Many tea drinkers get confused by labels saying “unfermented,” “semi-fermented,” or “fully fermented.” The terms are often used interchangeably, but "fermentation" and "oxidation" are actually two separate processes.

Oxidation: Enzymatic Chemical Reaction Oxidation happens when tea leaves are exposed to oxygen after harvest. The leaf’s own enzymes react with polyphenols, changing the colour from green to brown or black and creating bolder flavours (like malty or brisk notes). Producers control it by withering and rolling the leaves, then stop it with heat. It’s similar to an apple turning brown when sliced.

Fermentation: Microbial Biological Process

Fermentation involves microorganisms (bacteria or yeast) that convert compounds in the leaves, often in controlled humid or low-oxygen (anaerobic) conditions. This is a true biological step and doesn’t always need oxygen. True fermentation is common in post-fermented teas like pu-erh, where microbes create earthy, mellow flavours over time.

==> They Are Independent — See GABA Tea as an Example

Because the two processes are separate, teas can combine them differently. A great example is GABA tea. GABA teas use an anaerobic (zero-oxygen) step to boost gamma-aminobutyric acid (GABA) levels, linked to relaxation. This is a form of oxidation: 0%.

At the same time, the tea can have varying fermentation levels. Formosan Farms offers GABA Oolong (0% oxidation + 55%  fermentation) and GABA Red (0% oxidation + 100% fermentation). Formosan Farms’ GABA Oolong and GABA Red beautifully demonstrate how these two independent processes create distinctive, high-quality teas for different tastes and various purposes (e.g., warmup).

Note: further reading about GABA tea