How to Make Miso at Home: A Complete Guide to Homemade Miso Fermentation

Miso is one of the oldest fermented foods in Japanese cuisine, and making it at home is one of the most rewarding fermentation projects available to a home cook. The ingredients are three: soybeans, salt, and koji (rice inoculated with Aspergillus oryzae mold). The technique is labor-intensive for one day, then patient waiting for 3-12 months. The result is a miso with distinctly personal character that commercial products — however good — cannot replicate.

This guide produces a basic white to medium miso (shiro to shinshuu style). Longer fermentation times and more salt will produce darker, more intense miso.

Understanding the Components

Soybeans (daizu, 大豆): The protein source. Hydrated, cooked until very soft, then mashed and combined with koji and salt. Any dried soybeans work. Organic soybeans are preferred by many miso makers because the mold culture performs more predictably on soybeans without pesticide residues, but this is debated.

Rice koji (米麹, kome-koji): Rice that has been inoculated with Aspergillus oryzae mold and incubated for 48 hours until the mold has colonized the grain. Koji produces enzymes — proteases (break proteins into amino acids, including glutamic acid/umami) and amylases (break starches into sugars that feed fermentation microbes). Koji is the most important ingredient in miso making.

You can buy ready-made rice koji (fresh or dried) at Japanese grocery stores and increasingly from fermentation specialty suppliers. Making koji from scratch (inoculating rice with koji spores yourself) is an advanced technique covered in a separate project. For first-time miso making, buy premade koji.

Salt: Acts as selective antimicrobial pressure — high enough salt concentration inhibits dangerous pathogens and undesirable bacteria while allowing the beneficial lactic acid bacteria and koji enzymes to work. Standard table salt works; non-iodized salt is recommended (iodine can inhibit microbial activity at high concentrations).

Salt percentage: The ratio of salt to the total weight of soybean + koji is critical. Standard ranges:

• 8-10% total salt: Light, sweet, short fermentation miso (white/shiro style, 1-3 months)
• 10-12% total salt: Medium miso, 3-6 months
• 12-14% total salt: Fuller miso, 6-12 months

Higher salt = slower fermentation = more time for complex flavors to develop = more assertive result.

Equipment Needed

• Large pot (for soaking and cooking soybeans)
• Food processor or potato masher (for mashing)
• Kitchen scale (weight precision matters for salt ratio)
• Large mixing bowl
• Fermentation crock or food-grade plastic container with a lid
• Heavy press weight (to compress the miso during fermentation — a Ziploc bag filled with brine works)
• Parchment paper

The Recipe: Basic Home Miso

Yield: Approximately 1.2kg finished miso Fermentation time: 3 months (white miso) to 12 months (darker, fuller miso)

Ingredients:

• 500g dried soybeans
• 300g fresh rice koji (or 250g dried rice koji — dried koji is about 25% lighter than fresh)
• 130g non-iodized sea salt or kosher salt (this produces approximately 11.5% salt concentration — a medium miso)
• 1 tablespoon sake (optional — helps with initial moisture content)

Process:

Day 1: Soak and Cook the Soybeans

1. Soak: Rinse the soybeans. Place in a large pot and cover with 3× their volume of cold water. Soak 12-18 hours (overnight). The beans will roughly double in size.

2. Cook: Drain the soaking water. Cover with fresh cold water (again, 3× volume). Bring to a boil, skim foam that rises. Reduce to a gentle simmer. Cook 4-6 hours until the beans are very soft — they should crush completely between your fingers with almost no resistance. A pressure cooker significantly shortens this: cook at pressure for 45-60 minutes.

3. Reserve cooking water: When the beans are done, ladle out 1-2 cups of the cooking liquid. This may be used if the miso paste needs more moisture.

4. Drain and cool: Drain the beans (keeping the reserved liquid). Let cool to below 35°C (95°F) — too hot will damage the koji enzymes. You can speed this with an ice bath if needed.

Mixing: Soybean + Koji + Salt

5. Measure salt: Weigh out your salt precisely.

Reserve 1 tablespoon of salt (this will be used to salt the surface of the miso in the container as a protective layer).

6. Mash the soybeans: Using a food processor, potato masher, or meat grinder, mash the cooked soybeans to a smooth-to-slightly-chunky paste. Some texture is fine; large whole beans are not. The more thoroughly mashed, the more even the fermentation.

7. Mix koji and salt: In a large bowl, combine the rice koji and the remaining salt (reserving 1 tbsp). Mix thoroughly with your hands, squeezing and rubbing the koji and salt together. Let this mixture sit 5-10 minutes — the salt will begin to draw moisture from the koji. This step is called shio-koji mixing.

8. Combine soybean paste and koji mixture: Add the warm-but-not-hot mashed soybeans to the koji-salt mixture. Mix thoroughly with your hands, kneading until completely homogeneous. This takes 5-10 minutes. The mixture should be similar in texture to dense cookie dough — moist enough to form balls that hold their shape, but not wet. If too dry, add a little reserved soybean cooking liquid tablespoon by tablespoon.

9. Taste the raw mixture: It will taste very salty and bland — this is correct. The umami develops during fermentation.

Packing

10. Divide into balls: Divide the miso mixture into fist-sized balls (this is the traditional technique — the purpose is to pack more tightly than you could pouring it in, eliminating air pockets which can cause unwanted mold growth).

11. Pack the container: Slam each ball firmly into your crock or container, pressing out air as you pack. Continue until all the miso is packed, pressing firmly down with your hands after each addition. The surface should be as smooth and flat as possible — no air pockets.

12. Protect the surface: Sprinkle the reserved tablespoon of salt evenly over the surface. Press a sheet of parchment paper directly onto the surface, covering it completely.

13. Weight: Place a weight directly on the parchment. This pressure keeps the miso compact during fermentation and helps brine (tamari) rise to the surface. A Ziploc bag filled with salty water (saturated brine) works well — it conforms to the shape.

14. Cover loosely: Cover the container with a cloth or lid that allows some air circulation. Do not seal airtight — you want some gas exchange.

Fermentation

Storage location: Store at room temperature, away from direct sunlight. Ideal fermentation temperature is 15-25°C (59-77°F). Higher temperatures speed fermentation; lower temperatures slow it. Traditional Japanese miso is often made in winter and ferments through summer, which provides the slow-then-fast temperature change that develops complex flavor.

3-month check (white miso): After 3 months, open and taste. White miso at 3 months will be mild, slightly sweet, and somewhat salty with early miso character. If this is the target, it's ready to use.

6-month check (medium miso): At 6 months, the miso has developed significantly more depth, less sweetness, more umami.

12-month check (darker miso): A full year produces a miso comparable to commercial medium-red miso, with complex flavor and deep character.

What You May See During Fermentation

Tamari: Dark liquid that rises to the surface and pools. This is the original tamari (not the soy sauce sold as tamari today) — liquid that forms naturally from miso fermentation. Stir it back in or use it as a condiment (it's essentially aged soy sauce).

White mold on the surface: Aspergillus mold may form a white/cream fuzz on exposed surfaces. This is normal and harmless — scrape it off with a clean spoon and discard, then press the miso back down and re-cover.

Black or pink mold: These are contamination mold — less beneficial. Scrape off the affected area plus 1-2cm around it. These molds indicate an air pocket or insufficient salt; they are harmless in small amounts but indicate a fermentation issue.

"Turning" the miso: Some traditional recipes call for transferring the miso to a clean container halfway through fermentation. This redistributes fermentation activity and prevents the top layer from fermenting unevenly. Not essential but can improve consistency.

Finishing

When the miso is ready: The miso is "done" when it tastes right to you. There's no single endpoint. Transfer to clean jars and refrigerate, which halts further fermentation and preserves the character. Refrigerated homemade miso keeps for 1-2 years, continuing to slowly change.

If you want to homogenize the texture, blend or food-process the finished miso. If you prefer texture from the whole koji rice pieces, use as-is.

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Homemade miso has a vitality that commercial miso — however excellent — doesn't quite capture. The koji cultures that drove the fermentation are still present, the flavor compounds are at their most complex before any pasteurization, and the character reflects specific choices about salt percentage and fermentation time that make it unmistakably yours.

It is not a quick project. But the investment in that one hard day of cooking and the months of patient waiting produces something worth the wait.

Related reading: What Is Miso? Complete Guide | How Soy Sauce Is Made | Japanese Fermentation Guide