What the Heck is Oxidation in Tea?
Why it matters, what it does, and how it makes your tea taste the way it does
When you read about different types of tea, you often come across the word “oxidation.” Green teas are said to be unoxidized, black teas are fully oxidized, and oolongs are somewhere in between. But what does that really mean? And why does it matter so much for your cup of tea?
The Science in Simple Terms
Oxidation is a natural chemical reaction that happens when tea leaves are harvested and exposed to oxygen. It’s the same process you see when you slice an apple and watch it turn brown. In tea, oxidation begins when the cell walls of the leaf are broken—through rolling, bruising, or crushing—and oxygen interacts with the enzymes inside.
This reaction transforms the polyphenols (plant compounds) in the leaf. Two important groups of these compounds are catechins and theaflavins. Catechins are plentiful in fresh, green leaves. As oxidation takes place, they change into theaflavins and thearubigins, which are responsible for the rich color and bold flavor of black teas.
Why Oxidation Matters
Flavor: Oxidation is the single biggest factor in how tea tastes. Less oxidation means lighter, fresher flavors (like green tea). More oxidation creates deeper, maltier, sometimes fruity or spicy notes (like black tea). Partial oxidation produces the incredible variety of oolong teas.
Color: The chemical changes darken the leaves and deepen the color of the brewed tea, from pale yellow in white and green teas to amber, copper, or even deep red in black teas.
Aroma: Oxidation develops complex aromas. Think of the floral perfume of lightly oxidized oolongs versus the malty, wine-like fragrance of a strong Assam.
Body: Fully oxidized teas usually feel fuller and more robust in the mouth, while lightly oxidized teas feel cleaner and more delicate.
Controlling Oxidation
Tea makers carefully control oxidation to achieve the desired style. For green tea, leaves are heated right after harvest—by steaming or pan-firing—to stop oxidation before it even begins. For oolong, artisans carefully bruise and rest the leaves, monitoring oxidation level by smell and touch, sometimes over many hours. For black tea, the leaves are fully rolled and left to oxidize until they reach the perfect depth, before being fired to lock in flavor.
In Your Cup
The next time you sip tea, notice its color, aroma, and body, and remember that oxidation is the quiet alchemy behind it all. It’s what makes a grassy green taste so different from a malty black, even though both come from the very same plant.
Oxidation is nature’s magic in action—an invisible process that turns simple green leaves into an astonishing spectrum of teas. By understanding it, you can better appreciate the craftsmanship behind every cup and enjoy the remarkable variety the tea world has to offer.