2.2. Sample preparation
Thirty-four kinds of flowers were picked from Guangzhou, China and seventeen kinds of flowers were bought from local markets, Zhongshan second road, Guangzhou. The flowers were washed with deionized water, and given an airing at room temperature. The flowers were then ground into fine particles using a special grinder for food processing. An accurate amount (about 1.00 g) of these particles was weighed, and the fat-soluble components were extracted with tetrahydrofuran (5 mL) at 37 C for 30 min in a shaking water bath according to the literature. The sample was centrifuged at 4200g for 30 min, and the supernatant was collected. The residue was extracted with the same solvent twice, and the supernatants were combined, which would be used for the evaluation of antioxidant capacity and total phenolic content. Subsequently, the residue was extracted with a mixture (5 mL) of methanol–acetic acid–water (50:3.7:46.3, v/v/v). The extraction procedure was repeated twice, and the supernatants were combined for the evaluation of antioxidant capacity and total phenolic content. Furthermore, according to the literature , the residue was hydrolyzed with 10 mL of 2 M NaOH for 4 h at room temperature under a stream of N2. The mixture was acidified to pH 2 with 6 M HCl and extracted five times with hexane to remove fatty acids, which are released during alkaline hydrolysis. Bound phenolic compounds were extracted five times with diethyl ether and ethyl acetate (1:1, v/v). The combined extracts were evaporated to dryness at 30 C in vacuo and reconstituted in methanol and stored at 20 C until used.