A guide to make miso from Aya Aozuki in Tokyo
June 4, 2025
Miso. What begins as the simple bean, becomes something so much deeper. At its core, miso is a fermented paste made from soybeans, salt, and a fermentation started called koji: usually rice or barly inoculated with the mold Aspergillus oryzae.
Miso is more than just a pantry staple in japan and around the globe; it's a time capsule of fermentation, flavour and history. In this guide, we explore how miso is made by owners of Aozuki Izakaya in Tokyo, Aya & Ryota. They kindly invited us to their home while Aya was creating a new batch of miso from scratch.
From the humble soybean to the patient dance of koji, salt and time. Along the way, we'll sprinkle in some facts about the science and tradition behind it.
Soybeans: High in protein and fat - form the rich base
Koji: Rice mould, where the magic happens. Koji breaks down the soybeans, developing umami and sweetness.
Salt: It preserves, regulates fermentation, and balances flavour.
Time: Anywhere from 6 months to 3 years. The longer it sits, the deeper and darker the flavour.
1. Soak and cook the soybeans (or purchase pre-cooked)
Soak dried soybeans overnight, then boil or pressure cook until soft enough to mash between your fingers.
2. Mix Koji and Salt (Koji-kiri)
Mix the salt with the koji thoroughly.
3. Combine all ingredients
Mash soybeans and mix thoroughly with salt and koji. The texture should feel like firm cookie dough.
4. Transfer into container
Roll miso chunks into small balls called 'miso-dama' and throw into the container to flatten. Repeat this step for the entirety of your miso.
5. Seal air tight
Cover the miso with plastic and put a weight on top to completely air tight seal the miso. This is crucial, otherwise there is a chance mould can grow.
6. Ferment
Store somewhere cool and dark. Miso ferments slowly, the microbes work best with time and patience. Aya leaves hers for 6 months before trying and then she says it continues to develop in flavour after that.
Fermentation in miso is primarily enzymatic and microbial. Koji provides powerful enzymes like:
Amylase, which breaks starch into sugars (adding sweetness)
Protease, which breaks protein into amino acids like glutamate (the source of umami)
Salt regulates microbial activity, favouring beneficial organisms while preventing spoilage. Over months, this slow breakdown transforms dense beans into something complex and digestible. Depending on the environment, lactic acid bacteria and wild yeasts may also contribute sublt acidity and aroma - making each batch of miso slightly different.
The secret ingredient: your hands
Aya told us that it's important to not use gloves when making miso. When you make it by hand, you're doing more than mixing ingredients - you're introducing the unique microbes that live on your skin. Traditional miso makers believe these natural, harmless bacteria contribute to the character of the final product. Essentially, this means every hand made miso is completely unique to its maker and is not only a recipe but a fingerprint.
Not all miso is the same. While Aya showed us how to make one type there are many:
Shiro Miso (White Miso): Mild, lightly salty, slightly sweet. Often made with more rice koji and shorter fermentation.
Aka Miso (Red Miso): Deep Strong flavour. Longer fermentation, more soybeans, often higher salt.
Awase Miso: A blend of white and red miso - balanced and versatile.
Mugi Miso: Made with barley koji. Rich and rustic, often found in southern Japan.
Hatcho Miso: Pure soybean miso, aged for up to 3 years. Dense, dark and full of umami.
Each type brings a different flavour and mood - some better for soups, others for sweets and marinades.
