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Oxidation

Tea leaves on a bamboo basket and in hands

Oxidation

Oxidation describes the process of enzymes in the tea leaves reacting with oxygen. During oxidation, leaves undergo changes in their chemical composition turning them brown and changing the flavor and aroma profile.

Oxidation is an essential step in the processing of tea. Depending on the type of tea which is made, oxidation is either fully prevented or intentionally caused and stopped at a certain level. Whereas green, white and yellow teas are unoxidized or lightly oxidized, black teas are highly or fully oxidized. In between are oolong teas, which cover a wide range of oxidation levels from lightly oxidized to highly oxidized.

The process of oxidation is intentionally caused by crushing, rolling or tumbling the leaves. Thereby the walls of the cells are damaged, exposing the inside of the leaves with oxygen, resulting in a series of chemical reactions. In order to stop the oxidation at a certain level, leaves have to be heated or dried. Heat denatures the enzymes responsible for oxidation.

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Table of Contents

Tea leaves on a bamboo basket and in hands

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Oxidation vs. Fermentation

Oxidation is often confused with fermentation, which instead is a process using microorganisms to produce dark teas like Pu’er.

Oxidation Level by Tea Type

Green Tea — Minimally oxidized (approx. 0% - 10%)
White Tea — Slightly oxidized (approx. 10-15%)
Yellow Tea — Slightly to medium oxidized (approx. 10-40%)
Oolong Tea — Slightly to highly oxidized (approx. 10% to 80%)
Black Tea — Highly to fully oxidized (approx. 80% to 100%)

Further Reading

Continue learning with related articles

Loose leaves of black tea.

Black tea is a highly oxidized type of tea. It is higher oxidized than Oolong, yellow, white and green tea giving the leaves a dark color and the brew a strong flavor. The tea has its origin in East and Southeast Asia, with both the variety sinensis and assamica being used to make it. The western term black tea refers to the color of the oxidized leaves. Black tea is known as hong cha in Chinese which translates to red tea. It was named red tea because of the color of the liquid. Black tea should not be confused with dark tea which is a post-fermented tea called Hei Cha in Chinese which translates to black tea.

Loose leaves of green tea.

Green tea is a very lowly oxidized type of tea, unlike oolong and black teas which go through an oxidation process. Leaves of Camellia sinensis var. Sinensis are better suited for producing green tea than those from Camellia sinensis var. Assamica which are usually used to make black tea. Originating in China, production spread out into other areas of Eastern Asia. There are many styles of green tea that differ substantially in flavor, leaf look, and production process. Green tea is a type of tea with the highest antioxidant properties. There are many research studies on its health effects, but there is little evidence showing any effects on people's health.

Loose leaves of white tea.

White tea is one of the most recent tea types out there. It is the *least processed* tea type and has only been lightly oxidized. But just because of that, it does not necessarily mean that it is simple in taste. In fact, the processes it goes through are very meticulous and important for its taste development. Its production center is in China, especially in the *Fujian* province. In the last few years the production areas of white tea are increasing thanks to its gaining popularity.

Tie guan yin loose leaf oolong tea balls on bamboo basket

Oolong tea is a semi-oxidized type of tea ranging from approx. 10 to 80% oxidation. Being more oxidized than green tea and less oxidized than black tea it is often describes as an in-between type of tea. Due to the great range of oxidation, oolong teas have a widely varying flavor profile including sweet, fruity, honey, woody, thick, roasted, green and fresh notes. The leaves undergo the most complex production process of all tea types. It includes plucking opened buds, withering under the sun, bruising to start and heating to stop the oxidation, shaping into a strip or half-ball shape, drying and roasting. Originating in China, many oolong styles, especially ones from the Wuyi mountains, hold the Chinese famous tea title.

Tea leaf of the camellia sinensis plant

Tea Classification

Classification is the practice of grouping teas by similarities in the production process to understand relations, similarities and differences. Tea can be categorized in many ways, the most prominent way being the six tea types. In the tea world, terms are often used inconsistently and interchangeably which can be confusing. At Steeped we use the following classification terms:

Mengding huang cha yellow tea loose leaves in gaiwan

Yellow tea is a rare and expensive type of tea with a low oxidation. The processing is similar to that of a green tea but with an additional step of keeping the leaves in a hot environment. This leads to a more mellow taste and yellowish colored leaves. The tea is mainly produced in China and Korea.

Pu'er dark tea pressed leaves as tea cake

Dark tea is a microbially fermented type of tea. The fermentation continues several months to many years and the leaves are often pressed in cake or brick shapes. The most famous dark tea is Pu’er from Yunnan, China.