What had the woman done to attract all this ire? She heated her mug of water in the microwave, rather than using a stovetop kettle, leaving British viewers aghast. However, the culture clash between those who make tea with a microwave versus those who use a kettle isn’t just a point of personal preference, there’s science behind it.
Previous studies have proven microwaves leave liquids with an uneven temperature distribution, where the hottest parts of the liquid are at the top of the container and the cooler liquid remains at the bottom.
A new scientific paper is now weighing in on the heated debate. The team of scientists from the University of Electronic Science and Technology of China investigated the physics behind the uneven heating of liquids in microwaves and developed a potential solution.
This research won’t only catch the eye of tea drinkers though; improving the way in which microwaves heat is also of interest to numerous scientific fields. Last year, material scientists presented a fast and cheap way to prepare carbon nanotubes in a microwave oven, and chemical engineers have formerly shown interest in the use of microwaves for heating in processing industries.
The reason behind unevenly heated mugs of water is that microwave ovens suppress normal convection, a process which occurs within fluids wherein the hot liquid rises and heats up the cooler areas at the top by coming into contact with them.
Microwave ovens heat food and liquids with electromagnetic waves. Because this electric field is everywhere within the device, an unusual convection occurs for microwaved liquids. The hottest part of the liquid remains at the top of the glass and doesn’t drop to the bottom.
To study this unusual convection, the researchers developed a numerical model to determine the electric field and temperature distribution of a final heated object. In a glass of microwaved water, the simulations they ran showed the electric field was strongest where the temperature was highest.
In contrast, when the researchers placed the heat source at the bottom of the glass, they got water with a uniform temperature throughout.
When it comes to pursuing the perfect cup of tea, turning to scientific research is nothing new. Last year, food scientists from Cornell University published their study on the effect water composition has on the flavor of green and black tea. They tested bottled, tap, and deionized water against each other to see what tea drinkers preferred taste-wise. They concluded tea brewed with tap water has the most favorable flavor, however the water difference had the most drastic effect on the taste of green tea.
Interestingly, a 2013 Food Explainer column in Slate which took on the microwave versus kettle debate suggested the microwave might not be all bad. According to the article, water brought to below boiling temperatures in a microwave oven could be ideal for green tea, though not for black tea.
So is there a way to create a perfectly heated cup of water for all teas in a microwave oven?
The researchers were able to improve the heat distribution by using a waveguide — a device which they used to restrict the transmission of the electromagnetic waves. For their waveguide, they designed a microwave-safe metallic silver circular plating which went along the rim of the glass and covered it, like a lid. The waveguide weakened the electric field at the top of the glass, which kept it from getting hot.
By strengthening the electric field at the bottom of the glass, the researchers allowed normal convection to occur. The hot water at the bottom of the glass rose and heat up the cooler water at the top. This resulted in a final product with a much more uniform temperature. Without the waveguide, the temperature difference between the top and bottom of the glass was eight degrees Celsius, but with it the difference was only half a degree.
This research could potentially make the kettle versus microwave debate obsolete in the future. The team is now looking toward ways to perfect and commercialize their product, which could bring the perfectly heated cup of tea to people who prefer the microwave everywhere.