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Kombu

Kombu is dried kombu kelp – thick, dark and faintly mineral-smelling in the packet. Drop a piece into cold water and within half an hour it starts to change everything around it. The liquid turns pale gold. A fuller and more savoury flavour develops, because kombu releases glutamate, the amino acid behind umami. That simple soak is why kombu is used so often in Japanese cooking, especially as the base for dashi and simmered dishes.

Browse our range of kombu below, and discover an essential ingredient that will transform a huge variety of Japanese dishes.

What is kombu and how is it used in Japanese cooking?

Kombu is an edible kelp harvested mostly around Hokkaido, where cold water and long growing seasons produce thick, flavourful fronds. Once cut, it’s dried, pressed and sometimes aged so the taste deepens and concentrates.

What makes kombu kelp distinctive is its glutamate content. In 1908, Japanese chemist Kikunae Ikeda was studying why kombu dashi tasted so satisfying. He isolated glutamic acid as the compound responsible and named the sensation umami – a fifth taste alongside sweet, sour, salty and bitter. Kombu remains one of the richest natural sources of glutamate, which is why a single strip can turn plain water into stock with genuine depth.

The white powdery coating you’ll often see on dried kombu is mannitol, a natural sugar. It carries flavour and should be left on. Wipe the surface lightly with a damp cloth if needed, but don’t wash it under running water – you’d rinse away the very thing that gives the kelp its taste.

Types of kombu

Several varieties of Japanese kombu are harvested in Hokkaido, each with a different character. They’re broadly interchangeable for home cooking, but choosing the right one for a specific dish can make a noticeable difference.

Rishiri kombu produces a clear, refined dashi with a clean savoury edge and very little colour. It’s prized in Kyoto-style cooking where the broth needs to be transparent and understated, letting other ingredients speak. We stock Rishiri wild kombu harvested from open water.

Ma kombu is the most widely used variety. Thick, wide and light brown, it yields a slightly sweet, full-bodied broth and is versatile enough for dashi, simmered dishes, hot pot and tsukudani (kelp simmered in soy sauce and mirin). Our Ma kombu is available in large sheets, useful for making stock in volume or wrapping ingredients before cooking.

Hidaka kombu is softer in texture and cooks down more easily than other varieties, which makes it a good choice if you want to eat the kelp itself – in oden, simmered dishes, or sliced into salads. Its broth is milder and slightly sweeter.

Rausu kombu is thin, wide, and delivers an intensely flavoured, amber-coloured broth. It’s often used in hot pot and richer stock applications where subtlety matters less than depth.

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Frequently asked questions

What does kombu taste like?

On its own, dried kombu kelp has a mild, faintly briny flavour. Its real contribution is what it does to the liquid around it: it draws out savoury depth without adding a strong ‘seaweed’ taste. Dashi made from kombu tastes clean, savoury and subtly sweet, depending on the variety used.

How do I make kombu dashi?

Place roughly 10g of kombu per litre of cold water in a pot or container. For a cold soak, leave it in the fridge for 30 minutes to overnight. For a heat method, warm the water slowly and remove the kombu just before it boils. Either method produces a clear, umami-rich stock. For a stronger dashi, add katsuobushi (bonito flakes) or dried shiitake after removing the kombu.

What’s the difference between a cold soak and the heat method?

Cold soaking gives a cleaner, more delicate dashi and is very forgiving. The heat method is faster, but you need to warm slowly and remove the kombu before the water boils to avoid bitterness or a slimy texture.

How much kombu should I use per litre of water?

A good starting point is about 10g per litre. If you want a lighter stock, use a smaller piece. If you want it stronger, increase the kombu slightly or steep for longer rather than boiling.

Can I reuse kombu after making dashi?

Yes. Once you’ve lifted the kombu out, it still has flavour and texture. Slice it into thin strips and simmer with soy sauce and mirin for an easy tsukudani-style condiment or add it to simmered dishes.

Can I make plant-based dashi with kombu?

Absolutely. Kombu alone makes a light, savoury stock. For a deeper plant-based version, steep kombu with dried shiitake. The two work well together and give a fuller broth without using fish.

Can I cook rice with kombu, and will it taste like seaweed?

Yes. Add a small strip to the pot as the rice cooks, then remove it before serving. It won’t make the rice taste “seaweedy”. It simply adds a gentle savoury note that makes the rice taste fuller.

Is there a good kombu substitute?

The closest kombu substitute for dashi is kombu dashi powder, which dissolves in hot water. For umami from other sources, dried shiitake mushrooms, miso paste or soy sauce can partially fill the gap. Other seaweeds like wakame or nori won’t replicate kombu’s glutamate levels or its effect in stock.

Should I store dried kombu in the fridge?

It’s better kept in a cool, dry cupboard in an airtight container. Fridges add moisture, which shortens shelf life. If you see a white bloom during storage, that’s usually mannitol and it’s normal.

How long does dried kombu last?

Stored in an airtight container in a cool, dry cupboard, dried kombu keeps for months or even years. It can improve with age, becoming more concentrated in flavour. Avoid storing it in the fridge, as moisture will reduce its shelf life.