Knowledge Base

Miso

Legume ferment · Traditional · peer-reviewed

Soybeans (or other legumes) inoculated with koji mould, mixed with salt, and aged into a deep, savoury paste — Japan's foundational umami ferment.

Advanced to make
30–365 days ferment
protein, umami, live-cultures supplied

Ingredients

  • Dry soybeans500 g
  • Rice or barley koji (Aspergillus oryzae on grain)250 g
  • Non-iodised salt (roughly 12% of bean + koji weight)90 g

How to make it

  1. Step 1

    Soak soybeans overnight, then cook until soft enough to mash easily.

  2. Step 2

    Drain and cool to body temperature — heat kills the koji mould.

  3. Step 3

    Mash the beans, then fold in crumbled koji and salt until evenly combined.

  4. Step 4

    Pack firmly into a clean crock or jar, pressing out air pockets. Salt the surface lightly.

  5. Step 5

    Weight the surface and cover. Age cool and dark — weeks for sweet miso, months to a year for dark.

  6. Step 6

    Ready when the paste smells deep and savoury. Refrigerate once opened.

What it is

Miso is a salted, aged paste of cooked legumes (classically soybeans) fermented with koji — grain inoculated with the mould Aspergillus oryzae. The mould's enzymes break proteins and starches into amino acids and sugars; salt and time then select a community of yeasts and lactic bacteria that build the characteristic umami depth. Colour and flavour range from pale, sweet "white" miso (short age) to dark, robust red or hatcho miso (long age, higher salt).

The science

Koji is an enzyme factory. A. oryzae secretes proteases, amylases and lipases that hydrolyse soy protein into free amino acids (especially glutamate, the core of umami) and starch into fermentable sugars (Allwood et al., 2021). During ageing, salt-tolerant yeasts (e.g. Zygosaccharomyces rouxii) and lactic-acid bacteria further transform those substrates into organic acids, alcohols and aromatics. Salt concentration and temperature are the main levers: higher salt and cooler ageing slow the ferment and favour deeper, darker flavour over months to years.

Safety

Use food-grade koji and clean equipment. Salt at roughly 10–13% of the bean-plus-koji weight keeps spoilage organisms in check during the long ambient age. A white surface bloom of yeast can be skimmed; discard if you see fuzzy coloured mould, a putrid smell, or unexpected sliminess. Once opened, keep miso refrigerated.

Signs it worked / troubleshooting

  • ✅ Good: dense paste, clean savoury/miso aroma, colour appropriate to age and type.
  • ⚠️ Too salty or slow = salt high or room cold → normal for long-age miso; taste over months.
  • 🚫 Coloured mould or off smell = contamination → discard the affected batch.

How to store

Unopened crocks age at cool room temperature. After opening, refrigerate in a sealed container; miso keeps for many months and continues to deepen slowly.

References

  • Allwood JG, Wakefield LT, Grant H, et al. (2021). Fermentation and the microbial community of Japanese koji and miso: a review. Journal of Food Science 86(6):2194–2207. doi:10.1111/1750-3841.15773
  • Kusumoto KI, Yamagata Y, Tazawa R, et al. (2021). Japanese traditional miso and koji making. Journal of Fungi 7(7):579. doi:10.3390/jof7070579
  • Kumazawa K, Kaneko S, Nishimura O (2013). Identification and characterization of volatile components causing the characteristic aroma of Japanese soy sauce (shoyu) and miso. Journal of Agricultural and Food Chemistry — related koji-ferment aroma literature summarised in Allwood et al. (2021).

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