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Why is oxidation so important for tea?

Ever wondered what the difference is between green tea and black tea? One word; oxidation. This is a chemical reaction which changes the colour, smell, flavour and brewing parameters of tea leaves.

A simple example of the oxidation process is when rust appears on iron or steel, where water and oxygen react with the metal in a degrading process. In chemical terms oxidation occurs when electrons are stripped from the main component, causing a gradual decay. This reaction in foods is called enzymatic browning, where oxygen reacts with compounds in the food to change the chemical makeup of the product. This process is easily illustrated by cutting an apple in half and leaving it exposed to the air for short period of time, the flesh of the fruit begins to oxidise and brown pigments (melanosis) begin to appear.

Often this has a negative effect on the food, causing it to spoil and become inedible, however in certain instances it can help to create hidden flavours. Foods which  benefit from such reactions include raisins, figs, coffee, cocoa beans and, of course, tea. The ‘black’ in black tea comes from the colour created by melanosis after the leaves has gone through the oxidation process, however green tea does not go through any such process and thus keeps its green leafy colour and vegetal flavour. However we will continue to focus on the production of black teas and the role of oxidation.

Breaking the leaves

Once the leaves have been withered and their water content has been reduced, to produce a quick and consistent oxidation reaction, the leaves go through a process where they are slightly damaged or bruised. In technical terms this is called ;maceration’, ‘disruption’ or ‘rolling’. You can see in the image below the machine that does this process; a large plate slowly rolls and grinds the leaves round a serrated surface to damage the cell walls and allow oxygen to quickly react with the leaf in an even consistent manner.

The leaves are then left to oxidise for a very specific length of time according to the type of leaves and to achieve a particular flavour profile.  Generally speaking black teas are subjected to full oxidation but Oolong teas are only partially oxidised in a variety of ways depending on the type of tea and are halted through a process called “fixing” by applying a heat of at least 65°C

Drying the tea

For black tea once the tea leaves have hit the high point of oxidation, the leaves are put through dryers which blast them with hot air at temperatures of 80-90°C. The leaf darkens further and the humidity is reduced down to somewhere under 5%, taking on the dark brittle appearance we more accustomed to seeing.  The reduction of water content means that the shelf life of the product is significantly improved and it can be packed and shipped round the world without spoiling.

Black or Green?

The leaves of the Camellia Sinensis plant can be used to produce either Black tea or Green tea, the only difference is how those leaves are treated. If it goes through a process of full oxidation then it is a Black tea, if no oxidation occurs then it is a Green tea.

The oxidation process changes the colour, flavour and body of the tea, but also within the oxidation process how long the tea is left to oxidise and time taken to dry can play a factor in the further development of the flavour. So next time you have your cup of English Breakfast tea, just remember the chemistry that has gone on to create your brew.

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