Effect of Membrane Separation on Chemical Composition and Antioxidant Activity of Water Extracts from Different Kinds of Tea from Fujian

This study was done in order to investigate the changes of the major biochemical components and in vitro antioxidant activity of the water extracts from different kinds of tea during the process of their multistage membrane separation, ultrafiltration membranes with molecular masses cut-off (MMCO) of 20, 10, and 3.5 kDa, respectively were successively used to separate the water extracts from the major kinds of tea in Fujian including green tea, white tea, oolong tea from south Fujian, oolong tea from north Fujian, and black tea into fractions with different molecular masses. The fractions were spray dried and investigated for their ultraviolet (UV) spectra, biochemical compositions and in vitro antioxidant activity. The results showed that the UV absorption spectra of all samples were similar, exhibiting two obvious peaks at 210 and 274 nm. Absorbance at 274 nm exhibited a significant positive correlation with the contents of free amino acids (FAAs), caffeine (CAF), gallic acid (GA) and epicatechin gallate (ECG), and a significantly negative correlation with the contents of epigallocatechin (EGC) and epicatechin (EC). The distribution of biochemical components and the changes in antioxidant activity in vitro were affected by membrane separation, among which, the 20 kDa MMCO membrane had the most pronounced impact. The retentate from black tea after 20 kDa MMCO membrane ultrafiltration had the highest antioxidant activity in vitro, while the antioxidant activity of the permeate was lower than that of the permeate from the other kinds of tea. The biochemical components and antioxidant activity in vitro varied significantly among different types of tea samples, and the changes in biochemical components and antioxidant activity during the membrane separation process were not completely consistent. The water extract from black tea showed the greatest changes after 20 kDa MMCO membrane ultrafiltration. These results could provide a scientific basis for the membrane separation of aqueous extracts from different types of tea.