What is Oxidation in tea?

In the world of tea, you mad have heard about Oxidation, and it's no surprise as to why: the production of tea encourages or discourages oxidation, resulting in different categories of tea. Put simply, Green tea is 0% oxidized, and Red Tea (called Black in the West) is fully oxidized. But this doesn't answer the fundamental question: what is oxidation?

 
In the world of tea you made have heard about Oxidation, and it's no surprise as to why: the production of tea encourages or discourages oxidation, resulting in different categories of tea. Put simply, Green tea is 0% oxidized, and Red Tea (called Black in the West) is fully oxidized. But this doesn't answer the fundamental question: what  is  oxidation?
 

Oxidation in a nutshell

Any dictionary will tell you that Oxidation is a chemical reaction with oxygen in which atoms lose electrons. When plants are damaged and air gets in and enzymes rapidly oxidize causing that brown effect you see on cut apples. 

With tea, this happens when the leaf is plucked, or damaged, too.

  1. The leaf is plucked from the bush.
  2. Polyphenols reactions start, carried out by enzymes in the leaf.

When the tea leaf becomes Oxidized, some cool stuff starts to happen. Catechins begin to convert into Theaflavins & thearubigins.  Fats, proteins, and pigments break down into aroma and flavor compounds.

Fermentation or Oxidation?

True fermentation is the chemical breakdown of microbes, especially in regards to yeast, in a substance. Oxidization is a completely separate process, as it's a chemical reaction from Oxygen.

While there are some teas that do involve true fermentation, such as true Black Tea, and to a degree Raw Puers, most teas are not fermented. The majority of the time that you hear the word "fermentation" in regards to Wu Long (Oolong) teas or Red teas (Black Tea in the West) Oxidization is really what's being discussed.

Fermentation is not Oxidation, although the term is used in tea making interchangeably.

Why the confusion? When Tea was becoming popular in the west in the mid-19th-century wine terminology was applied to the mysterious and little-understood tea. That termination still exists today.

Oxidation in Tea Categories

The enzymes in the leaf mix with oxygen to oxidize. Different tea categories have different levels of oxidization changing the taste.

  • Green tea is not oxidized. 
  • White tea is 1-10% oxidized.
  • Wu Long tea is 10-85% oxidized.
  • Red tea (known as black tea in the west) is fully oxidized.

The vegetal and astringent notes of unoxidized tea (like Green Tea) convert into the rich, tannic, malty and fruity tastes of oxidized tea (like Red tea.)

Passive vs. Active Oxidization.

As mentioned before, the moment the tea leaf is picked the enzymes in the leaf begin to oxidize it. This is a slow, or passive, process - hardly efficient for Red tea which in which the enzymes are fully exhausted in the leaf.

To help speed things up, tea manufactures bruise or roll the leaf to rupture more cells, spilling the enzymes and juices through-out the leaf causing a speedy, or active, oxidization process.

Stoping Oxidation

What separates green tea from most other teas is it has effectively no oxidization. In order to achieve this, all the enzymes in the leaf must be denatured - and rendered inert. 

This can be done pretty easily with a processing method called "kill green." The wilted leaves are hit briefly over high heat that literally "kills" the enzymes from causing oxidation. By doing this, Green tea retains it's bright green color and original taste after processing and drying. 

It's important to note though, technically Green tea does have a negligible amount of oxidation. because as soon as the leaf is picked the process starts.

Encouraging Oxidization

As you've seen, Oxidization brings a lot of changing on to a tea leaf. Wu Long teas and Red teas are processed in a way that encourages or exhausts the enzymes in the leaf; usually with active oxidization methods like bruising and rolling.

  • In Wu Long, the amount of oxidization can range. The lighter the oxidization, the "closer" in flavor profile to a green tea we can expect. The darker the oxidization, the "closer" in flavor profile to Red tea.
  • Red tea, on the other hand, has effectively had all of it's enzyme exhausted and we can say that Red tea is fully oxidized. 

Enzymes are what we're talking about

One final, but important, note. When we talk about Oxidization in tea leaves, we are primarily speaking about the potential of the enzymes in the leaf to carry oxidization out.

After the enzyme is killed (like in green tea) or is fully exhausted (like in red tea) further oxidization can still occur - just not at the hands of enzymes, but from natural electron loss in the presence of oxygen. (Like rust.)

Okay, that's a mouthful, we can unpack that simply like this: tea goes stale. Green tea, left out in oxygen will oxidize and lose its luster. Red tea will oxidize too, and lose its taste and nuisance.

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