Which Probiotics Does Miso Contain?

Japanese miso soup in bowl and ingredients close-up.
A large bowl of miso soup with tofu. (Image: ALLEKO/iStock/Getty Images)

You may have heard of miso, a Japanese food made from fermented soybeans and some grains; if you haven't tried it, you're missing a flavorful addition to soups and other dishes. Usually available as a paste, miso contains probiotics, healthy microorganisms added to produce fermentation that are similar to those that live in your digestive tract. Miso probiotics may provide you with significant health benefits, but miso is also high in salt, so use it in moderation.

Fermentation Process

The production of miso requires addition of a fermentation starter called koji, which usually contains the fungal microorganism Aspergillus oryzae, although it's also possible to produce miso with other cultures, such as a yeast called Saccharomyces rouxii. These probiotic organisms activate the fermentation process in the raw materials, which are soybeans, alone or in combination with barley, brown rice or other grains. It can take up to three years to produce high-quality miso, with the product becoming smoother and acquiring a more complex flavor the longer it ferments.

Use of Miso

It's important to buy unpasteurized miso because pasteurization can destroy the live probiotic cultures in the paste. Miso is available in several varieties. The most common are: white or kome miso, made from soybeans and rice, which has a light, slightly sweet flavor; yellow or mugi miso, made from barley and soybeans, with a moderately intense flavor; and red or hatcho miso, made from soybeans alone, which is the most intensely flavored. When using miso in soup, stew or another cooked dish, don't add it until the end of the cooking process and remove the food from heat immediately; high heat can destroy the probiotic organisms.

Potential Benefits

Eating foods rich in probiotic microorganisms may have some significant health benefits, according to research summarized by experts at the Harvard Medical School. They report that many probiotics can prevent or improve diarrhea, especially when it develops after taking antibiotics that destroy healthy intestinal organisms. Probiotic-rich foods might also help if you have Crohn's disease or irritable bowel syndrome, two disorders that involve inflammation of the intestinal tract. Several research studies have looked for other possible benefits of miso, including one published in the June 2013 issue of the Journal of Toxicologic Pathology in which researchers found that laboratory animals fed three-month fermented miso failed to develop precancerous changes or colon cancer after exposure to a carcinogen. Other observations suggest that miso may protect animals from cancer of the breast, lung and liver, but these promising findings in animals still need confirmation in human subjects.

Some Cautions

Miso tends to be high in sodium, with about 630 milligrams in 1 tablespoon, or about 25 percent of the recommended daily amount of 2,400 milligrams. Consuming too much sodium can raise your risk of high blood pressure or, if you already have the condition, make it worse, so it's best to eat miso in moderation, fitting it into a healthy diet that's generally low in sodium. Avoid miso if you're allergic to soybeans because it could cause an allergic reaction. Miso also contains a compound called tyramine, an amino acid that can interact with some anti-depressant medications; discuss use of miso with your doctor if you take one of these medicines, to determine if it's safe for you.

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