Extraction and purification of red dragon fruit: Red dragon fruit was first separated from the pericarps and homogenized with 95% (v/v) ethanol by the ratio of 1:2 for 15 min. This step was to separate pectic substances. Subsequently the fruit pulp was sieved with gauze and centrifuged at 10,000 x g for 20 min to remove precipitates. Supernatant was then evaporated with a rotary evaporator (Buchi, Switzerland) at 160 Pa, 40°C until one forth from the initial volume. The fruit extract was used for further work.