Determining flavonoids in the skin of Citrus reticulata

n this work we studied the flavonoid composition of tangerine skins using 10 kinds of tangerines from various producers. The tangerine skins were peeled from fruit purchased from the market in Belgorod and dried at room temperature outside direct sunlight. The compositions of “green” ethanol-based extraction solutions and the “weighed portion of plant material: volume of extraction solution” ratios were chosen for extraction to ensure a high yield of flavonoids. For separation, reverse-phase HPLC on the Kromasil 100-5C18 stationary phase was used in environmentally friendly mobile phase compositions that contained ethanol as an organic modifier. In gradient elution mode all mobile phases contained 0.2 vol. % orthophosphoric acid. To assign extract components, we used samples of a number of substances and parameters of electronic absorption spectra in comparison with published data. It was found that the chromatographic profiles of the extracts of the studied samples were noticeably different. A number of the least lipophilic compounds, derivatives of cinnamic acids, were found in all extracts. After that, we discovered two flavanone glycosides, the main of which was hesperidin (hesperitin-7-rutinoside) with a content of is 26.0-39.9 mg per 1 g of dried raw material, and naringin (naringenin-7-neohesperidiside) in significantly smaller quantities, from 0 to 1.15 mg per 1 g. But of greatest interest were polymethoxylated flavones, among which the main component was nobiletin (5,6,7,8,3′,4′-hexamethoxyflavone) with a content from 0.03 to almost 11 mg/g in dried peel and tangeretin (5,6,7,8,4′-pentamethoxyflavone), found in slightly smaller quantities, 0.1-5.4 mg/g. It was established that nobiletin and tangeretin were mostly accumulated in the skins of small tangerines. On the whole, tangerine skin is a great source of highly biologically active flavonoids, so its processing is an important task for the food industry.