Submitted to: Proceedings of Florida State Horticultural Society
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/6/2006
Publication Date: 4/11/2007
Citation: Jaitrong, S., Rattanapanone, N., Manthey, J.A. 2006. Analysis of the phenolic compounds in longan (Dimocarpus longan lour.) peel. Proceedings of Florida State Horticultural Society. 119:371-375. Interpretive Summary: Longan fruit is a tropical fruit that has a very short shelf life, and compounding this problem, this fruit also readily shows chilling injury when stored at low temperatures. This chilling injury is expressed as a browning of the peel, and this browning is dependent on the phenolic compounds in the peel. Very little is known about these compounds in longan fruit, so an investigation was made to characterize some of the main classes of phenolic compounds in longan fruit peel. This investigation found 3 classes of phenolic compounds, including flavones, ellagic acid conjugates, and hydroxycinnamates. This work will help us understand the biochemical changes that occur in the peel of chilling injured longan fruit.
Technical 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.