Ku-jin tea (Acer tataricum subsp. ginnala or A. tataricum subsp. theiferum), an underestimated functional beverage rich in antioxidant phenolics

• The metabolic profile of Ku-jin tea extract was analysed using UPLC-DAD-QTOF-MS for the first time.
• Antioxidant capacities of the pure compounds isolated from Ku-jin tea were evaluated by DPPH, ABTS and FRAP assays.
• Five major gallotannins in Ku-jin tea infusions were quantified and ginnalin A was found to be the most abundant compound.
• Ku-jin tea could be an excellent functional beverage due to its high concentration of active phenolic compounds.

This study provides the scientific proof for Ku-jin tea (Acer tataricum subsp. ginnala or A. tataricum subsp. theiferum) as a functional beverage for the first time. Ku-jin tea is a polyphenol-rich drink and its metabolic profile was analysed using ultra high performance liquid chromatography-diode array detector-electrospray ionization-quadrupole time-of-flight mass spectrometry (UPLC-DAD-QTOF-MS). Thirty-five compounds were identified from the extract of Ku-jin tea, including twenty phenolics that are mainly flavonoids and gallotannins. Furthermore, fourteen pure phenolic compounds were isolated from Ku-jin tea and their antioxidant capacities determined by 1,1-diphenyl-2-picrylhydrazyl (DPPH), 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) and ferric reducing antioxidant power (FRAP) assays. The results demonstrated that the flavonoids and gallotannins are potent antioxidant compounds, and that the galloyl substituent group could improve the antioxidant activity of the flavonoids. Additionally, quantification of phenolic compounds in Ku-jin tea infusions was carried out with ultra-high performance liquid chromatography (UHPLC). Five gallotannins were found to be predominant, and ginnalin A was determined as the most abundant compound (up to 57.38 mg/g dry weight in most samples). These results revealed that Ku-jin tea could be an excellent functional beverage due to its high concentration of natural antioxidants and active phenolic compounds.