Oxidisation and fermentation of tea - huh?

« back to blog home | Thu 09 Feb 2012

One of the most important aspects of tea making is a process called 'oxidisation' - some teas need it, and other teas are ruined by it. Here's a quick intro for you on one of tea's most misunderstood words...

What is oxidisation?

Oxidisation is the process of getting oxygen into the tea leaves. Simple.

Well, it's a little more complicated. For tea to release its flavours, the cells of the tea leaf need to start breaking down and changing, and this only happens when oxygen is introduced. Oxidisation actually starts happening as soon as the tea leaf is plucked from the tree, but the tea master must quickly control that process if he wants to produce a good quality tea.

For example, if tea leaves are plucked and then crushed, they will start oxidising immediately, but the flavour will escape through the broken leaves. If the leaves are left out too long in the sun, they'll oxidise, but they'll also start to turn yellow which most people find unattractive (yep, we're suckers for the luminous green leaves).

Isn't oxidisation the same as rusting?

Yes and no - it's just the name for when oxygen reacts with something else. Actually we most commonly see it when we cut an apple in two and 30 minutes later the flesh has turned brown.

Fermentation is not the same

Fermentation is a different process, and it's most commonly needed for Puer teas (and definitely not Oolong or green teas). It's very similar to decomposition, and it's the work of bacteria breaking down the tea leaves (rather than a chemical reaction with oxygen).

When tea farmers produce green teas, especially those produced in Fujian, they try to avoid fermentation by drying and/or roasting the tea as it will ruin the taste of those particular varieties.

How tea is oxidised

Tea is oxidised in a variety of ways - the method, the length of time, the temperature and the humidity all contribute to the final taste.

For example, tea is laid to dry in the sun, where oxidisation begins. Afterwards, it is often gently heated and bruised in large tumblers, or left to dry on bamboo racks. The air needs to be changed regularly, and the heat and humidity carefully maintained to ensure the right level of oxidisation occurs.

Clearer now?

Hopefully that helps explain in simple terms what oxidisation actually means in relation to tea - and remember if someone offers you a fermented oolong, refuse it!

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