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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.

To explore the effect of room temperature storage on the flavor quality and biochemical composition of flower and fruit scented black tea, this study compared the sensory flavor quality of flower and fruit scented black tea produced in the years 2019, 2020, 2021 and 2022 and stored at room temperature, and it detected volatile and non-volatile compounds in the tea by headspace solid-phase microextraction-gas chromatography-mass spectrometry (HS-SPME-GC-MS) and ultra-performance liquid chromatography coupled to orbitrap high resolution mass spectrometry (UPLC-Orbitrap-MS). The results showed that the sensory quality of flower and fruit scented black tea was significantly affected by storage at room temperature, and the tea lost its floral and fruity aroma characteristics after three years of storage at room temperature; its quality characteristic was mature sour taste. The tea samples were divided into four groups of age by principal component analysis (PCA) and hierarchical cluster analysis (HCA). Totally 15 volatile differential compounds such as linalool, cis-β-ocimene, and hexanoic acid (VIP > 1 and P 2 and P < 0.05) were selected by partial least squares discriminant analysis (PLS-DA) and one-way analysis of variance (ANOVA). The content of volatile compounds such as linalool, hexanal, cis-β-ocimene, and 2-pentylfuran decreased with increasing storage time, while the contents of hexanoic acid, dihydroactinidolide 1-ethyl-2-formyl-1H-pyrrole, and β-ionone increased. The contents of non-volatile compounds such as most amino acids, nucleotides, sugars, tea polyphenols tended to decrease with increasing storage time, while the contents of organic acids and lipids showed an increasing trend. The results obtained from this study can provide a scientific basis for elucidating the quality changes of flower and fruit scented black tea during storage and help guide its rational storage and scientific consumption.