Fresh grapefruit (Citrus paradisi Macf.) were picked from Taizhou, China. The collected peel was first soaked in a water bath at 90 C for 5 min to inactivate enzymes, then drying was carried out in an oven with air circulation at 50 C. The dried peels were grounded by an electric grinder (Qijian Q-250A3, Shanghai Bingdu Electrical Appliance Co., Ltd., China) and passed through an 80 mesh sieve to obtain particles (<180 lm). Finally, the vacuum packed samples were stored inside desiccators until use. Ethanol, HCl, NaOH, phenol, H2SO4, foline phenol, gallic acid, NaNO2 and Al(NO3)3 (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), trifluoroacetic acid (Aladdin Reagents, Shanghai,China) and 1-phenyl-3-methyl-5-pyrazolone (PMP) (Acros Organics,USA) were analytical grade (purityP99%). Chemicals used for high-performance liquid chromatography (HPLC), including methyl alcohol and acetonitrile, were HPLC-grade (Tedia, USA). Rutin standard was purchased from Aladdin Reagents (Shanghai, China), and monosaccharides standards were purchased from Sigma–Aldrich (St. Louis, MO, USA).
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