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Blog 168: Why Do Some Black Teas Taste Sour?

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- Yunhan

In an ancient Chinese medical text, Inner Canon of the Yellow Emperor (黄帝内经), “Tastes” are categorized into 5 groups: Sour, Bitter, Sweet, Pungent/Spicy, and Salty. (Obviously, spicy is not a taste. But in the original Chinese writing, the author used the character “辛/Xin/Pungent” instead of “辣/La/Spicy”. Therefore, we use “pungent” as the preferred translation.)

There is a reason why sourness was listed as the No. 1 among all tastes. Sourness is the most common taste among unripe fruits. The right amount of sourness can be very refreshing. In traditional Chinese herbal medicine theories, sourness is also good for the spleen.

However, a strong sourness in tea is often an indication of bad quality or problems in the tea-making. Today, let’s discuss the causes behind a sour black tea.

The sourness is the direct result of black tea’s fermentation process. Black tea is fully fermented, and its fermentation is the key in the overall quality formation. Although there are many kinds of black tea products, the major tea-making steps are the same: withering, rolling, fermenting, and drying.

Naturally, the fermentation is the most important tea-making step. Scientifically speaking, black tea’s fermentation is polyphenols’ enzymatic action caused by the damage of leaf cell tissues. As a result, substances such as thearubigin and theaflavin get developed and transformed.

During the fermentation process, fresh black tea leaves develop more water-soluble pectin and soluble monose. Both substances help increase the sweetness and the thickness of the tea soup.

If leaves are under-fermented, the final product would taste very “green” and grassy. If leaves are over-fermented, it’s very likely that the final tea soup tastes sour.

There are a couple reasons why a black tea can be over-fermented, and we’ll focus on one most common reason: imperfect drying process.

Right after the fermentation, it’s the drying process. The purpose of the drying process is as straightforward as its name suggests: reducing the water content. For example, the water content must be reduced to no more than 7% for a standard Lapsang Souchong product.

Interestingly, the drying process can be exactly the reason a black tea gets over-fermented.

Drying requires heat, and heat needs to be created. If the heating up process takes too long, it’s possible that tea leaves get fermented again and develop the sour taste.

Furthermore, area of production also has an impact on the fermentation process. For example, Tongmu Village, the birthplace of black tea, has a high elevation and high humidity. The intensity of the normal drying process is usually not enough to reduce the tea leaf water content down to 7%.

Therefore, in Tongmu tea-making, a unique step called “Red Pan” is used to prevent leaves from over-fermentation. “Red Pan”, or 过红锅 (pronunciation: Guò Hóng Guō, meaning: passing the red pot), uses heat to kill off enzyme activity to stop leaves from fermenting. (Yes, it’s very similar to “kill-green/杀青” in green tea and oolong tea.)

Finally, we’d like to point out that even a perfectly made black tea might still carry just a little bit sourness. If it exists, the sour taste would be extremely light, and many tea drinkers probably wouldn’t even notice it.

We hope you enjoyed today’s blog. As always, if you have questions or suggestions, please leave a comment, tweet us @valleybrooktea or email the author directly at Please also follow us on Instagram @valleybrooktea and join our mail list to get our daily tea updates and our latest promotions!

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