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What’s in Miso Soup and Why Does It Move Around the Bowl?

Miso soup is a staple accompaniment to many a Japanese meal. It’s such a standard offering, in fact, that it’s all too easy to slurp it down mindlessly without giving it a second thought.

Yet you’ve almost certainly encountered something about miso soup that’s unique and, to be honest, a little freaky, even if you weren’t conscious of it at the time. While you may have registered miso soup’s signature cloudiness, did you also note how those clouds seem to move about the bowl entirely of their own accord?

I confess I’d never focused on this until a friend visiting Japan for the first time remarked on it with surprise. I couldn’t explain it then, but I knew I had to get to the bottom of miso mobility.

Sometimes the hardest part of researching topics is figuring how to restate the question in Japanese so I can get answers. This time, the word I most needed turned out to be “moya.” By itself, that means “haze” or “mist.” But when repeated — and I urge you to try this because “moya moya” is incredibly satisfying to say — it describes those ambulatory bits in miso soup in a way most Japanese will understand immediately.

So, what exactly is in miso soup? What are the cloudy bits and how does it move around the soup? Moreover, if you let it sit for a while, why do the particles gather in a “ball” in the middle rather than settling to the bottom?

This was going to take some investigating.

A bowl of miso soup with seaweed, tofu and spring onions in a wooden bowl on a bamboo place mat with a pair of wooden chopsticks to the left.
Miso soup when served /via Getty Images.
Two bowls of miso soup side-by-side. The broth has separated from the miso paste and the miso has gathered in the middle of each bowl like a ball. The bowl to the right has a rounder "ball," while the one on the left has more miso at the base and a smaller amount at the top.
Miso soup that has been left to sit for a little while. Why does this happen? © Alan Golding

What is miso soup and miso soup ingredients?

There are more than 1,300 types of miso, but the basic recipe calls for grinding up cooked soybeans and mixing them with salt, grains and a starter called kōjikin (usually Aspergillus oryzaea). This is allowed to ferment, resulting in a thick paste that can be mixed into clear broth to make a commonly served soup called miso shiru.

All miso pastes contain solids that don’t dissolve in the broth. I was pretty sure that’s what makes up the cloudy bits you see in miso soup, but I went by the Japan Miso Promotion Board in Tokyo to get details. General Manager Osamu Takanashi explained that fuyōsei busshitsu (insoluble material) makes up anywhere from 8 to 23 percent of miso paste and consists largely of parts of the soybeans and rice kernels that aren’t broken down during processing and fermentation, as well as sloughed off cells of fungus and yeast.

A spoonful of miso paste is held up to the camera with a spoon made of wood. The white bowl it has been scooped from is just behind it.
An example of miso paste /via Getty Images.

It is indeed the solids that go moya moya in your soup, Takanashi confirmed. What’s more, they’re what give miso soup its distinctive taste. “The solids contain a wealth of flavor and aroma compounds,” he explained. “If you let the soup sit too long, allowing it to separate so you get just broth without the solids, it won’t taste nearly as good.” This was solid information, but Takanashi couldn’t say why the ball forms, so I delved into the library in my further search for answers.

Why does miso “gather” in miso soup?

I spent the next few days slogging through scientific texts until I stumbled on a clue: In a book on water treatment, of all things, I found the statement that the aggregation of soybean particles in miso soup is caused by the sodium alginate in the dried seaweed used in making the broth.

In the food industry, extracted sodium alginate is used in the making of various food gels, includingand I’m sure you’re thrilled to learn this – the pimento stuffing in green olives. Given that information, we might reasonably assume that sodium alginate in the broth would cause miso soup to harden into some sort of mass or gel. So, isn’t that another mystery? Why do the soybean solids gather into a pleasant cloud rather than clumping together in a nasty, sticky mess?

Soon enough, I came across a theory that seemed to explain why the ball forms, and also provided a reason for the moya moya moving all about. That explanation focuses on tairyū (convection), which I hope you remember from physics class is the concerted, collective movement of bundles of molecules within a fluid.

Imagine a cup of tea. The interior is hotter than the top because the surface is exposed to cool air. To equalize the temperature, the liquid organizes itself into zones called “convection cells” that rise to the surface and give off heat. The heat loss makes the cells denser than those below them, so they fall back down into the liquid where they get warm again. This is called thermal convection.

With a clear liquid like tea, you can’t see the convection cells. But the solids in miso soup make it possible to track the movement. When the solids in your soup appear to be moving around, what you’re seeing is the columns of fluid rising, falling, and changing positions.

One physicist I consulted theorized that the miso solids may be settling in the region in the convection current where the upward force of current is offset by gravity. “It appears they are clumping together into a ball, but they are in fact just moving to the same area because they have the same properties of density, size and texture,” he suggested.

A steaming wooden bowl of miso soup with tofu and spring onions sits on a wooden table.
A steaming bowl of miso soup /via Getty Images.

Does outer space hold the final piece of the puzzle?

If that’s not complicated enough, there is another mechanism at work that causes some movement at the top of your soup.

Changes in temperature cause changes in hyōchōryoku (surface tension), so when one section of the surface gets cooler than another, there is flow away from the hot area toward the cold area. This is called the Marangoni effect.

That discovery launched me on another detour, this time straight into outer space. I read that experiments on exactly this effect were in progress on Kibo, the Japanese Experiment Module on the International Space Station, and shot right over to the Japan Aerospace Exploration Agency (JAXA) to find out more.

“On Earth, it’s impossible to measure the contribution of Marangoni convection because it occurs at the same time as thermal convection,” Tetsuya Sakashita, an engineer in the public affairs department, explained. “But in a microgravity environment such as space, thermal convection can’t occur because it depends on gravity. We’re therefore able to study Marangoni convection in ways that are impossible on Earth.”

In addition to advancing knowledge of fluid behavior, the experiments may well lead to better ways of manufacturing crystals for semiconductors and other high-tech products. The Marangoni convection causes quality problems during crystal growth, Sakashita explained. Once we understand more about how this phenomenon works, we may be able to reduce or eliminate such problems.

Before leaving, I just had to ask whether the Japanese astronauts laboring on these experiments get miso soup in space. “Of course,” Sakashita replied. “It’s freeze-dried and they drink it from a pouch, but miso shiru is definitely part of the in-mission diet.”

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