After any plant is harvested, a cascade of reactions begins in the leaves as they respond to being plucked from their source and separated from their connection to the soil.
These processes have common themes across all plants. For example, they will all begin to wilt, which causes the structure of their cells to become damaged, therefore allowing usually separate compounds to mix with each other and the air.
These chemical interactions are what we broadly call Oxidation in the tea world, and they change the taste and colour of the tea. It is way too simplistic to say that this is just Oxidation, so some people call it fermentation, but this suggests just microbial action, which is also not an accurate term.
In truth, we do not have any word that describes the multitude of reactions which happen as a leaf transforms after picking, so we will stick with ‘Oxidation’ for simplicity.
The six tea types are based on how the producers manage the process of transformation of the leaves after picking to steer the personality of the tea in various directions.
Oxidation is a very quick process – a tea leaf will transform from vibrant green to black and dry in a matter of days if treated roughly. So, the producers have to act immediately after picking, working non-stop with very little sleep (the leaves don’t stop transforming at night).
STOPPING OXIDATION (FIXING)
There are broadly two ways to stop the Oxidation (or, more accurately, SLOW the Oxidation to a very slow pace).
The first is to apply heat. If the leaf reaches temperatures above about 250 degrees C (around 500 F), this heat deactivates enzymes in the plant, which are present to speed up Oxidation. By stopping the enzymes from working, the Oxidation will take years and not days. This process is sometimes called the ‘kill-green’ or ‘fixing’ process.
The second method is to dry the leaf completely. If the plant has no water, then the chemical compounds can’t mix and react, and the leaf is in a stalled state. Of course, there is always some humidity in the air, so, just like with heating, the leaf never really stops changing.
This continuous change is why some teas should ideally be consumed within a couple of years of production whereas others can take advantage of years of storage and age beautifully.
THE MAIN PROCESSES
Here are some of the key steps used by producers when making tea. The sequence of steps they choose will determine the tea type.
Withering – spreading out the leaves after picking to wilt and transform. This gives an opportunity for the leaves to build different flavours and aromatics.
Shaking – picking up the leaves and tossing them around. This can be gentle or vigorous. There are a few purposes for shaking. One is to even out the water distribution (the water evaporates from the leaves and stays stubbornlly in the stems). Another purpose for shaking is to slightly damage the leaves to allow their compunds to have more exposure to air to increase Oxidation. A final purpose is to even out a pile of leaves so that those sitting at the bottom of the warm pile can be moved to the surface for exposure to air and regulation of temperature.
Rolling – vigorously rubbing and pressing the leaves to break the outer membrane and bring out the leaf juices.
Oxidising – piling the leaves (to increase temperature) and leaving them to darken. This is a natural extension of Withering but with more emphasis on accelerating Oxidation.
Fixing – heating the leaves to fix the oxidation level. It can also be useful for shaping the leaves and adding a toasted flavour. This process can also be called ‘Firing’ or ‘Kill-Green’, or ‘Sha Qing’.
Drying – reducing moisture level to less than 5%, usually by heating in stages at low temperatures (up to approximately 100c/212F)
Roasting – using heat to change the character of the taste or texture of the tea.