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How to make miso at home

Cooked soybeans, koji and salt, packed into a jar for weeks or months. What happens inside, how much salt each miso takes, how to pack it and what to do about what shows up on top.

8 min read Advanced Reviewed October 26, 2026 40 facts, each with its source

What miso is

Miso is a soybean paste fermented and matured with koji and salt1. There are hundreds of kinds, depending on what goes in and how it's made2, and they're told apart by color: red, yellow and white3. The most common in Japan is rice miso, made with rice koji and soybeans: it's 80% of what's made there4.

It's made in two fermentations5. First comes the koji: rice or barley covered in a mold, Aspergillus oryzae. You can buy it or grow it. Then comes the miso: the koji is mixed with cooked soybeans and salt, and left to mature. Everything about koji is in "Koji: what it is and how to use it."

What happens inside the jar

In miso, the koji mold stops growing. What does the work is its enzymes, which break down the proteins and starches in the soybeans6. The proteins give the amino acids behind the flavor; the starches, sugars. There are also yeasts and lactic acid bacteria, which come with the ingredients or are added7. The yeasts' alcohol is part of miso's aroma8, and since they need air, miso matures under a loose lid9.

Over the months, miso darkens. That's the Maillard reaction, the same one that browns bread crust, and it gives color and aroma10.

Miso isn't a sour ferment: its pH stays above 4.611. It drops from 6 to 5, and its water activity, a measure of the water free for microbes, drops from 0.98 to 0.8012. What preserves it is salt: the guides ask for more than 4%13, and for mature miso to reach a water activity of 0.85 or lower14. That's why pH won't tell you whether it's going well, and why the salt is always weighed.

Salt, on the total weight

In miso, salt is worked out on the weight of everything in it: koji, soybeans, water and salt15. It's usually 6 to 16%16. Store-bought miso has 5 to 13%, and usually 11 to 13%17.

With more koji, miso comes out sweeter; with less, saltier18. Fervida has two methods, each with its own salt and minimum. The minimums, and white miso's 2-month cap out of the fridge, are house rules:

Miso Salt on the total Minimum Out of the fridge
White or short 6–10%, 8% by default 6% 2–4 weeks, never more than 2 months
Red or long 11–13%, 12% by default 10% 3–12 months

There's no low-salt homemade miso. According to Japan's Ministry of Agriculture, with 25% less salt or more, miso spoils and safety becomes a problem19. Store-bought sweet misos, with 5–7% salt20, are made in a few days at a controlled 50 °C (122 °F) or so21. At room temperature they're no model to follow.

The soybeans: soaking, cooking and cooling

  1. Soaking. Rinse the soybeans and leave them in plenty of water for 15 to 17 hours22, in the fridge: a soak of more than 4 hours always goes in the cold23.
  2. Cooking. Drain them and cook until they crush between two fingers. Boiled or steamed, that's about 3 hours24, always at 74 °C (165 °F) or more25. A pressure cooker is the recommended option, because it does a better job against Bacillus spores26.
  3. Draining and mashing. Save the cooking water and mash the soybeans.
  4. Cooling. From 60 °C (140 °F) to 35 °C (95 °F) within 2 hours at most27. The guides set the limit at 628; if it took longer, Fervida throws it out (a house rule). Koji goes in with the soybeans at 35 °C (95 °F) or below29.

Chickpeas, black soybeans or white beans work too. Fervida treats them as variations, with the same salt, cooking and cooling (a house rule).

Mixing and packing

Mix the koji with the salt, keeping back a spoonful for the surface. Add the mashed soybeans and as much cooking water as it takes to get the texture of soft clay. Pack the paste into a clean container, pressing so there are no air pockets, and smooth the surface.

Sprinkle on the salt you kept back and cover the surface with plastic wrap or parchment30. Put a weight on top and leave the lid loose9. Never vacuum-sealed or in an airtight jar: the yeasts need air.

Glass, food-grade plastic and fermenting crocks all work. Don't use ceramic with a glaze you don't know (a house rule). And don't start the miso with miso from another batch: in ferments that aren't acidic, it isn't recommended31.

Maturing: weeks or months

Miso matures at room temperature for anywhere from a month to more than a year32. Sweet miso takes 10 to 15 days33; salty miso, 2 to 12 months34; all-soybean miso, 6 to 1235. At lower temperatures, it takes longer36. Industry matures it at 25–30 °C (77–86 °F)37.

At home, find a cool, dark spot: 15–25 °C (59–77 °F) for white miso and 10–25 °C (50–77 °F) for red (a house rule). Check it once a month and taste it. White miso from the second week; red at 3, 6 and 12 months. When you like it, into the fridge, which slows the fermentation down38.

What shows up on top

What you see What to do
A dark liquid Stir it back in or use it as a sauce (a house rule)
A smooth white film, or white or pale green mold Scrape it off with a margin and replace the wrap39
Pink, red, orange, blue or black mold, over the whole surface or inside the paste Throw out the whole miso (a house rule)
Slimy paste or a rotten smell Throw out the whole miso (a house rule)

The guides only say what to do about mold on the surface; the rest is Fervida's call. If you can't tell what it is, treat it as a colored mold: when in doubt, throw it out (a house rule). It's all explained in "A layer on top of the miso."

Miso, like soy sauce, can carry biogenic amines such as histamine40. If they disagree with you, see the guide on histamine.

The recipes

  • White miso: short and sweet, with 8% salt. Ready in 2 to 4 weeks.
  • Chickpea miso: the same short method, with chickpeas instead of soybeans.
  • Red miso: long and salty, with 12% salt. 3 to 12 months.

The koji can be bought or your own, grown from bought spores: see "Grow your own koji."

Where every number comes from

Every number in the text leads to its figure: the exact sentence from the official guide it comes from, in its own language.

  1. 1

    Miso is a soybean paste fermented and matured with koji and salt

    “Miso is defined as a semisolid paste primarily made from soybeans, which are combined, fermented, and matured with soybeans and/or grains cultured with koji mold and salt.”

    Journal of Fungi (MDPI), Kusumoto K.-I. et al. · Japanese Traditional Miso and Koji Making (2021) · read on October 25, 2026

  2. 2

    There are hundreds of kinds of miso

    “There are several hundred types of miso depending on substrates used and the processing conditions.”

    BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026

  3. 3

    There are red, yellow and white miso

    “There are red, yellow, and white miso according to the color of the finished product.”

    Journal of Fungi (MDPI), Kusumoto K.-I. et al. · Japanese Traditional Miso and Koji Making (2021) · read on October 25, 2026

  4. 4

    Rice miso is 80% of what Japan makes

    “Rice miso is the most common in Japan, accounting for 80% of production.”

    BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026

  5. 5

    Miso is made in two fermentations

    “describes miso as a two stage fermentation process.”

    BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026

  6. 6

    In miso the mold stops growing: its enzymes break down proteins and starches

    “During miso fermentation, growth of koji starter (Aspergillus mould) is retarded, enzymes in the koji starter break down proteins and carbohydrates.”

    BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026

  7. 7

    Miso holds yeasts and lactic acid bacteria, from the ingredients or added

    “Saccharomyces yeast and lactic acid bacteria may occur when naturally present on ingredients and/or as deliberately added ingredients during fermentation of miso”

    BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026

  8. 8

    The yeasts' alcohol is part of miso's aroma

    “A part of the fatty acids reacts with ethanol, which is generated by yeast fermentation, to form ethyl esters, the important constituents of miso aroma.”

    Journal of Fungi (MDPI), Kusumoto K.-I. et al. · Japanese Traditional Miso and Koji Making (2021) · read on October 25, 2026

  9. 9

    A weight on top and a loose lid: the yeasts need air

    “Weights are placed on top of a lid that is loosely applied (fermentation with yeast requires oxygen).”

    BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026

  10. 10

    The Maillard reaction gives it color and aroma

    “The Maillard reaction not only adds color, but also generates a range of aromatic compounds.”

    Journal of Fungi (MDPI), Kusumoto K.-I. et al. · Japanese Traditional Miso and Koji Making (2021) · read on October 25, 2026

  11. 11

    It isn't an acidic fermentation: the pH stays above 4.6

    “This is not an acidic fermentation. The final pH of koji and other koji products are above a pH of 4.6.”

    BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026

  12. 12

    pH drops from 6 to 5, and water activity from 0.98 to 0.80

    “The pH of miso will decrease from 6 at the start of fermentation to a pH of 5, and aw will reduce from 0.98 to 0.80 in mature miso.”

    BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026

  13. 13

    More than 4% salt holds mold back

    “Salt concentration should be >4%. Salt inhibits further mould, i.e., Aspergillus growth”

    BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026

  14. 14

    Mature miso at a water activity of 0.85 or lower

    “Matured miso should have aw of 0.85 or lower; finished miso usually has an aw of approximately 0.80.”

    BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026

  15. 15

    The percentage is worked out on the weight of all ingredients

    “Calculations of percent amounts to add are based on the weight of all ingredients.”

    BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 2: Starter cultures and fermented food standards (August 2024, updated December 2024) · read on October 25, 2026

  16. 16

    Salt is usually 6 to 16%

    “Typical salt concentration is 6 to 16%,”

    BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026

  17. 17

    Store-bought miso has 5 to 13%, usually 11 to 13%

    “The salt content varies widely from 5 to 13%, but typically it is in the 11 to 13% range.”

    Journal of Fungi (MDPI), Kusumoto K.-I. et al. · Japanese Traditional Miso and Koji Making (2021) · read on October 25, 2026

  18. 18

    More koji, sweeter; less koji, saltier

    “Higher proportions of rice or barley koji create a sweeter taste, while lower proportions of koji produce saltiness.”

    Journal of Fungi (MDPI), Kusumoto K.-I. et al. · Japanese Traditional Miso and Koji Making (2021) · read on October 25, 2026

  19. 19

    With 25% less salt, miso spoils and safety becomes a problem

    “○みそには、発酵に適切な塩分と水分の範囲が存在し、２５％以上の低減では変敗が生じ、安全”

    Consumer Commission, Cabinet Office of Japan (消費者委員会), with an annex prepared by MAFF (農林水産省) · 資料3-2: ナトリウムに係る低減された旨の表示（25％以上の相対差）の特例を設けることについて (15-oct-2014) · read on October 25, 2026

  20. 20

    Sweet misos have 5–7% salt

    “they are sweeter and lower in salt (5 to 7%)”

    Journal of Fungi (MDPI), Kusumoto K.-I. et al. · Japanese Traditional Miso and Koji Making (2021) · read on October 25, 2026

  21. 21

    And they're made in a few days at about 50 °C

    “completes within a matter of several days at around 50 °C”

    Journal of Fungi (MDPI), Kusumoto K.-I. et al. · Japanese Traditional Miso and Koji Making (2021) · read on October 25, 2026

  22. 22

    Soybeans soaked 15 to 17 hours

    “Soybeans are washed and soaked in water typically for 15 to 17 h.”

    Journal of Fungi (MDPI), Kusumoto K.-I. et al. · Japanese Traditional Miso and Koji Making (2021) · read on October 25, 2026

  23. 23

    Soaking over 4 hours, in the fridge

    “Soaking of rice, soy beans and other substrates in excess of four hours should occur in refrigerated conditions or in”

    BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026

  24. 24

    Soybeans boiled or steamed 3 hours

    “Steaming or boiling for minimum time of 3 hr (soybeans) and”

    BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026

  25. 25

    Always at 74 °C or more

    “Minimum cooking and steaming temperature steps should occur at 74oC, or higher.”

    BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026

  26. 26

    A pressure cooker is recommended against Bacillus spores

    “Generally longer cooking times are recommended to inactivate Bacillus spores which are common contaminants of rice, soybeans and other cereal substrates. Cooking under pressure is recommended for this reason.”

    BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026

  27. 27

    From 60 °C to koji temperature within 2 hours

    “Cooling from 60oC to target inoculation temperature should occur within two hours.”

    BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026

  28. 28

    2 hours, with 6 as the limit

    “CCP. 2 hours (critical limit is 6 hrs)”

    BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 2: Starter cultures and fermented food standards (August 2024, updated December 2024) · read on October 25, 2026

  29. 29

    Koji goes on soybeans at 35 °C or below

    “Koji should be added at or below 35oC”

    BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026

  30. 30

    The mixture is covered with parchment or cloth

    “the mixture is placed into a fermentation vessel, then covered with parchment or fabric.”

    BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026

  31. 31

    In ferments that aren't acidic, you don't start from an earlier batch

    “In summary, backslopping in non-acid fermentations or in fermentations where the culture creates low acid conditions are not recommended.”

    BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 2: Starter cultures and fermented food standards (August 2024, updated December 2024) · read on October 25, 2026

  32. 32

    It matures at room temperature for a month to over a year

    “Ambient temperature incubation for 1 month to more than one year.”

    BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026

  33. 33

    Sweet miso, 10 to 15 days

    “Sweet miso has a shorter holding time (10 to 15 days)”

    BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026

  34. 34

    Salty miso, 2 to 12 months

    “salt miso has a longer holding time (2 to 12 months)”

    BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026

  35. 35

    All-soybean miso, 6 to 12 months

    “Soybean miso is fermented from 6 to 12 months.”

    BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026

  36. 36

    At lower temperatures, it takes longer

    “Ambient temperature maturation is typical in smaller operations, is acceptable, with times to maturation of the miso likewise extended in lower temperatures.”

    BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026

  37. 37

    In industry, at 25–30 °C

    “Temperatures using automated equipment, or in controlled environments, range between 25oC to 30oC”

    BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026

  38. 38

    Finished miso goes in the fridge

    “Miso is stored and labelled to require refrigeration to limit”

    BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026

  39. 39

    Mold on the surface is scraped off and the cover replaced

    “Visible mould growth on miso surface (Aspergillus or others) should be scraped off, removed and covers replaced during the”

    BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026

  40. 40

    Miso can carry biogenic amines such as histamine

    “Biogenic amines such as histamine have been associated with soy sauce and miso products.”

    BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026

Keep reading

Koji: what it is and how to use it Salt: how much and why Fermentation weights Jars and lids Temperature and time Histamine and ferments Cooking with ferments