Kelp Pesto: A Coastal Twist on a Classic

Pesto is a technique disguised as a recipe: something green, something nutty, hard cheese, garlic, oil, crushed into a sauce. Once you see it that way, the Ligurians' basil is one answer among many — and kelp is a remarkably good one. Blanched kelp blends into a pesto that is silky, deeply savoury, faintly oceanic and a striking jade green. It keeps beautifully, freezes in ice-cube trays, and will change how you think about seaweed as an ingredient rather than a garnish.

This article gives you the master recipe, the why behind each step, and the variations worth knowing. If you are new to cooking with kelp generally, start with the edible seaweeds field guide.

Which kelp to use

Sugar kelp (Saccharina latissima) is the best choice: a thin-bladed, mildly sweet kelp farmed on longlines in Maine, Alaska, Scandinavia and Britain, and increasingly sold fresh-frozen and blanched through seafood markets and direct from farms. Its mildness is the point — you want green vegetable character, not iodine punch. Young kombu works, blanched an extra minute. Wakame works and makes a softer, silkier pesto. Avoid thick, mature kelp ribs; they stay fibrous no matter how long you blend.

You have three ingredient routes, all covered in the recipe:

• Fresh or frozen blanched kelp — the finest result. Many sea farms, including programs like Tidewright Harvest's, sell kelp already blanched and frozen, which is exactly what pesto wants.
• Fresh raw kelp — blanch it yourself first, following the method below.
• Dried kelp — completely workable: soak, then blanch, and proceed. The pesto will be slightly darker and more robust.

Master recipe: kelp pesto

Makes about 250 ml — enough for 500 g of pasta or a week of dolloping.

• 150 g blanched kelp (from about 200 g fresh), squeezed very dry and chopped
• 50 g toasted nuts — almonds are best here; walnuts and pine nuts both work
• 40 g finely grated hard aged cheese (or 2 tbsp nutritional yeast for dairy-free)
• 1 small garlic clove
• Zest and juice of half a lemon
• 120 ml good olive oil, plus more to cover
• Sea salt and black pepper

Step 1: Blanch the kelp (skip if already blanched)

Bring a large pot of unsalted water to a rolling boil. Drop in the raw kelp. Within 30–60 seconds it will shift from translucent brown to an astonishing, vivid emerald — that colour change is your signal. At one minute for thin blades (two for thicker kombu), lift it straight into ice water. The cold shock locks the colour and stops the slip from turning slimy.

Drain and then squeeze ruthlessly. Wet kelp makes a watery, separating pesto; this single step decides the texture. Chop roughly.

Step 2: Toast the nuts

Five minutes in a 180°C oven or a dry pan until fragrant and a shade darker. Raw nuts make a flat pesto; toasted nuts give the roundness that balances kelp's minerality.

Step 3: Blend

Food processor or mortar. Pulse the nuts, garlic and cheese first to a coarse crumb — this prevents garlic from going harsh and pasty. Add the kelp, lemon zest and half the juice, and pulse to a rough green paste. Only then stream in the oil with the motor running, stopping while the sauce still has texture. Pesto should be a sauce with body, not a purée.

Step 4: Season and rest

Salt carefully — the cheese and kelp are already briny-adjacent — then add black pepper and more lemon juice to sharpen. Rest fifteen minutes at room temperature: pestos noticeably settle and meld.

Why blanching matters (the science in one paragraph)

Raw kelp blades owe their brown colour to fucoxanthin, a pigment that masks green chlorophyll. Brief boiling denatures the pigment complexes and deactivates the enzymes that would otherwise dull and degrade the colour in the fridge — the same reason all green vegetables are blanched before freezing. Blanched kelp pesto holds its green in a sealed, oil-topped jar for up to a week refrigerated; raw-blended kelp pesto is khaki by the next morning.

Variations worth making

• Kelp-basil pesto: half blanched kelp, half basil. The best introduction for committed traditionalists — the basil perfumes, the kelp deepens.
• Kelp and walnut, no cheese, extra lemon: a sharp vegan version that is magnificent on roasted carrots.
• Kelp-dulse pesto: add a tablespoon of dulse flakes at the end for a smoky, cured note.
• Spicy kelp pesto: a fresh green chilli and a teaspoon of white miso in place of some cheese. Excellent on noodles.

How to use it

• Pasta: loosen a few spoonfuls with starchy pasta water; finish with lemon zest. It needs no other sauce.
• On fish and shellfish: a spoonful beside grilled white fish, or folded through steamed mussels at the end.
• As a spread: on toast under tomatoes; swirled into seaweed butter's opposite number — soft ricotta.
• In grains: stirred into warm farro or barley with roasted vegetables and white beans.
• Frozen: portion into an ice-cube tray, top each cell with oil, freeze, and bag. Six months of weeknight pesto, one cube at a time.

Storing and keeping it green

Press cling film directly onto the surface, or pour a 5 mm cap of olive oil over the top, and refrigerate. Oil-topped and sealed, it keeps a week; the oil cap is also the secret to freezing it without oxidation. Always spoon it out with a clean, dry spoon.

Key takeaways

• Pesto is a technique; blanched kelp is a superb green base — mild, silky, deeply savoury.
• Sugar kelp is the best species; blanch 1–2 minutes until emerald, shock in ice water, and squeeze very dry.
• Almonds beat pine nuts here; lemon juice keeps the sauce bright and green.
• Oil-capped in the fridge it lasts a week; frozen in cubes, six months.
• Start kelp-skeptics with a half-kelp, half-basil version.

FAQ

Can I make kelp pesto from dried kelp?

Yes. Soak dried kelp for 20–30 minutes until supple, then blanch for 1–2 minutes as usual. The resulting pesto is slightly darker and more intensely marine than fresh-kelp pesto — excellent with bolder partners like walnuts, miso or roasted garlic.

Why did my kelp pesto turn brown?

Three likely culprits: the kelp wasn't blanched (raw-blended pesto oxidises fast), the blanch wasn't followed by an ice-water shock, or the finished pesto was stored without an oil seal. The fix for a faded batch: stir in a handful of fresh blanched kelp and extra lemon, then eat it sooner.

Does kelp pesto taste fishy?

No — it tastes green, mineral and faintly oceanic, closer to marinated artichoke hearts or grilled greens than to seafood. Fresh, correctly blanched kelp is mild; if any batch tastes strongly marine, the kelp was likely old or over-boiled.

Is kelp pesto safe to eat often?

In normal amounts, yes — sugar kelp is moderate in iodine compared with kombu, and a serving of pesto contains only a few grams of kelp solids. As with all seaweeds, treat it as a regular vegetable rather than a supplement; our seaweed nutrition guide covers iodine specifics.

Where can I buy kelp for cooking?

Look for frozen blanched kelp from regional seaweed farms at seafood markets and specialty grocers, or dried kelp from any Asian grocer (kombu or dashima — both work, blanched a little longer). Farm transparency matters: buy from producers who share harvest location and testing.