Japan — Kanto/Tokyo tradition for soba tsuyu; kaeshi technique developed in Edo period
Tsuyu (つゆ) is the seasoned soy-based broth used both as a hot soup base (kaketsuyu) and as a cold dipping sauce (tsukejiru) for soba and udon. Premium tsuyu is made from kaeshi (a concentrated sauce of soy, mirin, and sugar, rested to mature) combined with dashi (typically katsuobushi + kombu dashi). The ratio of kaeshi to dashi determines the tsuyu's strength: for kaketsuyu (hot soup noodles) approximately 1:7–1:9 (1 part kaeshi to 7–9 parts dashi); for tsukejiru (cold dipping) approximately 1:2–1:3 (more concentrated, accounting for dilution from wet noodles). The distinction between kaeshi and fresh tsuyu is significant — kaeshi (the concentrated soy-mirin-sugar base) must be rested for minimum 3 days before use, during which time the flavours integrate and the alcohol from the soy fermentation mellows. The best soba restaurants make their own kaeshi and rest it for 2–3 weeks.
Properly aged kaeshi combined with good dashi: harmonious savoury-sweet-umami depth without any jarring notes; the mirin's sugar rounds the soy's saltiness; the dashi's umami amplifies the seasoning; the result should taste complete and satisfying immediately
Kaeshi preparation: heat mirin with sake briefly to cook off alcohol; add soy; bring to the verge of boiling without actually boiling; remove from heat; add sugar; cool and rest minimum 3 days (ideally 2 weeks) — the resting is not optional; combine with fresh dashi at service time; for soba tsuyu, the dashi is typically stronger-flavoured (mackerel-forward or sardine-forward) than for udon tsuyu (which is often lighter kombu-katsuobushi).
Home kaeshi formula: 360ml soy sauce + 60ml mirin (briefly heated to remove alcohol) + 50g sugar — heat mirin, add soy and sugar, bring to just under boiling, remove from heat, cool, rest in sealed container minimum 3 days; dashi for soba tsuyu: 1.5g katsuobushi + 1L water simmered 3 minutes — stronger than ichiban dashi; cold soba dipping ratio: 1:2 (1 part kaeshi : 2 parts dashi); the definitive test of any soba restaurant is the tsuyu — balance of savoury-sweet-umami without any harshness reveals whether the kaeshi is properly aged.
Using fresh (unrested) kaeshi (harsh, raw soy flavour dominates — the 3-day minimum rest is essential for flavour integration); using kaketsuyu concentration for cold dipping (too dilute — cold tsuyu must be stronger to account for dilution from wet noodles and gradual breakdown through the meal); reusing dipping tsuyu for soup (used dipping tsuyu has diluted and changed character — it should not be repurposed as hot soup).
Japanese Cooking: A Simple Art — Shizuo Tsuji