Oxidation

I briefly mentioned oxidation in my getting sorted post when I was talking about the different categories of tea and thought it might be worth talking about a little more.

Oxidation is basically what causes the leaves of the tea plant (that are green when they grow) to turn brown. It is a biochemical reaction which involves the absorption of oxygen (like when an apple is cut). Black tea is generally close to fully oxidised, Green teas are usually non-oxidised and Oolongs tend to be partially oxidised to varying degrees. Oxidation is pivotal to the processing of tea and will change its colour, smell and flavour.

Oxidation Chart

Oxidation Chart (Source)

Oxidised teas, are bruised (from lightly to extensively)  to break the  cell walls and allow the enzymes in the leaves to cause natural oxidation reactions. Heating the leaves stops oxidation by deactivating the enzymes. In this way the tea producer can decide on the extent of oxidation by introducing heat. Green teas are non-oxidised and so are heated early in the production process so that the oxidation process is skipped.

Sometimes tea-oxidation is called fermentation but no microorganisms are used so this is a misnomer. Generally when people talk about tea-fermentation they are talking about oxidation.

However, just to complicate matters, the proper micororganism fermentation does take place with pu-erhs.

There are exceptions to this, but here are the usual oxidation and fermentation levels:

Black tea – almost fully oxidised

Oolong tea – partialy oxidised (ranging from 12% – 80%)

White tea – minimal oxidation

Green tea – No oxidation

Yellow tea – No oxidation

Pu-erh – fermented (sheng pu-erh is not oxidised but shou pu-erh is)

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4 Responses to Oxidation

  1. M says:

    I dont get the point of this. Why you going into science on a tea blog?

    • admin says:

      Hi M, The point of this article was to talk about an aspect of tea that I find amazing – how is it that all tea comes from a single species of plant that has green leaves and yet some tea is black in colour, some is bright green, some is dark green, some is white/grey and some is brown? I understand that oxidation is getting kind of technical but it plays a big role in the differences between categories of tea. You definitely don’t need to understand oxidation (or processing) to enjoy tea but for me it adds to the fun 🙂

  2. JC says:

    Hello,
    I am confused now, you’re saying that yellow and green teas are less oxidised (in fact, not oxidised) than white tea, but your chart says otherwise. I’m confused… which is right?

    PS: to M, this is a matter of opinion. On the contrary I find this information fascinating. There is no reason why tea could not be approached with the same level of seriousness (“science”) as wine. But I agree that some people prefer to just drink wine without any interest in soil geology, while others will enjoy learning about the links between the character of a wine and its origin and the wine making process. The same applies to tea in my opinion.

    JC (from Dublin, Ireland)

    • admin says:

      Well spotted JC. Yes, the chart is slightly misleading by having white tea on the far left (and saying unoxidized underneath!). As a general rule, White teas undergo very little processing and are naturally more oxidised than Green tea. I didn’t have time to make up my own chart for the post but I’ll do that when I get a chance. You would not believe the number of charts out there with far worse inaccuracies!