Fermentation and Aging: Why Japanese Keeps Two Words
On hakkō, jukusei, and knowing who is doing the work
Someone asked me recently whether black garlic is fermented. It looks fermented: dark, soft, transformed, faintly sweet like something that has been through a long negotiation with time. Most packaging says “fermented black garlic.” That word is useful, but it is also imprecise.
The honest answer needs two words, and I only have them because I think about cooking in Japanese.
In Japanese kitchens we say hakkō (発酵) when microbes do the work. Miso is hakkō: koji mold and salt-tolerant yeasts eating soybeans, slowly, for months. Nukazuke pickles are hakkō: a living bed of rice bran where lactic acid bacteria hold territory against everything else that would like to move in. Sake, natto, soy sauce — all hakkō. The defining question is not what changes, but who is working. If the answer is “a population of living things I invited and now have to govern,” it is hakkō.
We say jukusei (熟成) when time does the work without new tenants invited to lead it. Dry-aged beef is jukusei: not a starter-led fermentation, but a controlled room where the animal’s own enzymes — the ones it carried in life — keep quietly dismantling proteins into savor, while air, surface life, and moisture loss shape the outside. A well-rested miso after its fermentation has settled, a katsuobushi in its final months, a fruitcake in autumn — jukusei. The defining question is what time is doing: enzymes already present, slow chemistry, water leaving.
Black garlic is jukusei wearing hakkō’s clothes. Hold whole garlic at around 60 to 70°C in high humidity for a few weeks, and what happens is not fermentation in the miso-or-sake sense — that temperature is a closed door to the cultures we’d normally invite. The main transformation is heat, humidity, slow browning, and time: the Maillard reaction running in slow motion, the same browning chemistry as a steak crust, stretched from ninety seconds to three weeks, alongside the garlic’s own enzymes softening its sharpness into something like dried fruit. It is not a living culture you govern. It is time under controlled heat.
I should be honest: the boundary leaks. That is part of why I love it.
Miso ferments first and matures after — hakkō handing the kitchen keys to jukusei somewhere in the second year, without ceremony. Katsuobushi alternates: molds are invited (hakkō), then sun and rest (jukusei), then molds again, in cycles, until the fish is harder than the plank you’d serve it on. A wheel of cheese does both at once — cultures still alive inside while enzymes age the paste around them. The two words are not a filing system. They are a pair of questions you can ask of anything on your counter: Who is working? What is time doing?
So why keep two words at all, if the categories blur?
Because the words tell you what to govern. This is the part I care about most, and the part that took me longest to learn.
With hakkō, you are governing the living. Salt measured in grams, not pinches, because salt is how you choose which microbes thrive. Temperature in degrees, because two degrees is the difference between the culture you invited and the ones you didn’t. And judgment — the daily look, the smell, the honest question of whether what’s growing is still what you started. When it isn’t, the answer is simple and non-negotiable: it goes out. Fermentation rewards attention and punishes sentimentality.
With jukusei, you are governing patience. There is almost nothing to do, which is its own difficulty. Stable temperature, right humidity, and the discipline not to open the lid, not to check, not to rush the only ingredient that can’t be added later. The failure mode of hakkō is invasion; the failure mode of jukusei is interference — usually mine.
I don’t think Japanese is unique in this. Craft languages grow words wherever hands need them. French, my other kitchen language, raises wine the way farmers raise animals — élevage, literally “rearing” — and gives cheese its own verb for the aging room, affiner, to refine. Every cuisine that has spent centuries negotiating with time has vocabulary for the negotiation. English simply put most of its precision elsewhere. When an English label says “fermented black garlic,” it isn’t lying so much as reaching for the only word on the shelf.
Here in Ho Chi Minh City, the negotiation runs hot. Heat is the great accelerant, and a tropical afternoon gives fermentation a shorter fuse than a cold winter ever did — what might take three unhurried days somewhere cool can rush into one here, and the margin for inattention narrows with it. That is the part the two words keep teaching me: fermentation is not a technique you perform once and then trust forever, but a relationship you keep judging — a look, a smell, the honest question of whether what is growing is still what you started. When the answer is no, it goes out, and it always costs a small sting to let it. That sting is the price of paying attention, and I have come to think of it as the most reliable ingredient I keep.
If you cook, you already have everything this letter describes — a counter, time, and the two questions. Next time something in your kitchen is changing slowly, ask them. Who is working? What is time doing? The answer decides everything you do next.
— Terumi
Ho Chi Minh City
My recipes, the failure-diagnosis index, and the longer essays live at terumimorita.com. This letter is where I think out loud between them.
A short note for my Japanese readers — on a third, gentler word the kitchen keeps.
英語では fermented のひとことで済まされてしまう場所に、日本語は「発酵」と「熟成」のふたつを置きました。そしてよく考えると、台所にはもうひとつ、いちばん優しい言葉があります —— 「寝かせる」。カレーを寝かせる、生地を寝かせる、味噌を寝かせる。働いているのが菌なのか酵素なのか時間なのか、問い詰めずに、ただ任せて待つ。分類の言葉ではなく、信頼の言葉です。科学の解像度と、この柔らかさを両方持てるのが、日本語で料理を考えることの贅沢だと思っています。