Edible seaweed: what it is, types and uses of the vegetable of the sea

What seaweed is: biology, classification and pigments

In the kitchen, edible seaweed is best understood as the vegetable of the sea: organisms that photosynthesize and fix carbon like land plants, but with a very different anatomy. Mastering this biological foundation is the first step towards cooking them with sound judgment, and it is a starting point in CIB’s Specialist Program in Advanced Culinary Techniques.

Photosynthesis without roots

What looks like a root is a holdfast (or rhizoid) that merely anchors the seaweed to the rock. Water and dissolved minerals are absorbed across the entire frond. That is why the origin of the water matters, and why all seaweed must be washed before use to remove salt and sand.

Macroalgae and phytoplankton

An estimated 72,500 species of algae exist, but only around 10,000 are multicellular macroalgae, visible and harvestable; about 250 of those are edible. The rest is single-celled phytoplankton, which produces roughly half of the planet’s oxygen. In the kitchen it is used as a concentrate: plankton paste, diluted, gives a base reminiscent of prawn and reinforces a rice dish or soup without using fish.

Pigments, depth and culinary behavior

Colour depends on the light that reaches each depth, and with it the composition and behavior in the kitchen change.

The chlorophyll in codium, for example, is very stable: although it looks brown raw, a brief blanching reveals an intense green. Nori, moreover, is a red seaweed, not a green one.

History of seaweed consumption and phycocolloids

Eating seaweed is an ancient practice. Remains found in Chile date back around 14,000 years, and around 2700 BC a Chinese medical treatise already described several species. In China and Japan they became part of the everyday diet, while along the Atlantic coast they were used as food, fertilizer and remedy, even in medieval monasteries. Their vitamin C content may have helped prevent scurvy among sailors.

Phycocolloids: the invisible texture

Phycocolloids are polysaccharides found in the cell walls of seaweed, capable of absorbing water and forming gels or thickening. They have become the basis of textures the food industry uses every day:

• Agar-agar (E-406): not a seaweed itself, but an extract of red seaweeds such as gracilaria. It sets from a hot liquid and withstands heat.
• Carrageenans (E-407): derived from red seaweeds such as Irish moss. They give body to dairy products, panna cotta-style desserts and ice creams.
• Alginates (E-400 to E-405): extracted from brown seaweeds, they are the basis of spherification and of many sauces and fillings.

This family of thickeners is also useful in pastry, a field covered in the Advanced Pastry Course. In industry they appear in ice cream, chocolate milk drinks, beer foam, cosmetics and toothpaste.

Why natural nori looks nothing like the supermarket kind

Industrial nori is a cultivated red seaweed, ground into a paste and dried on mats, hence the grid pattern on the sheet. Toasted and pressed, its flavor and texture have little in common with fresh, wild seaweed, which is far more vibrant, marine and tender.

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Galician Atlantic seaweed: tender and tough textures in the kitchen

Atlantic seaweed from Galicia offers one of the most varied marine pantries in Europe: Galician kombu, Atlantic wakame, sea spaghetti, sea lettuce, dulse, ogonori (gracilaria), codium and wild nori. Harvesting and cultivation are regulated in Galicia through permits and management plans, ensuring sustainable and traceable use.

The guiding principle: texture determines technique

Tender seaweeds call for brief heat, raw use or infusion; tough ones need hydration, long cooking or preservation.

Cross-cutting techniques

• Fat infusion: milk or butter captures the seaweed’s aromas; without blending, you obtain flavor without color.
• Vacuum pickling: reserved for tough or very salty seaweeds.
• Curing with kombu or wakame: seasons fish and marks it with umami.
• Crisp frying of fine seaweeds, which need a contained shape so they do not spread open.

The umami of kombu is the origin of dashi and led the Japanese chemist Kikunae Ikeda to identify glutamate in 1908. These techniques are explored in greater depth in the Gran Chef Diploma.

Iodine and food safety: how to use seaweed responsibly

Iodine in seaweed is the main safety variable when cooking with it. It is treated as a professional food-safety criterion, not as medical advice, and requires knowing the species, origin and dosage.

Variability between species

Brown seaweeds, especially kombu, concentrate very high amounts, which can reach several thousand micrograms per gram of dried product depending on species and origin. Nori and dulse contain considerably less, and wakame sits at an intermediate level. Chlorella, a freshwater microalga, provides practically no iodine.

References vary by authority, so it is advisable to always consult current regulations.

Good kitchen practice

• Dose kombu precisely, never adding extra “just in case”.
• Soaking and blanching removes part of the iodine, which passes into the liquid.
• Combine kombu with low-iodine seaweeds in blended dashis.
• Declare the use of seaweed on the menu, especially for guests with thyroid conditions.
• Avoid hijiki: AESAN (the Spanish food safety agency) recommends not consuming it because of its inorganic arsenic content.

This rigor in both product and service is part of the training in the Haute Cuisine Chef Diploma.

Halophyte plants: the coast on the plate

Halophyte plants (“salt-lovers”) are not seaweeds but land plants that tolerate saline soils. They grow in salt marshes, dunes and cliffs, without being submerged. They tolerate salt by accumulating it in vacuoles or excreting it through leaf glands, which is why many have fleshy, juicy leaves with a naturally salty flavor.

Common culinary species

• Samphire (salicornia): crisp, salty and fresh; excellent raw or blanched for an instant.
• Ice plant: leaves covered in vesicles that look like water droplets.
• Sea beet and sea spinach: more intense and salty than their land counterparts.
• Sea fennel: aromatic, anise-like and mineral.
• Sea purslane: fleshy and refreshing.

There are also acidic companions, such as oxalis (reminiscent of green apple) or sorrel (as sour as a lemon and best left unsautéed), useful for cleansing the palate.

Balancing marine flavor

Seaweeds contribute umami, iodine and depth; halophytes bring crunch, freshness and acidity. Combined, they build a coastal landscape narrative without resorting to more salt. They are used raw as a topping or briefly sautéed, and are best tasted from the mildest to the most intense. Add to this a sustainability argument: many are grown with saline water and compete neither for fresh water nor for fertile soil.

Frequently asked questions

Which seaweeds can be eaten?

Around 250 species of macroalgae are consumed. The most common in Atlantic cuisine are kombu, wakame, nori, dulse, sea lettuce, sea spaghetti, codium and ogonori. Each has a different texture, flavor and iodine content.

Are seaweeds plants, and do they have roots?

They are not plants in the strict sense, although they photosynthesize and are used in the kitchen as a vegetable of the sea. They have no true roots: the holdfast only anchors them to the rock, and they absorb minerals through the frond.

Can phytoplankton be cooked?

Yes, as a concentrated paste or powder. Diluted in water, it gives a marine flavor similar to prawn and is added at the end to rice dishes, soups or creams to reinforce the taste of the sea without fish.

Why does fresh nori differ from dried nori?

Industrial nori is ground, pressed into sheets, then dried and toasted, which changes its texture and aroma. Fresh seaweed is more tender and marine. It is a red seaweed, not a green one.

How do you choose a seaweed according to its texture?

Tender ones (sea lettuce, codium, dulse) are used raw, in infusion or with brief heat. Tough ones (kombu, salted seaweeds) require hydration, long cooking, vacuum pickling or infusion. These criteria are worked on in the Specialist Program in Advanced Culinary Techniques.

Can you eat seaweed every day?

It depends on the species and the amount. With kombu and other brown seaweeds, daily consumption can easily exceed EFSA’s tolerable upper intake level for iodine (600 µg per day). People with thyroid conditions should consult a healthcare professional.