95 mg·gDW−1), each of which showed a unique profile. Our results suggested the scope for effective utilization of the waste, including thinned fruits, from shiikuwasha and tankan, that major cultivars in Okinawa.