The Perfect Temperature for Soup, According to Science

Fudio/iStock via Getty Images
Fudio/iStock via Getty Images

Finding the right temperature for soup can be difficult. On one hand, few culinary experiences are as unsatisfying as sipping on a lukewarm broth. On the other hand, nobody enjoys burning their taste buds on nuclear-heated stock. Luckily, you can embrace your inner Goldilocks and find a soup temperature that’s “just right”—with science!

Studies confirm what chefs have long suspected: Temperature affects the taste of food. Cheddar cheese generally tastes more sour when warmed, while a savory ham will seem saltier as it cools. The reasons for these flavor differences are complex; sometimes they’re caused by receptors on the tongue and other times by chemical changes in the food itself. Research shows that some foods are epigenetically altered when heated or cooled. Tomatoes, for instance: The genes that help express a tomato’s full flavor profile are “turned off” when exposed to cool temperatures. That’s why some cookbooks warn not to refrigerate them.

The same principles also apply to soup. Different temperatures can accentuate, or dull, different aspects of a stock’s flavor profile.

In 2017, researchers in Spain published a study in the International Journal of Food Properties that tested the incidence of taste compounds—such as amino acids and nucleotides—in a traditionally cooked chicken broth. Samples were cooked for three to five hours, with temperatures ranging from 86°C to 103°C (that’s 185°F to 217.4°F). The team discovered that taste compounds, including those associated with umami, increased with temperature. Flavor compounds were also boosted by longer cooking times, but the effect was temperature-dependent.

In other words, the hotter the soup, the more flavorful it can be. It’s important, however, to make a distinction between cooking temperature and serving temperature. Nobody should serve soup at 217°F. Skin exposure to a liquid over 150°F can cause burns almost instantly [PDF]. There’s no point in boosting the umami of your soup if you can’t feel your tongue.

Woman in a cafe eating hot soup
Maksym Azovtsev/iStock via Getty Images

As soup cools, its flavor profile will change. According to a 2016 study in the journal Chemical Senses, umami flavors will deteriorate as a soup drops to (and below) room temperature. It will also taste saltier. This phenomenon is described in a handful of other studies, including a 2015 work published in the journal Appetite. In that study, researchers asked eight trained panelists and 62 untrained panelists to rate the saltiness of salt water, chicken broth, and miso soup. Temperatures ranged from 40°C to 80°C (104°F to 176°F). The trained panelists perceived no difference in the saltiness of the hot and lukewarm soups, but the Average Joes said the colder soups tasted saltier (the study didn’t go into the reasons why, however).

Temperature also affects other flavors. A 2012 study in Chemosensory Perception showed that sourness was most intense when a solution was warm and bitterness most intense when it was cold. Other studies show that our perception of sweetness is enhanced with cold foods, which may explain why frozen treats such as ice cream can taste sickly-sweet when melted [PDF].

But back to our original question: How do I find the ideal temperature for serving soup?

The annoying answer is: It depends! It depends whether you prefer a bowl that’s a pinch salty, a smidge umami, or something else. It also depends if you’re among the 20 percent of people who are “thermal tasters” most sensitive to food temperature. Among this group, “heating or cooling small areas of the tongue draws out a taste sensation without the presence of food or drink,” according to a press release about the Chemosensory Perception study.

Generally, the best serving temperature probably hovers around the pain threshold for the tongue, which is approximately 153°F [PDF].

There are a few reasons why. Most people will want to serve their soup at the warmest temperature possible without causing pain. Our taste buds contain small, heat-sensitive proteins called TRPM5 channels, which are important for the perception of umami and perform best when food is warm. High temperature foods also emit more aromas, an important factor that amplifies the intensity of taste. “As heat is applied to food, its essential oils, or volatiles, are released, which increases the food’s aroma and flavor,” food writer Amanda Hesser explains in The New York Times. As a hot dish cools, the flavors change and develop. She also suggests contrast, like topping hot chili with cool sour cream, to animate taste receptors.

Scientists have done a lot of research about where to draw the line between a liquid that’s “just right” and “too hot”—and a temperature ranging between 136°F to 162°F appears to be the best bet, according to a recent analysis in The Journal of Food Science. For soup-lovers, anything significantly warmer than 170 degrees will probably require tiny sips and spoon-blowing. Anything cooler than 130 might feel merely warm. Something in between should satisfy your taste buds without destroying them.

Arrokoth, the Farthest, Oldest Solar System Object Ever Studied, Could Reveal the Origins of Planets

NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Roman Tkachenko
NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Roman Tkachenko

A trip to the most remote part of our solar system has revealed some surprising insights into the formation of our own planet. Three new studies based on data gathered on NASA's flyby of Arrokoth—the farthest object in the solar system from Earth and the oldest body ever studied—is giving researchers a better idea of how the building blocks of planets were formed, what Arrokoth's surface is made of, and why it looks like a giant circus peanut.

Arrokoth is a 21-mile-wide space object that formed roughly 4 billion years ago. Located past Pluto in the Kuiper Belt, it's received much less abuse than other primordial bodies that sit in asteroid belts or closer to the sun. "[The objects] that form there have basically been unperturbed since the beginning of the solar system," William McKinnon, lead author of one of the studies, said at a news briefing.

That means, despite its age, Arrokoth doesn't look much different today than when it first came into being billions of years ago, making it the perfect tool for studying the origins of planets.

In 2019, the NASA spacecraft New Horizons performed a flyby of Arrokoth on the edge of the solar system 4 billion miles away from Earth. The probe captured a binary object consisting of two connected lobes that were once separate fragments. In their paper, McKinnon and colleagues explain that Arrokoth "is the product of a gentle, low-speed merger in the early solar system."

Prior to these new findings, there were two competing theories into how the solid building blocks of planets, or planetesimals, form. The first theory is called hierarchical accretion, and it states that planetesimals are created when two separate parts of a nebula—the cloud of gas and space dust born from a dying star—crash into one another.

The latest observations of Arrokoth support the second theory: Instead of a sudden, violent collision, planetesimals form when gases and particles in a nebula gradually amass to the point where they become too dense to withstand their own gravity. Nearby components meld together gradually, and a planetesimal is born. "All these particles are falling toward the center, then whoosh, they make a big planetesimal. Maybe 10, 20, 30, 100 kilometers across," said McKinnon, a professor of Earth and planetary sciences at Washington University. This type of cloud collapse typically results in binary shapes rather than smooth spheroids, hence Arrokoth's peanut-like silhouette.

If this is the origin of Arrokoth, it was likely the origin of other planetesimals, including those that assembled Earth. "This is how planetesimal formation took place across the Kuiper Belt, and quite possibly across the solar system," New Horizons principal investigator Alan Stern said at the briefing.

The package of studies, published in the journal Science, also includes findings on the look and substance of Arrokoth. In their paper, Northern Arizona University planetary scientist Will Grundy and colleagues reveal that the surface of the body is covered in "ultrared" matter so thermodynamically unstable that it can't exist at higher temperatures closer to the sun.

The ultrared color is a sign of the presence of organic substances, namely methanol ice. Grundy and colleagues speculate that the frozen alcohol may be the product of water and methane ice reacting with cosmic rays. New Horizons didn't detect any water on the body, but the researchers say its possible that H2O was present but hidden from view. Other unidentified organic compounds were also found on Arrokoth.

New Horizon's flyby of Pluto and Arrokoth took place over the course of a few days. To gain a further understanding of how the object formed and what it's made of, researchers need to find a way to send a probe to the Kuiper Belt for a longer length of time, perhaps by locking it into the orbit of a larger body. Such a mission could tell us even more about the infancy of the solar system and the composition of our planetary neighborhood's outer limits.

The Moon Will Make Mars Disappear Next Week

Take a break from stargazing to watch the moon swallow Mars on February 18.
Take a break from stargazing to watch the moon swallow Mars on February 18.
Pitris/iStock via Getty Images

On Tuesday, February 18, the moon will float right in front of Mars, completely obscuring it from view.

The moon covers Mars relatively often—according to Sky & Telescope, it will happen five times this year alone—but we don’t always get to see it from Earth. Next week, however, residents of North America can look up to see what’s called a lunar occultation in action. The moon's orbit will bring it between Earth and Mars, allowing the moon to "swallow" the Red Planet over the course of 14 seconds. Mars will stay hidden for just under 90 minutes, and then reemerge from behind the moon.

Depending on where you live, you might have to set your alarm quite a bit earlier than you usually do in order to catch the show. In general, people in eastern parts of the country will see Mars disappear a little later; in Phoenix, for example, it’ll happen at 4:37:27 a.m., Chicagoans can watch it at 6:07:10 a.m., and New Yorkers might even already be awake when the moon swallows Mars at 7:36:37 a.m.

If you can’t help but hit the snooze button, you can skip the disappearing act (also called immersion) and wait for Mars to reappear on the other side of the moon (called emersion). Emersion times vary based on location, too, but they’re around an hour and a half later than immersion times on average. You can check the specific times for hundreds of cities across the country here [PDF].

Since it takes only 14 seconds for Mars to fully vanish (or reemerge), punctuality is a necessity—and so is optical aid. Mars won’t be bright enough for you to see it with your naked eye, so Sky & Telescope recommends looking skyward through binoculars or a telescope.

Thinking of holding an early-morning viewing party on Tuesday? Here are 10 riveting facts about Mars that you can use to impress your guests.

[h/t Sky & Telescope]

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