Oxidation is a chemical process in which an atom, ion, or molecule loses one or more electrons, resulting in an increase in its oxidation state. This process often occurs simultaneously with reduction, where another species gains those electrons, in what is known as a redox (reduction-oxidation) reaction.
For example, when iron rusts, it combines with oxygen to form iron oxide.
In regards to tea, oxidation refers to the chemical process that occurs when compounds in the tea leaves interact with oxygen after harvesting. This enzymatic oxidation alters these compounds, which affects the tea’s color, flavor, and aroma. The degree of oxidation determines the type of tea produced, such as white, green, oolong, or black.