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Is the fermentation process in tea true fermentation or just oxidation?

We asked our boffin friend Professor Mike at NCSU. This is what he told us:

 

“I have done a bit of looking into your tea question. One issue is that the term ‘fermentation’ (originally coined by Louis Pasteur) has taken on several meanings. To most folks in the know, a fermentation is a specific type of microbial metabolism in which a sugar (or something similar) is partially degraded under anaerobic conditions (ie no oxygen from atmosphere). The fermentation process is typically described in terms of its major excreted byproducts (ie alcohol fermentation, lactate fermentation, acetone/butanol fermentation etc), rather than its starting materials. These are not single step transformations that could be brought about by individual enzymes but are all multistep processes that can only be brought about (and sustained) by intact organisms such as bacteria and yeasts.

 

The term fermentation has also been extended to include other biological processes that occur in a reactor. These reactors are typically called fermenters and the act of growing a microbial culture in one of these reactors (fermenters) is sometimes called a fermentation even if the biochemical process involved does not fit in with the biochemical definition of fermentation.

 

From what I can see there is another use of the term fermentation applied to tea. In this case the process is an entirely enzymatic process that takes a group of compounds called polyphenols and oxidizes these compounds to coloured products. These reactions are catalyzed by enzymes called polyphenol oxidases (among other names). This is exactly the same type of process that accounts for why apples go brown when you cut them and I think it is also the same process responsible for the rapid browning of avocados when you mush them up to make guacamole. While this might be called a fermentation, it does not fit the specific biochemical characteristics of this type of process. For one thing it is a single enzyme-catalyzed step rather than a multistep process catalyzed by a microorganism. Second, this tea fermentation is an obligately aerobic process, whereas a true fermentation is an obligately anaerobic process.

 

White, Yellow, Oolong TeaI looked into the different tea types and in essence the least fermented (tea fermentation) is white tea, followed by green tea followed by Oolong tea and then black tea. What I did find was that in most cases the fake tea fermentation (aka enzynmatic oxidation) process is terminated by heating the raw tea to inactivate or denature the polyphenol oxidases. The sooner the activity of these enzymes is stopped, the more polyphenols remain and the less "browned" the tea. Conversely, the more the tea is bruised and crushed before the heating step, the more mixing of the polyphenols and polyphenol oxidases occurs. This in turns leads this leads to the darkest teas. Stated a bit differently, white tea is treated minimally except to heat it to prevent true decay and putrescence, whereas black tea is serially abused until it is fit only for the English.

 

Compressed Puerh TeaAs far as Pu-erh tea is concerned, in summary this tea is made from unoxidized green tea. A form of this tea that is not further processed or moulded and is sold as loose leaf tea is called moacha. Pressed moacha that undergoes no further processing apart from moulding is called raw pu-erh. This tea can also be allowed to age and this process most likely involves some slow microbial degradation of the tea as well as some residual native biology in the leaves themselves. Another product is cooked or ripened pu-erh. This stuff is deliberately incubated under conditions that promote further oxidation and microbial biodegradation (again called a fermentation) that gives this tea a dark colour of the sort normally associated with black teas. This is akin to composting and my impression is this is a frowned upon practice that has only been fairly recently developed.

 

The long and the short of this is I think that to refer to the process that tea undergoes as ‘fermentation’ is probably right if you give the definition of fermentation quite a wide interpretation. However, the basic processing of teas is entirely an oxidation step, while the more subtle nuances of "aged" tea like Pu-erh looks like it does involve a further microbial degradation of tea. Whether or not it is fair to call this ageing a fermentation is up to you but probably would not pass muster from a strict microbiology or biochemistry perspective.”

 

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