What is fermentation?
Fermentation is what happens when micro-organisms change a food. In the process, enzymes, amino acids and organic acids appear that were not in the original ingredient. That is the whole definition — everything else is detail about which micro-organism, and on what.
Fermentation and spoilage are the same process
Micro-organisms break down a food and produce something new. When the result is something people want to eat, we call it fermentation. When it is not, we call it spoilage. The biology is not different; the judgement is ours.
What separates the two in practice is control — which organism you let in, at what temperature, with how much salt, for how long. Every fermented food in the world is an answer to that question, worked out over centuries by people without microscopes.
Kinds of fermented food
Fermented foods are usually grouped by what they are made from and which organism does the work.
- Plant-based, Japanese — miso, shoyu, sake, mirin, rice vinegar, amazake, tsukemono, natto
- Animal-based — yoghurt, cheese, cured meats, katsuobushi
- From elsewhere — kimchi, sauerkraut, doubanjiang, fish sauce, kombucha, sourdough
The organisms differ: lactic acid bacteria in kimchi and yoghurt, yeasts in bread and sake, moulds in cheese and in koji. Japan's distinctive contribution is the last one.
Koji — the reason Japanese fermentation is different
Koji is Aspergillus oryzae, a mould, grown deliberately on steamed rice, barley or soybeans. It is designated the national fungus of Japan. Left to work, it produces enzymes that break starch into sugar and protein into amino acids — which is why so much Japanese food tastes sweet or savoury without sugar or meat being added.
Almost everything in a Japanese kitchen runs through it. Miso is koji plus soybeans and salt. Shoyu is koji plus soybeans, wheat and brine. Sake is koji plus rice and yeast. Amazake is koji plus cooked rice and nothing else — no sugar is added at any stage; the sweetness comes from the enzymes. Shio-koji is koji plus salt and water, and it will season almost anything.
Koji is also visible. Under a microscope, the koji at the point of flowering — koji-no-hana — is one of the more startling things you can look at in a kitchen. We show it to people in our classes because a description does not do it justice.
Why Japan has so many fermented foods
Japan has four distinct seasons, high summer humidity, and regions with quite different climates. Those are good conditions for moulds and for the slow, controlled ferments that depend on them. Add rice, soybeans and vegetables in quantity, and you get a country where nearly every region developed its own miso, its own pickles and its own vinegar.
Miso alone varies enormously across the country — rice miso, barley miso, soybean miso, pale and dark, sweet and sharp. In our Fermentation Planner Course we taste them side by side, because reading about the difference does not teach you anything.
How to start
Four suggestions, in order of how easy they are.
- Use fermented seasonings. Miso, shoyu and vinegar are already fermented foods. Cooking with them is the least effortful way to eat more of them. Some are high in salt, so use them as seasonings rather than by the spoonful.
- Change your staple. Barley, brown rice and wholegrain bread carry more fibre than refined white rice or flour, and they go well with fermented seasonings.
- Vary the source. Natto, kimchi, tsukemono and yoghurt each carry different organisms. A little of several kinds is a better habit than a lot of one.
- Try amazake and shio-koji. Amazake is sweet without added sugar and works as a morning drink. Shio-koji draws out savoury depth, so a dish seasoned with it does not taste flat even with less salt. Both are good in dressings and marinades.
The habit matters more than the quantity. A small amount every day is easier to keep up than a large amount occasionally.
If you want to make it yourself
Reading about koji is not the same as growing it. If you are in Tokyo, we teach both a one-day lesson and a full four-level course, and there is a private session in English for visitors.
UMAMI & KOJI LAB (in English) → One-day fermentation lesson → Fermentation Planner Course →