Abstract
A characteristic substance was isolated from a methanol extract of kumquat by preparative HPLC using a reversed-phase column. The isolated substance was identified as 3’,5’-di-C-β-glucopyranosyl phloretin (DGP) by 1H and 13C NMR analyses. The antioxidative activity of DGP and phloretin, an aglycone, was measured using the 1,1-diphenyl-2-picrylhydrazyl radical scavenging assay. Phloretin exhibited higher activity than Trolox, a standard antioxidant, whereas DGP exhibited lower activity. The DGP content in four kumquat fruits was determined by HPLC and found to range from 6.1 to 10.9 mg/g dry weight (dw). The DGP content was determined as 17.4 mg/g dw in the peel, 12.8 mg/g dw in the sarcocarp, 7.8 mg/g dw in the juice, 17.9 mg/g dw in the fruit core, and 0.5 mg/g dw in the seed. The DGP content in solutions of kumquat fruit powder was analyzed by HPLC after treatment at 0 °C or 20 °C for 45 min and remained unchanged. However, the DGP content in solutions of kumquat fruit powder decreased significantly after treatment at 50 °C or 100 °C for 45 min. Processed kumquat food products, including kumquat jam and kumquat preserved in sugar, were prepared. The survival rate of DGP in kumquat jam in comparison to fresh kumquat fruit before processing was 42.1±3.2%, whereas that in preserved kumquat fruit (combination of candied fruit and syrup) was 87.6±5.6%, indicating a relatively high DGP content. This study revealed that the residual DGP content in kumquat food products depends on the processing method.
