A freshly brewed mug of tea is the product of many serendipities: ancient human ingenuity, meticulous harvesting, global trade and a whole lot of chemistry.
In her new book “Steeped: The Chemistry of Tea,” chemist Michelle Francl explores how science transforms humble leaves into this beloved beverage. She focuses on tea varieties that come from a single plant, including green, black, white, yellow, oolong and pu’er teas, as opposed to rooibos, chamomile or other herbal teas with much different origins.
Humans have a long history of fiddling with flora to access its desirable properties. Francl writes that the medical use of plants among modern humans dates back at least 60,000 years.
Modern tea culture boils down to one specific plant dubbed C. sinensis. It’s not quite clear how the tea plant — or the practice of tea-making itself — came to be, Francl says. But we do know that today’s teas exist thanks to millennia of experimentation, plus intricate production processes that require a dizzying number of precise steps.
Michelle Francl is the Frank B. Mallory professor of chemistry at Bryn Mawr College, an adjunct scholar of the Vatican Observatory and author of the new book “Steeped: The Chemistry of Tea.” Photo by Aaron Windhorst/Bryn Mawr College
If you’re a newcomer to the world of tea, Francl recommends two types to try starting out with: green snail tea, which she describes as “sweet” and “grassy,” and a pu’er tea named “Celestial Tribute.”
Francl, who is a professor of chemistry at Bryn Mawr College and an adjunct scholar of the Vatican Observatory, spoke to PBS NewsHour about the chemistry that goes into making and flavoring tea, and how to wield this “science of change” to brew yourself the perfect cup.
This interview has been condensed and edited for clarity
Where does tea come from?
Camellia sinensis is a single plant that grows in China. It’s been cultivated so long that they can’t find a wild version of it. So it’s a plant that’s been cultivated for a very long time, and that one plant gives rise to all the different varieties of tea.
The compounds in the tea vary with the time that they’re picked. There’s [a] first blooming in the spring, later blooming in the spring. And those will also have different properties.
How is tea processed? Why has it been referred to as a work of art?
It’s the chemistry that changes the plant into all these incredibly different teas [after the leaves are picked]. There are enzymes in the tea that can cause oxidation — the same thing that causes browning in apples or bananas. So you pick it, and you’re very careful not to bruise things. For green tea, what you want to do is heat it up really quickly so you destroy those enzymes, so they can’t oxidize things and turn it brown. With black tea, you want to capitalize on that. You let them wilt a little bit to start kind of softening up the cells, and then you deliberately bruise them to get that oxidation started. And that’s what eventually leads to those beautiful colors in a cup of black tea, which is almost ruby red.
In the end, what you’re getting is something that you’re going to drink. And that whole experience from the color in the cup to the scent to the taste — as well as the caffeine content — all those things are sensory experiences. So that’s where the art comes in — you want to have the sensory experience that you desire at the end.
Why do tea plants produce caffeine?
[Plants use] trashed pieces of DNA to make the caffeine, and then it uses it for its own purpose, which is as a pesticide and an herbicide. So when the leaves fall around the plant, it kind of acts to keep other plants from growing. And it [makes the plant less] attractive to insects. If you think about chewing on a bitter leaf, we wouldn’t want to chew on a bitter leaf. And so this decreases pests both around the plant, as well as the ones that want to feast on the plant.
If people don’t want a caffeine buzz, how can they decaffeinate tea at home?
If you want to decaffeinate tea, you can [steep it for] 30 seconds, dump the water and now resteep it. It’ll taste a little flat because the caffeine has taste — it’s bitter. So it won’t taste quite as good as a cup of tea with caffeine in it. But if you want a cup of tea in the late afternoon and you’re afraid it’s going to keep you awake, that works. And for me, I like all these different kinds of teas, and it lets me decaffeinate a tea that doesn’t have a decaffeinated version.
What are the steps to making the perfect cup of tea?
The very first thing you want to do is pre-warm the pot. So you want to fill your pot or your mug up with hot water while you bring your water to a boil in a kettle.
Use loose leaf tea in a big tea infuser [that has enough room for the tea to expand]. I have this very prosaic basket that’s roughly the size of the mug. Agitate the tea leaves, stir them with a spoon. I just bounce the infuser up and down, because you want all that contact between the solvent — the water — and the tea leaves.
Time it. I set a timer on my watch. Four minutes, pull it out. It gets a little more bitter, but [if you want a little bit] more of the antioxidants, squeeze a little of the excess water out of the end. And to me, that’s the perfect cup of tea.
What happens if you make tea using a microwave instead of a kettle?
[A colleague] makes her tea [by] heating it up in the water in the microwave, and she wanted to know, why does it always get the white film on it? When she makes it at home and boils it in the kettle, she doesn’t get that. So I started [looking] around through the literature to discover it’s called tea scum. What happens is when you boil it in the microwave, you don’t get enough of the oxygen out because it boils so fast. And so that helps to promote the development of tea scum. And the tea scum reduces the flavor, in addition to just kind of floating on the top of your cup, so you get a less flavorful cup. And it’s just like the scum in bathtubs — it’s positively charged ions and organic compounds.
To deal with it, you can use citric acid. The citrate in it will bind the ions and prevent the formation of the tea scum. So the same solution you would use in the bathtub — some kind of chelating agent — will work in the tea, too. A little squeeze of lemon.
When should you add milk, if you’re so inclined?
[Warming milk before adding it] helps to keep it from curdling. The proteins will denature if you toss them into hot water, so if you warm them up a little bit, there’s less thermal shock and the proteins are less likely to curdle.
Warming milk can help prevent it from curdling when you pour it into a mug of hot tea. Photo via Getty Images
If you are brewing tea in a cup with a tea bag, you definitely don’t want to add the milk until the end, because you’ll reduce the temperature of the water by putting in cold milk, and you reduce it enough to get less caffeine out.
Why shouldn’t you reuse tea bags?
Because that caffeine comes out so fast. So after you’ve used the bag and taken it out, there is really not much caffeine left. You also lose lots of the antioxidants. If you brew it a second time, you’re getting no caffeine, really — you’re still getting some antioxidants. By the third time, what you’re getting is colored water.
What does adding a pinch of salt do for a cup of tea?
I add the pinch of salt at the end, so after everything’s all brewed and it’s sitting in my mug. And the reason for doing that is if you want to moderate the bitterness — it won’t get rid of all the bitterness, but it will reduce it. If I’ve let my tea brew a little too long and I’ve gotten distracted, that will help. The reason it works is the sodium ions in the salt block some of the bitter receptors [on our tongues].
“The Classic of Tea” by Lu Yu, which is an 8th-century Chinese manuscript, [is] where the whole addition of salt thing comes from. Of course, they didn’t know about sodium ions and they didn’t know about receptors, but they did know what made the tea taste good.
Has your research on tea changed the way you think about chemistry?
It’s made me think about the scale of things. So, I have a part-time appointment at the Vatican Observatory. And there’s something, to me, kind of awe-inspiring about [the fact that] there are as many molecules in my cup of tea as there are stars in the universe.
So that gives me a different perspective on chemistry. I think physics [has the reputation of being] the kind of beautiful science, right? And a lot of that comes from astronomy and looking at the stars. But I think that there’s as much richness in that cup of tea as there is looking out at the night sky.