Thearubigins are a group of polyphenolic compounds that are formed during the oxidation and fermentation of tea leaves, particularly in black tea. They are responsible for the dark color and astringent taste of black tea. Thearubigins are complex and heterogeneous, lacking a definitive chemical structure, which makes it challenging to assign a specific molecular weight or formula. They are formed from the oxidation of catechins, which are flavonoids present in tea leaves. Thearubigins contribute to the antioxidant properties of tea, although their exact chemical nature and health effects are still under investigation. They are moderately soluble in water and alcohol, but insoluble in fats, reflecting their polyphenolic nature.