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g00p said

Why do tea leaves sink and float in consistently?

Most of my teas float. Just purchased a new batch of organic peppermint and while the leaves float initially, they sink the the bottom after a few minutes of steeping. My google searches have turned up little on why this occurs and if it reflects anything on the quality of the tea. Thoughts?

8 Replies
Vito said

The Short Answer: Variable buoyancy in tea leaves depends on how much of various lighter-than-water compounds are in the leaves before and after infusion.

The Long Answer: Flotation of tea leaves depends on something called the coefficient of thermal expansion. Like nearly everything else, tea leaves contain substances that expand in volume as their temperature increases. Some of those substances occupy a volume greater than the same amount (mass) of water occupies at the same temperature, which causes them to float.

Of course, if those substances are still trapped inside the leaves (before they’ve gone into solution in the surrounding water), then when their buoyancy causes them to rise, they take the tea leaves with them.

Eventually, the lighter-than-water substances go into solution, which is what colors and flavors the water, turning it into brewed liquor. When the leaves finally become depleted of those substances, what’s left is totally waterlogged organic material that’s slightly heavier than water, and the leaves sink to the bottom of the infuser.

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teataku said

Yay chemistry! :D

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DukeGus said

Nice post vito,cheers!

The strange up-down of white tea?(silver needs p.e.)

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Vito said

Right; certain teas do an up-down thing, wherein leaves that are at the bottom will gradually rise, while others near the top will gradually descend.

It’s the same phenomenon described in my previous post, but the different behaviors are due to the fact that the leaves are not all completely uniform in composition and density in the first place.

Additionally, the temperature profile of the water in the infuser gradually changes after the initial turbulence (from pouring in the water) settles down. After it sits for a while, the water temperature acquires a gradient, with the hottest water at the top and the coolest water at the bottom. The cooler water is slightly denser, so leaves that previously wouldn’t float in hotter (less dense) water will gradually become more buoyant as the water around them cools and becomes more dense.

Meanwhile, the tea that has been floating at the top (in hotter water), has been giving up more of its essence to the water, becoming less buoyant, and therefore starting to sink while the other leaves are starting to rise. Hence, the Dance of the Leaves, wherein some are rising and some are falling.

Actually, there’s more to it. There are convection currents, varying rates of solubility, and other subtleties in the thermodynamics…

…er, well, that’s enough. (heh)

DukeGus said

Cheers again Vito, I imagined something like that but it was fun asking you :)

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Ellyn select said

I love the chemistry! Thank you.

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Vito said

Ellyn and teataku: Thanks for your feedback. Truth be told, I know very little about the chemistry of tea. My comments reflect some elementary physical principles applied to what I’ve observed happening in my infuser.

Tea is an endlessly fascinating subject. It’s still full of mystery for me, and probably always will be…which is part of its allure.

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seule771 said

Adjunct comes to mind when reading these posts and reference to Chemistry.

I place cover/saucer as lid on tea cup and hope and pray that the leaves floats downward; and most time they do with some being rather difficult and for this the spoon is handy is scooping out those not adhering.

Is it cheers or kudos! Don’t have mine own horn to toot.

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