Japan — mustard seed use in Japan dates to the Nara period (8th century), when Brassica seeds were cultivated for oil and condiment use. The karashi tradition as a specific condiment developed through the Edo period alongside the oden and nabe traditions that it accompanies. S&B brand (est. 1923) standardised the commercial karashi paste market.
Karashi (芥子, 辛子, Japanese yellow mustard) is Japan's traditional spicy condiment — a hot, pungent yellow mustard paste made from the seeds of Brassica juncea (brown mustard), ground and mixed with water to produce a sharp, sinus-clearing heat that is instantaneous rather than lingering. Unlike Western prepared mustards, karashi contains no vinegar or other taming agents — it is simply ground mustard seed and water, sometimes with a small amount of flour or turmeric. The result is a mustard with extremely direct, immediate pungency (the allyl isothiocyanate release is fast and intense) that disappears almost as quickly as it arrives. Karashi is used as the standard condiment for tonkatsu, oden, natto, gyoza, and various nimonoand miso preparations.
Karashi's flavour is almost entirely structural: it exists to create a contrast in the meal. Its own flavour — direct, flat, slightly bitter — is less important than its effect: the immediate sinus-clearing pungency that resets the palate after the richness of tonkatsu's panko coating, the gentle warmth of oden broth, or the ammonia-forward character of natto. Karashi doesn't add flavour so much as reset the flavour register — a momentary sharp contrast that makes the next bite of the primary food taste fresher and more vivid.
Mixing karashi: dry powder + small amount of water (mix with a fingertip to paste consistency); cover the bowl and invert for 1–2 minutes — the steam activates the glucosinolate conversion to allyl isothiocyanate (pungency), then expose and use immediately. Pre-mixed karashi from tubes (the convenience standard) lacks the fresh intensity of freshly activated powder. The appropriate amount: karashi is used in very small quantities as a punctuation — a pea-sized amount on the edge of a plate rather than as a coating sauce.
The karashi-natto combination: breaking down the sticky natto with karashi paste before adding soy sauce is the standard preparation method. The karashi's heat cuts through natto's fermanted ammonia character and provides a clean pungency that transforms natto from challenging to compelling. Karashi mentaiko (spicy pollock roe) uses karashi as part of the chili-based marinade — the mustard's flat, direct heat complements the togarashi chili's more complex fruitiness. For oden: karashi is the single condiment applied to daikon, konnyaku, and eggs — the direct sharp heat against the gentle long-simmered flavours is the essential oden contrast.
Using too much — karashi is aggressively pungent; a small amount is the correct measure. Not using the inversion technique for fresh karashi — the steam produced under the inverted bowl is essential for pungency development. Confusing with Western prepared mustard — karashi has no vinegar and is approximately 4–5× more pungent by volume than Dijon mustard.
Japanese Cooking: A Simple Art — Shizuo Tsuji; The Japanese Pantry — Nancy Singleton Hachisu