Blog 98: Visible Caffeine on Tea Leaves?

There are few teas have white tea leaves.


In fact, among all major teas, only white tea has white leaves. White tea’s white color doesn’t actually come from leaves. It’s the small white fuzz on green leaves that give white tea’s name.



Recently, we came across an online post asking why his oolong tea has some “white leaves” mixed in the package.


Specifically, there are “white substance” on tea leaves. Many comments speculated that the tea in question has either gone bad or was badly made.


However, neither answer is correct. After reviewing the photo he posted, we determined that the “white substance” on tea leaves is simply crystalized caffeine.



Many tea and coffee drinkers talk about caffeine, but few have actually seen caffeine in a physical state. Crystalized caffeine has a frost-like appearance, and its formation is similar to frost as well.


In cold days, when water vapor coming in contact with a solid surface whose temperature is below freezing, a thin layer of ice forms over it. And we call it frost.


Frost on our tea plants

Caffeine is naturally presented in tea leaves. Caffeine starts to sublimate at a temperature about 120℃/248℉. When sublimated caffeine getting in contact with a cold surface, it condenses into a crystalized state. Rapid changes in temperature are the key in the formation of crystalized caffeine.


As we’ve introduced before, oolong tea has a roasting process. During this process, the sublimation and the condensation of caffeine is often observed in the roast room. Crystallized caffeine can also be found on the walls and ceilings of a roast room. Each time we clean up the room, we can get tens of pounds of crystallized caffeine. (Fun fact: because of the purity and the quality of crystallized caffeine found in roast rooms, it becomes quite a high-demand byproduct of the tea-making.)


Our old roast room before renovation. You can see the caffeine accumulation on the walls and ceiling.

But if you’re familiar with oolong tea, you’d notice that not all oolong tea leaves have visible crystallized caffeine. In reality, it’s rare to see oolong tea leaves with crystallized caffeine.


That’s because the level of caffeine content decides whether a oolong tea develops crystallized caffeine. It’s also affected by tea plant cultivars, the growing environment, and particular tea-making skills. Many oolong tea products outside China are actually lightly roasted, and they don’t develop crystallized caffeine.


The accumulation of crystallized caffeine is associated with the roasting temperature. If roasting temperature is too low, caffeine wouldn’t sublimate; if roasting temperature is too high, caffeine would evaporate into the air. Only a certain range of the roasting temperature can give us crystallized caffeine.


Traditional roasting methods are more likely to produce crystallized caffeine. Charcoal roasting is done in a closed environment (in a roast room). No matter where caffeine goes, it always stays in the room.


Roasting oolong leaves

Oolong leaves with crystallized caffeine often means tea leaves are more throughly roasted. These leaves can be stored for a longer time.


Nevertheless, crystallized caffeine is NOT an indication of an oolong’s quality.


As we’ve explained earlier in this blog, as long as there’s a rapid change of temperature during the roasting process, crystallized caffeine would appear on the surface of leaves. If crystallized caffeine means high quality oolong, we only need to control the temperature to create this physical phenomenon.


The presence of crystallized caffeine only shows that a tea is throughly roasted. It’s NOT a standard or indication of quality. Some good teas have crystallized caffeine, and some bad teas have them, too.


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 zhang@valleybrooktea.com. 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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