Topic hub — 2026-09-19
Bitterness in offal
Read through the preparation steps for offal dishes and one operation keeps recurring: remove the bile, the gall bladder, the bile ducts, the veins. In France, in Germany and in Italy alike, what gets named as the source of bitterness is not the organ but the liquid attached to it.
Yet in the sources from northwestern Vietnam, from Guizhou and from northern Thailand, that same liquid is added as an ingredient. The recipe for sa states plainly: if you like it bitter, add nam di — bile. This page sets the removing and the adding at the same height, and keeps alongside them the records where measurement found no difference at all.
On this page (8 sections)
What this page establishes
- Bitterness in offal is usually attributed to the bile rather than the organ. A wholesaler's comment that the faint bitterness of saury guts comes from the bile is one example.
- The removing operation is shared across regions: the veins of foie gras, the bile ducts of liverwurst, the gall bladder in Tuscan liver paste, all taken out in preparation.
- Some dishes add the same thing. The recipe for sa says to add bile if you like it bitter, niubie adds cattle bile, and nam pia uses the digestive fluid taken from the junction of the small and large intestine.
- The process can make bitterness too. In canned and jarred liverwurst, bitterness is attributed to the Maillard reaction during high-temperature sterilisation.
- Some measurements found nothing. Comparing ankimo with tiger puffer liver, a sensory test with 8 panellists found no significant difference on any attribute, including appearance, odour, oiliness and taste.
Taking the bile out
First, where the bitterness is attributed. A wholesale-market executive is recorded as commenting that the faint bitterness of saury guts comes from the bile. What is named is the liquid, not the organ.
Scope: A comment from a wholesale-market source interviewed in a Weathernews article (September 24, 2021)
Note: A comment from a single interviewee (a seafood wholesale industry source); no chemical analysis or other corroboration is presented.
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ingredient-sanma-harawata —has_perceived_bitterness_associated_with→ "Takahisa Utsumi, division head at Yamasue Co., a seafood wholesale intermediary at the Funabashi Municipal Wholesale Market, comments that the mild bitterness of sanma (Pacific saury) guts comes from bile."In foie gras, what is removed are the veins. Residues in the veins of raw foie gras are linked to bitterness, and are carefully removed in the déveinage before cooking.
Scope: Home-cooking guides from 2 French foie gras producers
Note: Debates over animal welfare and production regulation are not addressed here; only descriptions concerning bitterness are taken up. 2 independent producers agree in attributing the cause of bitterness not to the gallbladder itself but to the veins (veines).
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ingredient-foie-gras —has_perceived_bitterness_associated_with→ "Components remaining in the veins (veines) of raw foie gras. Carefully removed in the pre-cooking preparation step (déveinage) before heating or cooking."Liverwurst has the same step. The preparation describes roughly removing the bile and bile ducts, along with adhering visceral fat, from the raw liver.
Scope: The production section of the German Wikipedia article "Leberwurst".
Note: There is no footnote.
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ingredient-liverwurst —is_prepared_by→ "A preliminary preparation that roughly removes bile, bile ducts (Galle, Gallengänge), and attached visceral fat from the source liver."In Tuscany the operation is written in finer detail: the gall bladder of the chicken liver is removed inside a vessel of water, taking care not to burst it. That phrase — not to burst it — is the heart of the step.
Scope: The recipe for crostini di fegatini di pollo in the 1895 edition of Artusi's 『料理の科学と美味しく食べる技法』 (in translation: "Science in the Kitchen and the Art of Eating Well").
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dish-crostini-di-fegatini-toscani —is_prepared_by→ "Carefully removing the chicken-liver gallbladder (vescichetta del fiele) in a basin of water without rupturing it."Modern accounts repeat the same caution, with removing the gall bladder without bursting it written as the means of holding the bitterness down.
Scope: A home-cooking blog article about crostini di fegatini.
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dish-crostini-di-fegatini-toscani —has_perceived_bitterness_suppressed_by→ "Carefully removing the gallbladder (sacca del fiele) without rupturing it."Removing all of it is not the aim, though. The sensory description of the same dish is a persistent aroma with a slight bitterness (amarognolo). The bitterness released by bursting is avoided; the bitterness that stays is recorded as a characteristic.
Scope: An article on the flavor of chicken-liver pâté (crostini di fegatini toscani) paired with Tuscan Chianti.
Note: This is the food-article writer's sensory description, not a measurement from a sensory panel.
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dish-crostini-di-fegatini-toscani —has_sensory_attribute→ "A persistent, slightly bitter (amarognolo) aroma."Putting the same thing in
Here the direction reverses. Nam pia, from northwestern Vietnam, simmers the organs of ruminants — buffalo, cattle, goat — together with pịa, the digestive fluid taken from the junction of the small and large intestine, and with bone broth.
Scope: The Tai (Thái) people of northwestern Vietnam (Sơn La province, Mộc Châu, and elsewhere)
Note: The anatomical definition (the junction of the small and large intestine) follows a journal article (Vương Xuân Tình, 2025). The ingredient makeup also agrees across state-media outlet articles.
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dish-nam-pia —is_defined_as→ "A dish in which the internal organs (heart, liver, lungs, stomach, intestines, and so on) of ruminants such as water buffalo, cattle, or goats are simmered together with a digestive fluid (pịa) drawn from the junction of the small and large intestine, and bone broth. It is scented with mắc khén seeds (hạt mắc khén), lemongrass, chili, and cilantro."Its bitterness is recorded not as something avoided but as the property that defines the dish.
Scope: As above.
Note: Sources share a common description in which bitterness gives way to a fatty sweetness (“vị đắng ban đầu nhanh chóng chuyển sang vị béo ngậy, ngọt bùi đặc trưng”, in translation: "the initial bitterness quickly turns into a distinctive rich, fatty sweetness"). An academic journal article likewise states “có vị đắng” (in translation: "has a bitter taste").
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dish-nam-pia —has_defining_sensory_theme→ sensation-bitterNiubie from Guizhou has the same structure. Cattle are fed medicinal plants and forage shortly before slaughter; the undigested plant matter is taken from the stomach and small intestine, the liquid pressed out, and cattle bile, Sichuan pepper, ginger, dried tangerine peel and herbs are added before reducing it over low heat.
Scope: The Qiandongnan Miao and Dong Autonomous Prefecture in Guizhou Province (counties including Liping, Congjiang, Rongjiang, and Jinping) and parts of the Guangxi Zhuang Autonomous Region (including Sanjiang and Rongshui)
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dish-niubie —is_defined_as→ "Made by feeding cattle (sheep in some areas) medicinal herbs and forage just before slaughter, taking the not-yet-fully-digested plant residue (forage material) from the stomach and small intestine, pressing out the liquid, and simmering it on low heat with added cattle bile, Sichuan pepper, ginger, dried tangerine peel, and herbs. Also called "baicao tang" (hundred-herb soup)."The timing is specified. About half an hour before slaughter the animal is fed young shoots and vines of kudzu, mung bean, codonopsis, fleeceflower and angelica; after slaughter the contents are taken out, boiled with water for about 30 minutes, cooled, and the liquid pressed out. This is not a process for removing; it is a process for extracting and reducing.
Scope: Dong and Miao residential areas of Qiandongnan, Guizhou.
Note: The source does not establish differences in quantities or medicinal-plant species across households and regions. Descriptions about hygiene and safety are excluded under this rule.
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dish-niubie —is_prepared_by→ "About half an hour before slaughter, cattle are fed young shoots and vine leaves of medicinal plants including kudzu root, mung bean, dangshen, shouwu, and danggui. After slaughter, undigested contents are removed from the stomach and small intestine, boiled with water in a large pot for about 30 minutes, and cooled before the liquid is squeezed out. Cattle bile, Sichuan pepper, ginger, dried tangerine peel, and aromatic herbs are added to the liquid, which is then reduced further over low heat."This dish too carries bitterness as a defining property.
Scope: As above.
Note: Descriptions of the bitterness differ by source: one describes a shift from bitterness to sweetness, “刚入口时虽苦，但紧随而来的是爽口的甘甜” (in translation: "bitter at first, but immediately followed by a refreshing sweetness"), and another describes a bitter-pungent-fragrant sequence, “先苦，再辣，然后是牛肉的香” (in translation: "bitter first, then pungent, then the aroma of beef"). The intensity or mechanism of the bitterness is not generalized.
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dish-niubie —has_defining_sensory_theme→ sensation-bitterThe clearest case is sa, from northern Thailand. Its main ingredients are raw buffalo or beef and offal, the offal boiled before use.
Scope: A home-cooking recipe published on a regional introduction page for Ban Kwang, Sung Men District, Phrae, Thailand.
Note: The page itself cites a 2007 interview and a printed encyclopedia published in 1999 (edited by รัตนา พรหมพิชัย, 『สารานุกรมวัฒนธรรมไทย ภาคเหนือ』 (in translation: "Encyclopedia of Northern Thai Culture"), p.6806). The encyclopedia and original interview were not consulted for this entry.
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dish-sa —is_defined_as→ ""ส้าจิ๊น คือส้าเนื้อ ส่วนผสมหลักคือ เนื้อควายหรือเนื้อวัวสด และเครื่องใน สำหรับเครื่องใน นิยมนำมาต้มก่อนปรุง ใช้เครื่องปรุง เป็นพริกลาบ เช่นเดียวกับลาบชนิดต่างๆ" (in translation: "sa chin is sa neua, meaning meat sa. Its main ingredients are raw buffalo or beef and offal. The offal is commonly boiled before preparation, and phrik laab is used for seasoning as in other kinds of laab"). The specific ingredients are 200g of buffalo meat, 200g of offal, 3 tablespoons of phrik larb, 2 tablespoons of น้ำเพี้ยต้ม, salt, shallots, garlic, lemongrass, ผักไผ่ (a type of Vietnamese coriander), cilantro, and scallions."And the method reads: if there is no nam phia (digestive fluid), the broth from boiling the offal may be used instead; if you like it bitter, add nam di — bile. What Tuscany takes such care not to burst is here offered as an option to add.
Scope: As above: the recipe introduction page for Ban Kwang, Sung Men District, Phrae.
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dish-sa —is_prepared_by→ ""หากไม่มีน้ำเพี้ย ใช้น้ำต้มเครื่องในแทนได้ หากชอบรสขม ให้ใส่น้ำดีลงไป" (in translation: "If there is no nam phia, offal cooking liquid may be used instead; if a bitter taste is preferred, add nam di [bile]"). The method explicitly says 「ละลายน้ำเพี้ยและพริกลาบเข้าด้วยกัน ใส่เนื้อ」 (in translation: "dissolve nam phia and phrik laab together, then add the meat"), mixing nam phia with phrik larb before tossing in the raw meat."Bitterness made by the process
Bitterness in offal does not sit only on the ingredient side. In liverwurst the raw material is named first: using beef liver is linked to bitterness.
Scope: The ingredients section of the German-language Wikipedia article "Leberwurst"
Note: Note that this passage in the article carries no footnote and is presented as general culinary/food knowledge.
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ingredient-liverwurst —has_perceived_bitterness_associated_with→ "Using beef liver (Rindsleber) as a raw material."Beyond that, the processing itself produces it. During the high-temperature sterilisation of canned and jarred liverwurst, bitterness is attributed to the Maillard reaction between carbohydrates and free amino acids or the epsilon-amino groups of protein-bound lysine. The same reaction that appears on the scorched rice hub is written here as a maker of bitterness.
Scope: University of Hohenheim Food Materials Science project "Hocherhitzungsgeschmack bei Leberwurst" (period 1990-1997).
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ingredient-liverwurst —has_bitterness_generated_by→ "A Maillard reaction between free amino acids and the ε-amino groups of protein-bound lysine and carbohydrates such as glucose during canning or jarring (high-temperature sterilization)."Measured, and no difference
Even things named as bitter offal can show no difference when measured. Those records are kept rather than dropped.
For ankimo, the finding that no source explicitly describes bitterness is itself recorded as a claim. Three Japanese dictionaries and two regional-cuisine pages from the Ministry of Agriculture all stop at calling it prized, without naming bitterness.
Scope: The 3 dictionaries consulted this time and 2 Ministry of Agriculture, Forestry and Fisheries local-dish resources
Note: An honest record of a negative finding, based on NIGAMI LAB's collection discipline of not bending a source that only says "koku" (rich depth of flavor) into a claim of bitterness. A wider set of sources (academic journals, specialist cookbooks, and the like) might turn up a different description.
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ingredient-ankimo —has_insufficient_evidence_for→ "A statement explicitly naming bitterness or mild bitterness. The 3 Japanese-language dictionaries consulted this time (Digital Daijisen, Selected Edition of the Nihon Kokugo Daijiten, and the Dictionary of World Cuisine), as well as 2 ankimo dish entries in the Ministry of Agriculture, Forestry and Fisheries' "Uchi no Kyodo Ryori" for Ibaraki and Fukushima Prefectures, all stop at descriptions such as 「珍重」 (in translation: "prized") and do not explicitly state bitterness."Sensory testing found no difference either. Serving 15 g each of ankimo and tiger puffer liver heated for 7 minutes, 8 panellists rated on a 5-point scale the appearance, odour, oiliness and taste along with liking for appearance, aroma, taste, texture and overall, and found no significant difference between the samples on any attribute.
Scope: The comparison of commercial Chinese-produced ankimo and land-farmed tiger pufferfish liver in Mineki et al. 2009, with 8 female teaching staff
Note: 「味」 (in translation: "taste") is a broad evaluation item, and bitterness intensity was not measured independently. This is limited to an 8-person single-institution panel and a single commercial ankimo sample, and the absence of a significant difference is not proof of equivalence.
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ingredient-ankimo —has_sensory_response→ "In a 5-point sensory test in which 15 g each of ankimo and tiger pufferfish liver, heated for 7 minutes, were presented, 8 female teaching staff in their 20s to 50s evaluated appearance, odor, oiliness, and taste, along with preference for appearance, aroma, taste, texture, and overall impression, and there was no significant difference between the two samples for any item"Italy has a record of the same shape. For cibreo, the claim on file is that Artusi's cookbook makes no mention of bitterness at all.
Scope: The cibreo recipe text in Artusi's 『料理の科学と美味しく食べる技法』 (in translation: "Science in the Kitchen and the Art of Eating Well"), 1895 edition.
Note: While the recipe for the chicken-liver dish crostini di fegatini (dish-crostini-di-fegatini-toscani) in the same author's book records a gallbladder-removal step, the cibreo recipe uses chicken liver, combs, and undeveloped eggs without using a bitterness term even once. This is a negative observation about whether bitterness is mentioned; it does not establish that bitterness is not actually perceived.
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dish-cibreo —has_insufficient_evidence_for→ "Mention of bitterness (amaro/amarognolo)."Its characterisation in fact points the other way: a simple but delicate and refined stew, suited to women without appetite and to convalescents. An offal dish described through gentleness rather than strength.
Scope: An introductory sentence in Artusi's 1895 cookbook
Note: It is recorded as the cookbook's own discourse and does not claim actual health effects or digestive efficacy.
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dish-cibreo —is_presented_as→ "Artusi's cookbook characterizes this dish as 『単純だが繊細で上品な煮込みで、食欲のない婦人や病後の人に適したもの』 (in translation: "a simple but delicate and refined stew, suited to women without appetite and people recovering from illness")."The Arctic experiment needs the same reading. A fermented sauce made from ptarmigan intestine scored significantly higher than one made from the meat on only 2 of 33 attributes: black olive and game. Bitterness was not one of the two.
Scope: A CATA sensory evaluation by 12 trained cooks and researchers at the Basque Culinary Center. It assessed modern reconstructed garum, not traditional raw orunit.
Note: No significant differences were reported for the other 31 attributes, including bitterness. This was a single study with a small panel, and the results are not generalized to the sensory properties of traditional raw consumption.
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process-arctic-gastrophagy —has_sensory_attribute→ "An experimental fermented sauce (garum) made from intestines (orunit) was rated significantly higher than garum made from meat for the 2 attributes 『ブラックオリーブ』 (in translation: "black olives") and 『ジビエ風味』 (in translation: "game flavor") (these 2 attributes alone differed significantly among 33)."Keeping the gut as it is
There are also records where the gut is neither removed nor added but kept as it is. Woodcock is recorded as being treated more ceremoniously than other game, cooked whole without drawing, and the trail is prized as the finest part.
Scope: A description in an American cookbook and catering manual from the late 19th century.
Note: The original expression 『美食家の栄光、料理人の誇り』 (in translation: "the glory of the gourmet and the pride of the cook") praises the whole woodcock, not the trail (intestines) specifically. It is recorded as the cookbook’s own judgment.
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process-becasse-trail-eating —has_cultural_role_of→ "Woodcock is described as being treated more carefully than other game because it is cooked whole without removal of the intestines, while the trail (intestines) is prized as 『この上ない部位』 (in translation: "an unsurpassed part")."In the Arctic, eating ptarmigan intestine and its contents raw is a recorded practice, and the record adds that the intestine was once prized above the meat.
Scope: Inuit/Kalaallit food culture around Nuuk Fjord, Greenland. Positioned under the academic term gastrophagy (eating gut contents).
Note: The paper’s generalization 『非工業化食文化で一般的』 (in translation: "common in non-industrialized food cultures") is recorded as that paper’s own claim, without editorial generalization.
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process-arctic-gastrophagy —includes→ "The traditional practice of eating ptarmigan intestines and intestinal contents (orunit) raw. The intestines (orunit) were formerly described as more prized than the meat."For caribou, the fermented rumen contents, the boiled intestine and the caecum are eaten. In eating the stomach contents of a ruminant, this stands on the same side as nam pia and niubie.
Scope: Traditional Inuit land hunting around Kangerlussuaq, Greenland (food items listed alongside dried musk ox and caribou meat).
Note: Does not include a claim about sensory properties or nutritional effects. Limited to the food list in a microbiome analysis paper.
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process-arctic-gastrophagy —includes→ "Consumption of fermented caribou rumen contents, boiled intestines, and cecum."The scale of the fermentation is recorded too. Igunaq sews walrus meat and fat inside the animal's own hide and buries it in the ground or the permafrost for several months.
Scope: An Inuit traditional preserved food of the Canadian Arctic and Greenland
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dish-igunaq —is_prepared_by→ "Chiefly walrus meat and fat, along with seal, caribou, and similar ingredients, are sewn into the animal's own skin, buried in soil or permafrost for several months, and fermented."Kiviaq is larger still: 300 to 500 little auks packed whole into a sealskin with no gaps, sealed with seal fat, and fermented for 3 to 18 months under stones.
Scope: A winter preserved food of the Inughuit in northwestern Greenland
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dish-kiviaq —is_prepared_by→ "300–500 little auks (tuulligjuait) are packed whole and tightly into seal skin, sealed with seal fat, and fermented for 3–18 months under stones or similar locations."On the Japanese side, one entry carries a compositional analysis. In both the raw material and the dried product of kuchiko, free glutamate is markedly abundant in the extract, with alanine and glycine relatively high and a fair amount of AMP, and these are held to be the principal cause of its umami and sweetness. The explanation comes from the umami side, not the bitter one.
Scope: Laboratory analysis of raw and sun-dried gonads of Japanese sea cucumber (corresponding to Apostichopus japonicus).
Note: These are analyses of gonads (kuchiko or konoko), not measured values for konowata itself, which uses the intestines.
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ingredient-kuchiko —has_composition→ "Raw gonads (sample A) have more ash and less protein than the gonads of other seafood. Extracts of both the raw material (A) and dried konoko (B) contain markedly high free Glu, relatively high Ala and Gly, and considerable AMP; these are described as the main sources of umami and sweetness. Sun drying greatly changes flavor and texture, but few changes in analytical values other than moisture content have been reported."Finally, one record that is not a measurement but cannot be left out. Satō Haruo's "Song of the Pacific Saury" describes a girl offering the guts of the saury to a man who is not her father, and asks whether the saury is bitter or salty. The bitterness of the guts carries a relation between people inside the poem.
Scope: The full poem as held by Aozora Bunko.
Note: 「さんま苦いか塩っぱいか」 (in translation: "Is the saury bitter or salty?") is a self-mocking poetic question about the taste of saury eaten while hot tears fall; it does not directly describe the innards themselves as 「苦い」 (in translation: "bitter"). The poem does not establish why the girl gives away the innards. It is not used as direct evidence for bitterness of the innards.
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ingredient-sanma-harawata —is_documented_in→ "Haruo Satō's poem 「秋刀魚の歌」 (in translation: "Song of Saury"), written in October 1921, first published in the November issue of the third volume of 「人間」 (in translation: "Humanity"), and included in the collection 『我が一九二二年』 (in translation: "My Nineteen Twenty-Two") in 1923, depicts a girl giving saury innards to a man who is not her father in the dinner-table scene: 「愛うすき父を持ちし女の児は／小さき箸をあやつりなやみつつ／父ならぬ男にさんまの腸（はら）をくれむと言ふにあらずや」 (in translation: "The girl with a father of little affection struggles with her small chopsticks and offers the saury's innards to a man who is not her father"). Near the end is the line 「さんま、さんま、／さんま苦いか塩つぱいか。／そが上に熱き涙をしたたらせて」 (in translation: "Saury, saury, are you bitter or salty? Hot tears fall upon it")."Questions people ask
Where does the bitterness of offal come from?
In this material it is attributed to the bile more often than to the organ. A wholesaler comments that the faint bitterness of saury guts comes from the bile; foie gras has its veins removed, liverwurst its bile and bile ducts, Tuscan liver paste its gall bladder. None of these is a measurement; they are records of process and explanation.
Is bile something to be removed?
It depends on the kitchen. On the removing side, lifting the gall bladder out unbroken is written as the way to hold bitterness down. On the adding side, the recipe for sa says to add bile if you like it bitter, niubie adds cattle bile and reduces it, and nam pia uses the digestive fluid from the junction of the intestines. The same substance is an accident in one place and an ingredient in another.
Is ankimo bitter?
No source on this page says so. Three dictionaries and two ministry pages on regional cuisine stop at calling it prized. A sensory test with 8 panellists comparing ankimo with tiger puffer liver, both heated for 7 minutes, found no significant difference on any attribute. It is not that bitterness has been ruled out; it has not been recorded.
Is the bitterness of an offal dish the bitterness of its ingredients?
Not necessarily. In liverwurst, bitterness is attributed to the Maillard reaction during high-temperature sterilisation of the canned and jarred product. Bitterness from the ingredient and bitterness made by the process are handled separately within the same product.
What is not known
- Attributing bitterness to bile rests, within this page, on process descriptions and practitioners' comments. Nothing here measures bitterness with and without the bile.
- How intense the bitterness of the bile-adding dishes (nam pia, niubie, sa) actually is has not been measured. Bitterness is recorded only as a defining property.
- The ankimo sensory test used a panel of 8 and compared against tiger puffer liver. It is not a comparison against an untreated control or against other offal.
- That bitterness showed no significant difference in the ptarmigan intestine sauce is not proof of its absence; only 2 of 33 attributes differed at all.
- The Maillard bitterness in liverwurst is an account of how it forms, not a measurement of how much or how intense.
- The flavours of igunaq and kiviaq come from journalists' and researchers' descriptions rather than systematic sensory tests. Where bitterness sits in them is unknown.
Read in the encyclopedia
- Nam pia — the dish built on digestive fluid from the junction of the intestines
- Niubie — a process starting half an hour before slaughter, with cattle bile added
- Sa — the method that says to add bile if you like it bitter
- Foie gras — the preparation that draws the veins
- Liverwurst — bile ducts removed, yet the processing makes bitterness
- Crostini di fegatini — a step whose heart is the phrase "without bursting it"
- Saury guts — the part whose faint bitterness is attributed to bile
- Ankimo — the entry that records finding no source naming its bitterness
- Cibreo — an offal dish with no mention of bitterness, described through gentleness
- Arctic gastrophagy — eating intestine and its contents, and a sensory test across 33 attributes
All entries in this hub (14)
Every encyclopedia entry that belongs to this hub, grouped by category. Entries without an English version open in Japanese.
ingredientLophius litulon · foie gras · liverwurst · Cololabis saira · kuchiko (dried sea cucumber ovary)
dishIgunaq · Kiviaq · crostini di fegatini (toscani) / crostini neri · Sa (Saa Chin / Saa Neua) · Cibreo · Nậm Pịa (Nặm Pịa) · Niúbiě
Go deeper in the research notes
- Bitterness in offal — gathering the records part by part, and where bitterness has been placed
- Fermentation writes and erases bitterness — how fermentation works in both directions
- When bitter is a compliment — when bitterness becomes a dish's defining property
- Aku-nuki and unmeasured bitterness — what to do with a process whose endpoint is never written
Source ledger
The primary sources and limitations for the claims on this page can be checked from the source links inside each card. The full list is on the Japanesesource ledger.
How to cite this page
Aoki, M. (NIGAMI LAB) (2026). Bitterness in offal. https://nigamilab.com/en/topics/offal-bitterness/
Each claim card has a stable anchor (#clm-…). The dataset is CC BY 4.0; see the data page. Dataset DOI: 10.5281/zenodo.22739101