Abstract: In order to reveal the primary structural characteristics and chemical composition of theabrownins in Pu-erh tea, the differences in the chemical composition of theabrownin fractions with different molecular masses from the tea were studied using membrane ultrafiltration and mass spectrometry (MS). The contents of proteins, polysaccharides, total polyphenols and active groups and color difference in the theabrownin fractions were analyzed, and their structural characteristics were analyzed by Fourier transform infrared spectroscopy (FTIR), ultraviolet (UV) absorption spectroscopy, gas chromatography-mass spectrometry (GC-MS) and Curie-point pyrolysis combined with gas chromatography-mass spectrometry (CP-Py-GC-MS). The results showed that the smaller the molecular mass of theabrownin fractions, the lighter the color of their aqueous solutions, while no significant difference was observed in the content of active groups. The contents of proteins, polysaccharides and total polyphenols showed a downward trend with decreasing molecular mass. All the fractions contained a large amount of –OH groups, saturated C–H peaks and C=C benzene ring peaks, and exhibited an absorption peak at 200 nm. Ribose, rhamnose, arabinose, xylose, mannose, glucose and galactose were all contained in the fractions with molecular mass > 300 kDa and in the range of 500 Da–10 kDa. The main pyrolysis products derived from the theabrownin fractions with different molecular masses were methyl palmitate, octadecenoic acid methyl ester, methyl octadecadienoate, tetracosane, pentacosane, furfuryl methanol, and tocopherol.

Key words: Pu-erh tea; theabrownin; physicochemical properties; Curie-point pyrolysis combined with gas chromatography-mass spectrometry