Flavone Glycosides of Tea: Structure, Isolation, Identification, Health Benefit and Sensory Property

Flavones and their glycosides (FGs) are an important class of secondary metabolites that are abundant in tea, with a series of health benefits. They not only regulate the physiological functions of tea plants but also contribute to the formation of unique flavor qualities in tea. FGs, therefore, have a practical application value for the development of tea processing, natural food colorants, and health-promoting phytochemicals. This review summarizes the existing knowledge about FGs from the aspects of chemical structure, separation, and identification methods, as well as their changes during tea processing. The biosynthesis and metabolic pathways of FGs in tea plants are also concluded. In addition, the sensory characteristics and biological activities of FGs were also discussed. The flavone glycosides in tea mainly exist in the form of O-glycosides and C-glycosides. There are various types of flavone glycosides in tea, which are classified into kaempferol, quercetin, myricetin, and apigenin glycosides based on their glycosides. Currently, over 100 flavonoid glycosides have been identified through mass spectrometry and nuclear magnetic resonance spectroscopy methods. In addition, qualitative and quantitative variation of flavone glycosides during different processing techniques is also summarized. The taste threshold concentration of flavonoid glycosides is extremely low, which not only offers tea infusion with a dry and soft astringent feeling, but also enhances the bitterness of caffeine. As the main contributing component of the yellow-green color of green tea infusion, flavonoid glycosides also have health benefits, including anti-aging, metabolic regulation, and immune-enhancing functions.