Hiyashi dashi (冷やしだし) — cold-brew Japanese stock — is the simplest, most forgiving way to make dashi. It requires no timing, no temperature monitoring, and no risk of the bitterness that can come from overheating kombu.
The method: submerge kombu in cold water, refrigerate overnight, remove kombu, use the liquid.
That's it. The result is a clean, bright, deeply savory stock with a subtly different character than hot-extracted dashi — slightly sweeter, more transparent in flavor, excellent for dishes where you want dashi presence without any earthiness.
The Science of Cold Extraction
Dashi's flavor comes primarily from kombu (Saccharina japonica), which contains:
- Glutamic acid: The original umami compound — the "savory" sensation that makes dashi feel complete
- Mannitol: A mild natural sweetener (the white powder on dried kombu surfaces)
- Iodine and minerals: Minor contributors to flavor complexity
Why temperature matters:
Hot extraction (60-80°C) is fast and pulls more compounds from the kombu, including both glutamic acid and some less desirable compounds that contribute to a slightly muddy or earthy quality if the kombu overheats past 80°C.
Cold extraction is slow (8-12 hours minimum) but selective. At refrigerator temperatures (4-6°C), glutamic acid extracts cleanly while many of the compounds that create off-flavors remain locked in the kombu's cell walls. The result is a purer glutamate extraction — arguably higher quality for delicate applications.
The difference:
- Hot dashi: Richer, deeper, slightly more complex, faster (30 minutes active + 15 min rest)
- Cold dashi: Cleaner, more transparent, sweeter, more delicate — better for clear soups and dishes where subtlety matters
Neither is universally superior. They're different tools.
Method: Basic Hiyashi Dashi (Kombu Only)
Ingredients:
- 1 liter cold water
- 15-20g dried kombu (about 10cm piece)
Equipment:
- Glass jar, pitcher, or container (glass preferred — less likely to absorb odors)
Method:
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Wipe kombu gently with a dry cloth to remove any surface dust. Do not wash — the white powder (mannitol) is flavor-contributing. Rinsing washes it away.
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Submerge kombu in cold water in the container. The kombu will immediately begin to soften and release a slight viscous quality into the water.
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Refrigerate for 8-12 hours (overnight is ideal; up to 24 hours is fine and will produce a stronger result). There is no danger of bitterness from cold extraction the way there is from hot extraction — the compounds that cause bitterness don't extract effectively at cold temperatures.
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Remove kombu. The dashi is ready to use immediately. It keeps refrigerated for up to 5 days.
Yield: 900-950ml (slight reduction from kombu absorption)
Strength Calibration
Cold dashi strength is primarily controlled by:
- Kombu quantity: More kombu = stronger extraction
- Time: Longer = stronger (up to about 24 hours before diminishing returns)
- Water temperature: Standard refrigerator temperature is ideal; colder is fine
Standard ratio: 15-20g kombu per 1 liter water. For a stronger dashi (for simmered dishes or sauce bases where dashi needs to hold up against competing flavors), increase to 25-30g per liter or extend to 20+ hours.
Hiyashi Dashi Variations
Katsuobushi Cold Dashi
For a more complex stock with both glutamate (kombu) and inosinate (katsuobushi) umami synergy:
- Make cold kombu dashi as above
- The next day, heat the kombu dashi to just under a simmer (about 60°C)
- Remove kombu (or use it for kombu tsukudani — see below)
- Add 20-30g katsuobushi (dried bonito flakes), remove from heat
- Let steep 3-5 minutes
- Strain through fine mesh
This hybrid method uses cold extraction for the kombu base, then a controlled hot steep for the katsuobushi. The result is an ichiban dashi (first dashi) with the clarity benefits of cold extraction plus the depth of katsuobushi.
Kombu + Shiitake Cold Dashi
For an entirely plant-based dashi with both glutamate (kombu) and guanylate (dried shiitake) umami synergy — the vegetarian option with excellent depth:
- Add 2-3 dried shiitake mushrooms alongside kombu in cold water
- Refrigerate 12-24 hours
- Remove both kombu and shiitake
This dashi has pronounced depth and slight earthiness from the shiitake. Excellent for vegetarian ramen broth, mushroom-based dishes, or any preparation where you want plant-based umami richness.
What to Do with the Spent Kombu
After cold extraction, the kombu is soft and flavorful — significantly more edible than after hot extraction (where long cooking and high temperature can make it gummy or overly gelatinous).
Kombu tsukudani: Slice spent kombu into strips and cook in a small saucepan with:
- 2 tbsp soy sauce
- 1 tbsp mirin
- 1 tbsp sake
- 1 tsp sugar
- Optional: sesame seeds, dried chili flakes, katsuobushi
Simmer until liquid is almost entirely absorbed (10-15 minutes). The result is a deeply savory, slightly sweet kombu condiment that works as a topping for rice, a filling for onigiri, or a component of Japanese lunch boxes. Cold-brew kombu makes better tsukudani than hot-extraction kombu because the texture is better preserved.
Best Uses for Hiyashi Dashi
Cold-brew dashi's cleaner, more delicate profile makes it ideal for:
Clear soups (suimono): The most elegant Japanese soup format — a crystal-clear broth with a few carefully selected ingredients (clams, mitsuba, yuzu peel). Cold dashi produces an exceptionally clear stock with no turbidity.
Chawanmushi: Japanese steamed egg custard. The ratio of dashi to egg determines the final texture; cold dashi's clarity lets the egg flavor come forward without competing earthiness.
Cold noodle dipping broth (tsuyu): For zaru soba or hiyashi somen, diluted cold dashi is the base of the dipping sauce. Cold dashi's sweetness and clarity work better than hot dashi here.
Dashimaki tamago: Japanese rolled omelette (tamagoyaki). Many recipes incorporate dashi directly into the egg mixture. Cold dashi gives a cleaner flavor than hot dashi in cooked eggs.
Wafu dressing: Japanese salad dressing using dashi, soy sauce, mirin, and rice vinegar. Cold dashi's sweetness balances the soy and vinegar effectively.
Ochazuke: Hot green tea or dashi poured over leftover rice with toppings (pickles, salmon, nori, wasabi). Cold dashi heated gently is the preferred base for a refined ochazuke.
Storage
Refrigerator: Up to 5 days in a sealed container. The flavor may intensify slightly over time as extraction continues (if kombu is left in) or remains stable (if kombu is removed).
Freezer: Cold dashi freezes exceptionally well. Freeze in ice cube trays for small portions (1 cube = approximately 30ml), then store in a freezer bag. Keeps up to 3 months. Frozen dashi cubes are convenient for adding dashi to pan sauces or small preparations without making a full batch.
Practical Workflow
The most practical approach to cold dashi for regular home cooking: maintain a jar in the refrigerator, refreshing it every 3-4 days.
Routine:
- Sunday evening: fill jar with water + kombu
- Monday morning: remove kombu, jar is ready
- Use throughout the week for miso soup, simmered vegetables, egg preparations
- By Friday, make a fresh batch
This means dashi is always ready without planning or waiting — the cold-brew method removes the friction from Japanese home cooking.
Cold-brew dashi is a genuine gateway technique. Once you maintain it as a kitchen constant — always a jar in the refrigerator — the barrier to making proper miso soup, proper Japanese simmered dishes, and proper Japanese egg preparations drops to nearly zero.
Related reading: Kombu Guide | All Types of Dashi | Japanese Miso Soup Guide
The full recipes live in the book.
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