Black tea gets its dark color and bold flavor from oxidation, the same process that turns a sliced apple brown.

The primary chemical process behind black tea is enzymatic oxidation, specifically involving enzymes called polyphenol oxidase and peroxidase. When tea leaves are crushed or rolled, these enzymes react with oxygen and polyphenols like catechins. This reaction converts simple catechins into complex compounds known as theaflavins and thearubigins. Theaflavins are responsible for the bright, brisk flavor and golden-yellow color of the tea. Thearubigins are larger molecules that provide the deep reddish-brown color and the characteristic body or 'thickness' of the brew. In contrast, green tea production involves 'killing the green' by heating leaves to 150 degrees Fahrenheit (65 degrees Celsius) to denature these enzymes. Research from the Tea Research Association indicates that black tea typically undergoes oxidation for two to four hours at temperatures between 25 and 30 degrees Celsius. This precise timing ensures the perfect balance of chemical compounds. If the process continues for too long, the tea loses its briskness and develops a flat, dull taste. This biological phenomenon is nearly identical to the browning seen in fruits like apples and bananas. In both cases, the plant's defense mechanism is triggered by cellular damage. For tea drinkers, this natural reaction results in a more robust caffeine profile and a shelf-stable product that retains its flavor for years.