An autoclave with 500 mL working volume was used for pectin extraction by subcritical water, in which a thermocouple and a pressure gage were used to assay the temperature and pressure inside the reactor. The raw material and distilled water were added into the reactor with the solid to liquid ratio of 1:30. The extraction temperature was set at 130
C, 150
C, 170
C for apple pomace pectin extraction and 100
C, 120
C, 140
C for citrus peel pectin extraction according to preliminary experiments. The extraction time was set for 5 min. All the experiments were performed in triplicate, with the average value reported.
After subcritical water extraction, the water soluble portion was retrieved by filtration and the filtrate was collected for alcohol precipitation. The precipitate was washed by 5% (v/v) HCl in 60% isopropyl alcohol and anhydrous ethanol in turn for three times, whereafter it was dried at 105
C for 24 h. The pectin yield was calculated according to equation