Gamtae Green Laver

Ulva prolifera

Chlorophyll and dietary fibre · a green alga gathered from winter mudflats

Gamtae is a green alga of the sea lettuce group, gathered from the mudflats of the Korean west coast in winter. It grows as tangled threads raked up by hand, and because it thrives only where fresh water meets the flats, both its range and its season are narrow. Pressed onto frames and dried like laver it becomes gamtae-gim; fresh, it is dressed with vinegar. A faint bitterness on chewing is its signature, and it runs finer and softer than ordinary sea lettuce. One thing confuses buyers — the gamtae that appears on supplement and cosmetic labels is Ecklonia cava, a brown alga of the seas off Jeju, an entirely different plant from the green alga covered here. The Donguibogam also lists it in the Vegetable section as 甘苔, records the alternative name 靑苔, and says it grows in the sea and is spread and dried like dried meat before eating. The season runs December to March, when both fresh gamtae and dried sheets appear together in west coast markets.

Year-Round Calendar

Harvest

Where It Grows

Main producer Major producer Wild population Latitudes at 5–15℃ Zoom and pan to explore · temperatures are rough zonal averages

Main producer Effectively the only place edible gamtae is produced. It is gathered in winter from the west coast tidal flats of Seocheon and Taean in South Chungcheong and of Muan, Sinan and Haenam in South Jeolla. Harvest December, January, February, March
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2. 2
3. 3
12. 12
Major producer At the mouth of the Shimanto River in Shikoku and elsewhere, it is gathered and dried under the name sujiaonori. Harvest December, January, February, March
1. 1
2. 2
3. 3
12. 12
Wild population It is widely distributed along the coasts of the Yellow Sea and the East China Sea. There, however, it is treated as a species that spreads in bulk in summer and causes green tides, and it is not counted as a food-producing area.

It grows attached to brackish water along the East Asian coast — that is, on the estuary tidal flats where fresh water and seawater meet. The same species holds a different place from country to country: on Korea's west coast it is a seaweed gathered from the winter flats and eaten, while on China's Yellow Sea coast it is treated as the cause of large summer green tides. The gamtae that shares its name — Ecklonia cava, of the genus Ecklonia — is a brown alga and an entirely different seaweed.

Optimum water temp.: 5~15℃
Range: 0~25℃
Hardiness: Not applicable
Day length: Not applicable

What sets its growing season is not the air temperature but the water temperature and the tides. From late autumn, once the water falls below 15℃, it grows attached to the floor and the stones of the tidal flat, lies thickest in midwinter, and then breaks down and vanishes as the water warms in spring. It favours estuary flats where fresh water mixes in and the salinity drops.

In Korea Coastal areas

Confined to the tidal flats of the west coast. Seocheon and Taean in South Chungcheong and Muan, Sinan and Haenam in South Jeolla are the known sites, and it grows well only on the estuary-side flats where fresh water moves in and out.

How to grow

Light: Not a crop anyone grows. It is gathered from winter mudflats, and the tide and the water temperature rather than the sun fix the season
Soil and depth: Nothing is planted in soil. It grows attached to the bed and the stones of flats where fresh water meets the sea
Spacing: Do not strip one patch. Each flat has its own permitted season and area, so follow the local rules
Watering: Preparation is where water matters. Fill a large bowl, swirl it, pour off the settled grit and change the water several times
Feeding: No feeding. It lives on what is dissolved in the flats and the seawater
Harvest: Fresh gamtae runs from December to March. Choose deep green material with fine, lively fronds and clumps that are not sodden. For dried sheets, choose ones pressed evenly thin with few holes

What makes the difference. Wash it the day it is bought, squeeze it dry and freeze it in single-use portions. Keep dried sheets sealed somewhere cool and dark

Why this one is harder. The season is three winter months and the growing area is narrow. Out of season only dried sheets are available, so if you want it fresh, look through west coast markets between December and March

Common problems

Sand and mud You will see Grit crunches between the teeth What to do Swirl it through several changes of water, repeating until the last water runs clear
Faded colour You will see The green has gone yellow and there is no aroma What to do It is old or has been heated. Choose fresh material that is deep green with the fronds still thread-fine and lively
Limp dried sheets You will see The dried sheets sag and stick together What to do They have taken up moisture. Store sealed with a desiccant, and discard anything whose smell has changed

This page describes how a crop is grown and used. It does not claim health effects for any food, nor is it intended to prevent or treat disease. Component and study notes are references only and do not mean the same result occurs in people. Consult a medical professional about health concerns.

Health Benefits

The old record — gamtae (甘苔). The Donguibogam lists it in the Vegetable section as 甘苔, with the vernacular name recorded in the same form still used today. It gives a cold nature and a salty taste. This entry carries no statement that the plant is free of toxicity. It records the alternative name 靑苔, and says the plant grows in the sea and can be spread and dried like dried meat before eating. This is a historical medical record and differs from modern medical judgment.

An overlapping name. The name gamtae is used today both for the green alga of the west coast flats, Ulva prolifera, and for the brown alga of the Jeju seas, Ecklonia cava. That one name covers two unrelated species is itself the reason to read the botanical name when buying seaweed or looking up a source.

Drying as a method. The line in the Donguibogam about spreading and drying it like dried meat connects to the way gamtae sheets are made now. It shows that pressing the gathered alga thin on a frame and drying it is an old handling rather than a recent one.

Nutrition

Chlorophyll (the deep green pigment of the alga) — the pigment that colours the fronds green
Dietary fibre (the polysaccharides that build the frond) — a mix of soluble and insoluble fibre
Minerals (potassium, calcium and others) (concentrated from seawater) — minerals common in seaweeds

The bitterness is the signature Chewed, gamtae carries a faint bitterness. Ordinary sea lettuce has almost none of it and its blades are broader and coarser. If dried sheets of the two are hard to tell apart on a shelf, the thread fineness and that aftertaste settle it.

How thoroughly to wash it Gathered off the flats, it comes with sand and mud in it. Fill a large bowl, swirl it through several changes of water and pour off the settled grit each time. Working it hard breaks the thread-fine fronds and turns them slack, so swirl rather than scrub.

Colour and chlorophyll The deep green comes from chlorophyll. Kept too long or hit with strong heat, the colour fades yellow and the aroma goes with it. Dressed dishes use no heat, and dried sheets keep their colour and scent better torn straight from the pack than toasted.

Fibre and minerals Being a seaweed it carries dietary fibre and minerals. It is a side dish eaten in small quantity, though, so the per-100g figure in a composition table does not translate into one meal. This entry stays with the compositional record.

Pairings

○ Vinegar and sugar — dressed fresh — Wash the fresh alga, squeeze it dry and dress it with vinegar, a little sugar and soy sauce. No heat is used, so the colour survives intact. Eat it the day it is dressed.

○ Sesame oil and soy sauce — dried sheets — Brush dried sheets thinly with sesame oil and serve soy sauce alongside. The aroma holds without toasting, and wrapped around rice the bitterness comes through as it is. If you do toast them, let the heat barely graze the sheet.

○ Oysters and radish — A pairing of things in season at the same time. Salt shredded radish, squeeze it out, and dress it with oysters and gamtae together. Rinse the oysters under running water and add them last so they stay whole. Korean Radish (Mu)

△ A different seaweed under the same name — The name gamtae also attaches to Ecklonia cava, a brown alga from the seas off Jeju. The one appearing on supplement and cosmetic labels is usually that brown alga, while this entry covers the green alga of the west coast flats. The two differ in form and in where they grow, so check the botanical name, the origin or the form when buying.

△ Grit left in — Gathered off the flats, it holds mud and sand. Swirl it through several changes of water, repeating until the last water runs clear. Grit that survives ruins the entire dish the moment it is bitten.

△ Keeping it too long — Fresh gamtae goes slack and loses colour quickly even refrigerated. Wash it the day it is bought, squeeze it dry and freeze it in single-use portions. Dried sheets go limp once they take up moisture, so keep them sealed somewhere cool and dark.

Varieties

Gamtae (Ulva prolifera) (seaweed) — Fine as thread and soft. The faint bitterness is its signature
Maesaengi (seaweed) — Finer still than gamtae and slippery. Mostly simmered as a soup
Sea lettuce (parae) (seaweed) — Broader-bladed and coarser than gamtae. Mixed into laver sheets or dressed as a side

Source: Donguibogam, Vegetable Section 甘苔 · National Institute of Fisheries Science