If you've ever had a furious debate about the ungodly act of microwaving your cup of tea and how "it's the same" as boiling the kettle, you're about to lose — not only to Britain but to science.
Researchers have explained the process your zapped cuppa goes through in a new study published in the American Institute of Physics' peer-reviewed online journal AIP Advances, and why you might not be getting the best results from making it this way over the traditional kettle/stove method.
The study, conducted by researchers at the University of Electronic Science and Technology of China, looked at how heating liquid works in a microwave, and how the electric field that acts as a warming source causes the liquid to end up different temperatures at the top and bottom of the cup. A good cup of tea is all about getting uniform temperature throughout your water and, though many scholars have studied uniformity and how to solve it within the microwave itself, these researchers have offered up a different possible solution (more on that later).
Typically, the study describes, if you're warming a liquid like water on the stove or within a kettle, the heating source warms the container from below. This is when a process called convection happens, when the liquid at the bottom of the container warms up, diminishes in density, and moves to the top, letting the remaining cooler liquid to get access to the sweet, sweet heating source below. This results in even, uniform temperature throughout the container.
But if you're throwing your cup of water in the microwave for 90 seconds, like the researchers did, the device's electric field heats it from all angles, not just from below, so while the top part of the cup's water may be sitting at boiling point, the bottom may not. "Because the entire glass itself is also warming up, the convection process does not occur, and the liquid at the top of the container ends up being much hotter than the liquid at the bottom," reads the study.
So, your microwaved cup of tea is hotter at the top than the bottom. Let's take a 30-second mournful-staring-at-the-floor break.
OK, so you've got a cup full of disparately warmed, microwaved water, and while I'd personally just cast this back into the fiery chasm from whence it came, if you do use it you should be aware that this is going to affect the way your tea develops. Though you might have palmed this off as something only your snobbiest friend thinks, the temperature of your water when brewing tea is really important, which is why making sure it's uniform is vital to a good cup — the water is needed for the dried tea leaves to expand, unfurl, and start brewing.
According to the UK Tea Academy's white paper on water (yeah, they're hardcore, they write papers about tea water), different teas need their own specific water temperatures to brew properly. "This is because the bitter components in tea (caffeine and polyphenols) are extremely soluble in boiling and very hot water," reads the paper. "When we brew tea in boiling water or water at approx. 90 – 95ºC, more of the bitter tasting ingredients are drawn out quickly into the water, giving a robust, sometimes quite aggressive brew. This can be fine for strong black teas, dark oolongs, and fermented ‘dark teas’, but when brewing more delicate teas such as white, yellow, green and the greener jade oolongs, the bitter components can easily overwhelm the subtle sweet and aromatic character of the tea."
Notably, the Academy wrote that teas steeped in cold or iced water release fewer of their bitter ingredients and more of the sweet, so it doesn't always have to be hot stuff. But whatever tea you're making, all of this inevitably gets thrown off balance when your cup of water is not the same temperature all the way through, say, if you warm the water in the microwave and dunk a teabag in it as opposed to pouring kettle-boiled water over the tea in a pot or bag. You've got a small window for perfect brewing temperature, whether you're making green tea (70°C), oolong tea (90°C) or black tea (95-98°C). So if the water goes from hot to less hot levels in the cup, it will brew differently — and when you start jiggling the teabag around the temperature can change too, so it might be less warm than you need it to be.
Interestingly, the researchers came up with a possible solution by designing a glass with a seven-centimeter metallic silver lid that's meant to redistribute the electric field, conducting the heating away from the top of the cup. This means heating happens from the bottom of the cup upwards, and effectively simulates the convection process.
"The experimental results show that when the modified glass cup with 7 cm metal coating is used to heat water in a microwave oven, the temperature difference between the upper and lower parts of the water is reduced from 7.8 °C to 0.5 °C," reads the study. "The modified glass cup is placed in the center of the ceramic plate, far away from the cavity wall, and there is no spark ignition."
So, no microwave fires (yay!) and the temperatures at the top and bottom of the cup were more similar than they would have been without the metal lid. Pretty neat. Probably wouldn't touch the metal straight away, though.
Whether or not this becomes an actual device people can buy and pop on the top of their cups remains to be seen, but if it does, some form of it could be an option for those who opt for the microwaved cuppa and still want a well-developed brew. Until then, however, just know that your tea might be brewing weirdly if you zap it, and with what you paid for the lid, you probably could have bought a rapid boil kettle.
Look, microwavers, I get it, it's quicker. And if you want something to come back at your opinionated, traditionally-made tea drinker pals, a food scientist from the University of Newcastle in Australia reckons microwaving your cup of tea is the key to getting more health benefits from the beverage (note: health, not taste, benefits).
I'll be sticking to my kettle method, but you do you.
Just know Britain is judging you.
