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Food Science


It’s fall: All the warm liquids, please

by Leigh Krietsch Boerner
September 27, 2020 | A version of this story appeared in Volume 98, Issue 37


A smiley face with bowls of tomato soup for eyes, a tomato for a nose, and a banana for a smile.
Credit: Shutterstock
Souper: Hot tomato soup might turn that frown upside down.

Hot soup to fill the void in your cold soul

Does it seem like warm soup makes you happier? It may be so. A study from sensory scientists Han-Seok Seo and Asmita Singh from the University of Arkansas found that people who slurped hot tomato soup reported more positive emotions than when they ate lukewarm or room temperature soup (Food Qual. Prefer. 2020, DOI: 10.1016/j.foodqual.2020.104005).

Volunteers tasted tomato soup samples at 25, 40, 55, and 70 °C and then chose words from a list to describe how the soup tasted and how they were feeling emotionally. The researchers found that soup sippers picked more positive words, such as happy, nostalgic, or affectionate, after eating the two hottest soups, and they were still generally positive after the 40 °C lukewarm soup. However, room temperature soup was apparently a drag: tasters selected words such as worried, disgusted, or bored after supping on this soup. The flavors the taste testers identified in the soups were not as affected by temperature. The researchers also did the same test with water and found that the participants’ emotional reactions weren’t as strong as the soup testers’.

Seo has done similar studies in the past with coffee and tea, inspired by his own grapple with enthalpy. Akin to many of us, Seo likes a hot coffee while working but tends to forget it’s there. “Fifteen minutes later, I feel that this is not the coffee that I had previously tasted,” he tells Newscripts. “It’s different not only in temperature but also flavor and mouthfeel.”

Seo was surprised that hot soup puts people in their happy place. He suggests that since people in the US usually eat their soup hot, the positive emotions might have something to do with familiarity.

“Personally, I like tomato soup served at hot temperatures, at 70 °C or higher,” Seo says. But just like with his coffee, Seo’s treasured temps tend to dissipate quickly and are typically gone within 5 min, he says. Thermodynamics can indeed be a drag.


For superior tea, put the kettle on

A photo of a young person drinking a cup of tea standing over an isolated red background.
Credit: Shutterstock
Tea-rrible: Microwaved water makes a cruddy cuppa.

Are you a tea novice? Have you ever had a tea purist purse their lips at you for making the drink in a microwave? No one likes to be shamed, but it turns out these connoisseurs may have a point.

Zapping water in a microwave for tea just doesn’t cut it, and now scientists know why. Baoqing Zeng and coworkers from the University of Electronic Science and Technology of China found that boiling a glass of water in a microwave oven results in the top of the water being almost 8 °C hotter than the water at the bottom of the glass (AIP Adv. 2020, DOI: 10.1063/5.0013295).

Water boiled from below, such as on a stove or in an electric kettle, will be roughly the same temperature throughout, the researchers say. Thanks to something called heat convection, the hot water molecules at the bottom, near the heat source, get pushed up in a column toward the surface. This movement forces the cooler water down the sides, where it comes into contact with the hot surface. Repeat ad nauseam until the water boils.

In a microwave, the sides and top of the glass are being heated, not just the bottom. This method does not create convection, the researchers say, so the water does not reach the same temperature throughout. When brought to boil in the microwave, the water on the surface reaches 100 °C, Zeng says, but the water lower in the glass will be cooler.

So if you boil water in the microwave for your tea, even if you pour it from one mug to another, the temperature is not going to be boiling, Zeng says, since the hot water at the top and the cooler water at the bottom will mix together. Same goes if you mix the microwaved water with a spoon. Zeng hasn’t experimented with the ultimate sacrilege to a tea drinker: putting a tea bag in cool water and microwaving the whole thing. However, this will not “make a perfect tea,” he tells Newscripts. The region where the tea bag is likely won’t get to boiling, plus, while the microwave is running, the water temperature goes up very slowly. Neither of these tea-steeping conditions is ideal, according to pretty much every company that makes tea.

Zeng and coworkers did find a way to force convection in a microwave: they applied a band of silver around the rim of the glass (don’t try this at home, please). This effectively shields the surface of the water from the waves. After microwaving water in this special glass, the team measured only a 0.5 °C difference between the top and the bottom.

In the future, this technology may be coming to a microwave near you. But until then, if you want to make tea, use a pot or a kettle.

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