日本語版

Research note 018 — 2026-08-31 (expanded 2026-09-08)

How Are Bitter Leaves Brought to the Table?

「苦い葉を食べる」 (in translation: "eating bitter leaves") is a single line to write. But the real path to the table is long: identifying the plant, choosing the part eaten, discarding the boiling water, and pairing it with another taste. Rather than just listing more leaf names, this note follows plant species, edible part, prep, and dish as separate things.

What this page establishes

• Sadao (young neem shoots and flowers) is recorded across multiple Thai sources, each with a different prep procedure — blanching and discarding the water, or heating in rice-rinse water, among others.
• A cooking column from Suan Dusit University shows, in a single source, that boiling sadao in hot water for about 3-5 minutes can adjust the strength of its bitterness.
• Which plant in Vietnam the dish name 「ラウダン」 (rawdan) refers to has not yet been confirmed from this source alone.
• No source has yet been found that measures how much bitterness utazi leaves carry in cooking.

Sadao Meets a Sweet Fish-Sauce Dip

Thai government sources record examples of sadao's young shoots and young flower clusters being blanched, simmered in rice-rinse water, or wilted over heat to adjust bitterness, then eaten with sweet fish-sauce dip or grilled fish. This is not a 「甘味が苦味を何％下げる」 (in translation: "experiment measuring by what percent sweetness lowers bitterness") test. It is a cultural record of turning a bitter plant into a dish through prep and pairing.

A cooking column from Suan Dusit University's Faculty of Home Economics explains the same sadao through a procedure of boiling it in hot water for about 3-5 minutes, adjusting the boiling time depending on whether more or less bitterness is wanted, then transferring it to cold water to set the color and keep the texture. The adjustment of boiling time is written up as a step tied to how bitterness is felt, and the cold-water step as a separate step for keeping color and texture. Set alongside the government sources' steps of simmering in rice-rinse water or wilting over heat, the underlying shape — passing the leaves through hot water before moving to the finishing step — is shared, but the liquid used partway through and the time specified differ from source to source.

Khilek Keeps Its Bitterness Even After the Water Is Discarded

Mahidol University's Institute of Nutrition describes gaeng khilek as boiling the leaves and flowers, discarding the water, boiling again until soft, and then adding them to a coconut-milk curry. The same page explains that coconut milk cuts the bitterness, but this is not a trained-panel comparison. Even so, a culinary logic can be read here: not eliminating the bitterness to zero, but moving it, through the two stages of boiling-and-discarding and fat, into an edible profile.

「ラウダン」 (rawdan) Is Not Yet One Plant Name

An ethnobotanical survey in northeastern Thailand records Glinus oppositifolius as a wild plant that adds bitterness to dishes. However, the culinary plant or soup called 「ラウダン」 (rawdan) in Vietnam does not always point to the same species. Rather than forcing the candidate name and the confirmed plant identification into one, the unconnected part is kept open here.

An item page from a Thai vegetable database published by a Japanese national research and development agency states plainly, for this plant, only that the young stems and leaves have a bitter taste. Rather than the whole plant, this narrows the edible part described to the young stems and leaves.

An interview survey in Pa Tio District, Yasothon Province, records the same species under the local name ผักแก่นขม (Phak Kaen Khom), and states that it is used as a wild edible plant for adding bitterness to dishes. This also preserves the difference between sources that record only the plant's identification and sources that go further, into the part used and the purpose of use.

Utazi Is Both a Vegetable and a Spice

Sources from southeastern Nigeria describe utazi as the leaf of Gongronema latifolium, used as a flavoring spice in fish, chicken, and goat-meat pepper soup, Nkwobi, and Abacha. But the same sources do not measure the bitterness intensity of utazi alone. So the culinary use and the plant identification are confirmed here, while the earlier, broader candidate claim of 「苦味食材」 (in translation: "bitter ingredient") is kept unverified.

There is no single path of prep. Even where the underlying shape — passing the leaves through hot water before moving to the finishing step — is shared across sources, the liquid used partway through, the time, and the part specified differ from source to source in how they are written. No ranking of which prep is more elaborate or more refined can be drawn, either between the two Thai sources or against the Nigerian source. The further one traces the path bitter leaves take to reach the table, the more it looks like a branching road rather than a single line.

What is not known

• Whether the culinary plant called 「ラウダン」 (rawdan) always points to the same species (Glinus oppositifolius) has not yet been confirmed.
• No quantitative measurement comparing how much the blanching time or the choice of liquid changes the strength of bitterness in sadao's prep has been found in the sources.
• The explanation that coconut milk cuts the bitterness in khilek curry has not yet been confirmed by any source through a trained-panel comparison.
• No source has been found that measures the bitterness strength of utazi alone, or the amount used in a dish.

Related: The Names of Wild-Greens Dishes Do Not Point to One Plant / Does any seasoning remove bitterness? — Sugar and salt / Hypothesis intake box