What is Kill green?

Kill green or killing the green is the step in tea processing that keeps the leaves green, thus making green tea. I have seen other more general names such as fixing, fixation or de-enzyming.

In normal circumstances a picked tea leaf will turn brown, it is similar as the browning of a cut apple or the forming of rust. This process is called oxidation. For a green tea we dont want this brown color. We need the leaves to stay as green as possible. So instead a tea manufacturer is going to use a trick.

First an introction about oxidation. In the world of chemistry an oxidation reaction takes place, when substance that can be oxidated and some oxygen come in contact with each other. The forming of rust is a reaction of Iron and Oxygen and sped up by water and salt.In the case of tea leaves, the oxidation reaction is a reaction of polyphenols and oxygen accelarated by enzymes. The two enzyms most known to be responsible for this reaction are PolyPhenol Oxidase (PPO) and Peroxidase.

In living plants the different parts of the reaction are seperated from each other in the cells of the plant. From the moment a leaf is plucked, cell walls are damaged and all three chemicals can start reacting.

What tea manufacturers do to slow down this reaction, is the kill green method. They heat up the leaves to a temperature above 65°C of 150 °F to denaturize or break the enzym. This means that the reaction will slow down dramatically. Just like the forming of rust can take many years if iron doesnt get wet. If the enzyme is not present to speed up the reaction, it can take many years. This also explains the shelf life of green tea, your, green tea will not stay green for ever.

Japanese green tea

Then why is there still a difference between some green teas ? Well, for example in China they pan-fry the leaves and in Japan they steam the leaves. Heating accelerates every kind of reaction, the enzymatic reaction above but also the same reactions that occur during cooking. The longer you heat the leaves and the longer you need to reach the temprature to break the enzym, the more reactions will take place. The steaming method in Japan is the shortest method to reach the temperature, the shortest heating period and therefore it loses less green.

Does this mean that Japanese green tea is better? of course not, You might say that this is a choice made by the tradition and it is the tea masters choice which flavour balance he seeks.