Analysis of the phenolic compounds in Longan (Dimocarpus Longan lour.) Peel

Somkit Jaitrong, Nithiya Rattanapanone, John A. Manthey

Abstract


Longan fruit are susceptible to chilling injury, where the injured peel exhibits discoloration due to water-soaking and enzymatic browning. This peel discoloration is dependent to a large degree on the composition of the phenolic compounds. Yet, the main classes of phenols in longan peel remain largely uncharacterized. In order to further characterize these classes of compounds, and to begin investigating the differences in these compounds resulting from chilling injury, two varieties ('Daw' and 'Biew Kiew') were subjected to cold storage. Peels of the stored fruit were freeze-dried and extracted with 80% aqueous methanol. HPLC, coupled with photodiode array (PDA) and mass spectral (MS) analysis, provided evidence of a large number of UV-absorbing compounds in longan peel, the majority of which are phenolic compounds. Subsequent fractionation of these compounds by size-exclusion chromatography effectively separated several main classes of phenolic compounds, including ellagic acid conjugates, flavone glycosides, and a set of other phenols with unknown structures, but which were grouped together because of their similar MS, UV and IR spectra. HPLC analysis of acid hydrolyzed extracts of longan peel showed that the majority of the flavones were glycosides of quercetin and kaempferol. Similar analyses also showed evidence of ellagic acid glycosides. The spectral properties of these compounds are reported, and several of the main classes of phenols in longan peel are described.

Keywords


flavonoid; ellagic acid; quercetin; kaempferol; HPLC-MS; FTIR; hydroxycinnamates

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Proc. Fla. State Hort. Soc.     ISSN 0886-7283