Soba and udon developed as distinct culinary traditions in different regions of Japan: soba (buckwheat noodles) in Edo (Tokyo) and the mountain regions; udon (wheat noodles) in the Kansai region around Osaka and Kyoto. The broths reflect these regional differences: Edo soba broth is darker and more assertive (more soy, darker kombu extraction); Kansai udon broth is pale, sweet, and delicate (usukuchi soy, very light). [VERIFY] Tsuji's treatment of regional broth differences.
The seasoned broths for soba and udon differ from suimono and nimono dashi in concentration, seasoning balance, and the specific type of soy used. Kaketsuyu (the light broth for warm noodle soup) must be carefully calibrated to season the noodles — which are already salted from their making — without producing an over-salty result. Mentsuyu (the concentrated dipping sauce for cold soba and cold udon) is fundamentally different — a strong, sweet-salty concentrate served cold, with the noodle dipped in small amounts rather than submerged.
**Kaketsuyu (warm noodle broth):** - Ichiban or good niban dashi, seasoned with soy and mirin — the ratio varying by region and noodle type. - Soba broth: darker, more soy-forward, made with katsuobushi-rich dashi. - Udon broth: paler, sweeter, made with kombu-heavy dashi and usukuchi soy. - Temperature: piping hot — the bowl should be warmed before service. **Mentsuyu (cold dipping sauce):** - A concentrate: a much higher ratio of soy to dashi than kaketsuyu. Used a tablespoon at a time, diluted with the cold dashi to each diner's preference at the table. - Contains mirin (often reduced — kaeshi) and soy combined with a smaller amount of katsuobushi-steeped dashi. - Made in advance — mentsuyu improves with resting 24 hours as the flavours integrate.
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