Miso is a salted paste made from fermented soybeans — and it is one of the most important ingredients in Japanese cooking. It forms the base of the soup served at virtually every traditional Japanese meal, seasons everything from marinades to dressings, and has been a cornerstone of the Japanese diet for well over a thousand years.
In this article: what miso is, how the different types compare, how it's traditionally made (including the three-year process still used by heritage producers), what the research says about its health properties, and how to use it.
What Is Miso?
Miso is a traditional Japanese fermented paste made from soybeans, salt, and a fermentation culture called kōji (麹). The type of kōji used — and the grain it's grown on — determines what kind of miso you get. There are three main types produced in Japan, plus a fourth category for blends.
| Type (Japanese) | English Name | Key Ingredients | Market Share |
|---|---|---|---|
| 米味噌 (Kome-miso) | Rice-malt miso | Soybeans, rice-malt, salt | 82% |
| 麦味噌 (Mugi-miso) | Barley-malt miso | Soybeans, barley-malt, salt | 4% |
| 豆味噌 (Mame-miso) | Soybean-malt miso | Soybeans, soybean-malt, salt | 4% |
| 調合味噌 (Chōgō-miso) | Blended miso | Mixture of different types | 10% |
Source: Ishii Miso Co. Ltd informational display, Nagano Prefecture
How Is Miso Made?
Miso is made by combining cooked soybeans with salt and kōji (a mould culture grown on rice), then fermenting the mixture in wooden barrels. Mass production accounts for over 80% of output and takes weeks. Traditional methods ferment for up to three years, producing miso with noticeably deeper flavour and complexity.
One of the finest examples of traditional production is Ishii Miso Co. Ltd, established in 1868 in Nagano Prefecture (historically known as the Shinshū region). Their speciality is Shinshu San-nen Miso (信州三年味噌) — a miso fermented for a full three years.
The Three-Year Process
Traditional production involves four key stages, each recorded on tags attached to the fermentation barrels.
Stage 1 — Shikomi (仕込): Initial Preparation
The soybeans are cooked, combined with rice-malt kōji and salt, and packed into large wooden barrels called kioke (木桶). These barrels are bound with braided bamboo hoops and covered with cloth lids weighted with stones.
Stage 2 — Tenchi-gaeshi (天地返し): The Turning
Several months into fermentation, the miso is turned — literally "heaven-earth return" — flipping the contents of the barrel to redistribute the fermenting paste and ensure even fermentation throughout.
Stage 3 — Ikae 1 (移替①): First Transfer
The miso is transferred to a new barrel. This mixes the paste, encourages changes in the balance of microflora, and discharges methane gas that builds up during fermentation.
Stage 4 — Ikae 2 (移替②): Second Transfer
The transfer is repeated a second time during the three-year cycle, for the same reasons.
To give a sense of the timescales: Batch No. 623 at Ishii Miso, made with Nagano Prefecture soybeans, was started on 23 February 2024. Its tenchi-gaeshi was performed on 5 September 2024, and the first transfer took place on 8 January 2025. Its second transfer isn't scheduled until 23 December 2025 — just under two years after initial preparation. Meanwhile, Batch No. 642 was started on 21 February 2026, with all subsequent stages still pending. One batch nearing completion, the next just begun.
Source: Barrel production tags, Batch No. 623 and Batch No. 642, Ishii Miso Co. Ltd
The Fermentation Vessels
Traditional miso fermentation uses large wooden barrels (kioke) bound with braided bamboo hoops, covered with cloth lids and weighted with stones. The kioke are a defining feature of traditional production and their continued use at producers like Ishii Miso is part of what distinguishes craft miso from industrial.
What Are the Key Ingredients in Miso?
Miso is made from three core ingredients: soybeans, which provide protein; kōji (麹), a mould culture grown on rice that produces the enzymes driving fermentation; and salt, which makes up around 12% of the finished paste.
Soybeans (大豆)
Soybeans provide the protein backbone of miso. In Japan, soybeans have particular historical significance: a carnivore ban (肉食禁止令) meant that soybean-based products and fish were the primary sources of protein for much of Japanese history. This is the context in which miso, tofu, soy sauce, and nattō all became essential rather than optional.
Rice-Malt Kōji (麹)
Kōji is the engine of fermentation. It's a mould culture grown on rice and produces the enzymes that break down the soybeans during the long fermentation period.
| Enzyme | Function |
|---|---|
| Protease | Breaks down protein |
| Amylase | Breaks down starch |
| Lipase | Breaks down fat |
These enzymes work alongside yeast and lactic acid bacteria, which develop during fermentation and contribute to miso's complex flavour. The interplay between these microorganisms over months or years is what gives traditionally fermented miso its depth.
Source: Ishii Miso Co. Ltd manufacturing information panels
Salt
Salt content in miso sits at around 12%. It serves two purposes: seasoning and preservation. A traditional Japanese meal of rice and miso soup contains approximately 2.0g of salt total (rice 0.8g, miso soup 1.2g) — comparable to a bread-and-side-dishes meal at 1.2g. The sodium contribution is moderate when miso is used in normal quantities.
Source: Salt intake comparison display, Ishii Miso Co. Ltd
Health Properties of Miso
Research suggests miso may offer several health benefits, including associations with reduced rates of certain cancers and lower blood pressure. Findings should be read as indicative rather than conclusive — no single food is a medical intervention.
Functional Properties
Japanese research has investigated miso across a broad range of potential health applications. A poster published by the Miso Health Committee (みそ健康づくり委員会) lists the following attributed properties: cancer prevention, anti-ageing effects, cholesterol control, gastric ulcer prevention, removal of radioactive substances, digestive promotion, and skin health benefits.
Research Highlights
Gastric cancer: A 1982 study by Hirayama (published in Nutrition and Cancer, 3, 223–233) examined mortality rates from gastric cancer against frequency of miso soup consumption. The data suggested an inverse correlation — higher consumption associated with lower mortality rates.
Breast cancer: Research found that frequent miso soup consumption was associated with a reduced relative risk of breast cancer. This finding formed part of the JPHC (Japan Public Health Centre-based Prospective) Study, a large multi-purpose cohort study conducted across multiple centres in Japan.
Blood pressure: Research at the Ishii Miso factory compared blood pressure readings with miso extracts at two concentrations (1.5g/cup and 0.75g/cup) against a control group. The data suggested miso extract may help reduce blood pressure in those with high blood pressure.
Sources: Hirayama, T. (1982) Nutr. Cancer, 3, 223–233; JPHC Cohort Study; Miso Health Committee (みそ健康づくり委員会) poster; research display panels, Ishii Miso Co. Ltd
A Brief History of Miso
Miso has been part of the Japanese diet for over 1,300 years, first documented in the 7th century as a provision for military rations. Its high protein content and long shelf life made it essential in a culture where a carnivore ban restricted meat consumption for much of history.
7th century — Miso is first documented as a provision for military rations. Protein-dense, shelf-stable, portable.
12th century — Miso soup enters the samurai diet as a regular part of daily meals.
15th–16th century — During the warring states period, miso becomes widely consumed across all levels of Japanese society, reinforced by the ongoing carnivore ban.
Source: Historical display, Ishii Miso Co. Ltd, Nagano Prefecture
How to Use Miso
Miso is most commonly used to make miso soup (misoshiru), but it works equally well as a marinade for fish and meat, stirred into dressings and sauces, or dissolved into stews and braises for umami depth. Add miso towards the end of cooking — prolonged boiling damages the living cultures produced during fermentation.
White (shiro) miso is milder and sweeter — good for dressings, light soups, and delicate fish. Red (aka) miso is stronger and more savoury — better for hearty dishes, marinades, and robust soups. If in doubt, start with a blended (awase) miso, which balances the two.
Frequently Asked Questions
What does miso taste like?
Miso has a deep, savoury umami flavour — salty with complex undertones that develop during fermentation. White miso is milder and slightly sweet; red miso is stronger and more intensely savoury. A three-year aged miso will taste markedly different from one made in weeks.
Is miso vegan?
Traditional miso paste is vegan. The three core ingredients are soybeans, kōji (a fermentation culture grown on grain), and salt. Always check the label on commercial products, as some may contain additional ingredients.
How should I store miso?
Store miso in the fridge once opened. It keeps well for months — the high salt content acts as a natural preservative. The colour may darken slightly over time; this is normal. Keep the lid tightly sealed to prevent drying out.
What is the difference between white miso and red miso?
White miso (shiro miso) ferments for a shorter period, producing a milder, sweeter paste suited to dressings and light soups. Red miso (aka miso) ferments longer, producing a stronger, more savoury flavour for hearty soups, marinades, and robust dishes. Blended (awase) miso combines both.
Is miso high in salt?
Miso contains around 12% salt, but a serving of miso soup contains approximately 1.2g — comparable to the side dishes in a bread-based meal. A full traditional Japanese meal of rice and miso soup totals about 2.0g. Used in normal quantities, the sodium contribution is moderate.
Where to Buy Miso in the UK
Our store stocks authentic Japanese miso — white, red, and specialist long-fermented varieties — with next-day delivery available across the UK.
Technical data sourced from Ishii Miso Co. Ltd, Nagano Prefecture, Japan. Established 1868. All factual claims drawn from factory display panels, production records, and published research cited at the Ishii Miso facility.
