Abstract: In order to investigate the changes of physicochemical of different tea samples in the process of multistage membrane separation, ultrafiltration membrane with different pore sizes were used for separation. Fifty samples of 4 classifications of the green，white，Fujian oolong ( southern and northern varieties) ，and black teas were collected and extracted with boiling water for the membrane separation. The water extract of teas were separated by ultrafiltration membrane (20, 10, 3.5 kD), and tea powders were prepared by spray drying respectively. The physicochemical properties of tea powders were analyzed and compared including UV absorption spectrum characteristics, the main biochemical components, and antioxidant activities in vitro which included DPPH free radical scavenging capacity, the ferric reducing antioxidant power ( FRAP), and total antioxidant capacity (T-AOC ). The analytical results on the tea powders showed the UV absorption spectra of tea powder samples were similar, with two obvious absorption peaks at 210 nm and 274 nm wavelength. The contents of FAAs, CAF, GA and ECG exhibited the strongest and significant correlations with the UV absorbance of each tea powder at 274 nm. On the other hand, EGC and EC correlated inversely with the absorbance. The distribution of biochemical components of tea powder and the change of antioxidant activity in vitro were affected by membrane separation, among which, 20 kD ultrafiltration membrane had the greatest impaction. The tea powder prepared by 20 kD ultrafiltration membrane retentate of black teas had the highest antioxidant activity in vitro. While other teas had the highest antioxidant activity in the permeate portion of the 20 kD ultrafiltration membrane. The biochemical components and antioxidant activity in vitro vary among different types of teas, and the changes of which were not completely consistent during the membrane separation process. Among them, the 20 kD ultrafiltration membrane had the greatest influence on the distribution of each component and the largest change in antioxidant activity. The results could provide a scientific basis for the membrane separation of different types of teas.

Key words: Tea, Aqueous extract, Membrane separation, biochemical composition, antioxidant