White, Green, oolong, and black tea all come from the same plant (camellia sinensis) but are differentiated by the level of oxidation they undergo.
What is oxidation?
In layman's terms, oxidation is a chemical reaction involving oxygen1. In tea and food, this reaction results not only in a physical browning of the substance (like the banana peel above) but also in the creation and unlocking of new compounds at a molecular level. Sometimes, oxidation is a good thing and leads to the creation of compounds like theobromine, which is used as a bronchodilator in pharmaceutical drugs (i.e. asthmatic medicine). Other times, oxidation is not a good thing; for example, the weight loss compound EGCG found in green teas quickly degrades by oxidation. The following formula summarizes the biological oxidative reaction:
enzymes or light + oxygen + biochemical compound = oxidation
This reaction occurs best in a humid and warm environment (under 95°F)
Oxidation ≠ fermentation
Although disproven for sometime, the process of oxidation is mistakenly referred to as fermentation in regards to tea. Until the advent of newer technology it was assumed that the browning of tea leaves was through fermentation, which would entail microbiological activity like yeast and beer. This has been disproven for sometime, however there is still much literature in the tea world that confuse the two or use the words equivalently.
Two forms of oxidation
Passive oxidation is natural oxidation without an outside stimulus, such as the greening of copper, or the rusting of metal. This process is excruciatingly slow in comparison to active oxidation. In regards to tea, the moment the leaf is plucked off, the bush oxidase (oxidative enzymes) are activated resulting in passive oxidation. During this passive oxidation/withering stage, aromatic compounds are especially keen to develop. This method of oxidation continues on through the withering stage until it is halted by the enzyme kill stage, where the tea leaves are briefly flashed with heat.
Controlled oxidation is where the conditions for oxidation have been augmented by raising room temperature, humidity, and physically breaking leaf epidermis. The degree of oxidation can be measured by various parameters. For example, in black tea, this process is usually controlled to maintain a ratio of theaflavins and thearubigins of around 1:10 to 1:12.2
Black tea ≠ fully oxidized
On a related note, black tea is often incorrectly referred to as being “fully oxidized”; this is untrue, as oxidation halts when the right parameters are reached. A fully oxidized tea would more appropriately be termed a “stale” tea, a tea way beyond its shelf life.
How is oxidation stopped?
There are two things that can be done to halt oxidation: 1) denature oxidation enzymes or 2) displace oxygen.
1) Denaturing the oxidation enzymes entails treating raw tea leaves with heat. This occurs in two different stages: the “enzyme kill” stage and the “final firing” (drying). The earlier stage, “enzyme kill”, is a high heat quick process lasting under one minute. This can be done by steaming, pan-frying (tossing in a wok), or by baking the leaves. The later stage, “final firing”, is done only by baking the leaves for about 20-60 min with the goal of reducing water content in the leaves to 3-5%.
The use of heat during the final firing step actually results in an initial rise in enzymatic activity where 10-15% of theaflavins will form in the first 10 minutes.3
2) The second way to halt oxidation is to remove oxygen. This is mainly the issue for the storage of tea. I strongly recommend storing tea in an air-tight and light proof container will minimize the opportunity for passive oxidation. This is why Eli Tea tea’s are sold in tins.
In the tea trade, wholesalers of tea usually package tea in vacuum sealed, nitrogen enriched bags. Sealing bags with nitrogen would displace and or diffuse any oxygen from reacting with the tea. These stringent measures for preservation increase shelf life of the tea to up to ten years, compared to tea simply sealed in an airtight container, which has a shelf life of 6-24 months.
Continuing on the topic of nitrogen, Japanese researchers in 1987 developed a new technique for the “enzyme kill stage” where the tea was given a nitrogen treatment instead of the traditional heat treatment methods. Treating the tea with nitrogen increased gamma aminobutyric acid and therefore these teas have adopted the acronym suffix of GABA. (i.e. GABA oolong, GABA green etc.) GABA can be naturally found in the brain and functions as neurotransmitter.
Health benefits of oxidation in tea.
Tabulating the health benefits of oxidation of tea is not an “apples-to-apples” argument; white tea has many health benefits due to a lack of oxidation, and conversely, black tea has many health benefits because of the extra oxidation. There are two categories of compounds to show this sometimes good sometimes bad oxidation relationship.
Catechins are an organic compound of interest for their involvement in metabolic reactions in the body. In layman's terms, these are compounds that encourage weight loss, while xanthines, like caffeine, are stimulatory compounds. Oxidation causes the loss of catechins and the gain of xanthines. The take-away message from this example and post is that there is value in each of the teas (white, green, oolong, black) that make up this spectrum.
I hope this post shed some light on the relevance of oxidation in tea processing. As always, there is much more that can be said about the topic, so if you have any comments or questions, please leave them below.
Footnotes and References.
The definition of oxidation as found in the The American Heritage® Stedman's Medical Dictionary 1. oxidation ox·i·da·tion (ŏk'sĭ-dā'shən) n.
2. Spiller, Gene A. Caffeine. Boca Raton, FL: CRC P, 1998. 47.
- The combination of a substance with oxygen.
- A reaction in which the atoms in an element lose electrons and the valence of the element is correspondingly increased.
3. Preedy, Victor R. Tea in health and disease prevention. Amsterdam: Elsevier/Academic P, 2013. 958.
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