日本語版

Topic hubs

The encyclopedia builds evidence one entry at a time; the research notes follow one question each. A topic hub sits between them: it answers the first question, then leads to the related entries, notes, and sources. These are the hubs available in English.

• What Is IBU? Beer Bitterness Units vs. the TasteIBU (International Bitterness Units) is an absorbance measurement, not the bitterness you actually taste. Iso-α-acids from hops, IBU rising while iso-α-acids fall with dry hopping, and how storage changes the quality of bitterness — a topic hub gathering what IBU measures and what it misses, built from the encyclopedia's claim data.
• The Words for BitternessDo dictionaries across languages define bitterness as the same taste, and does it carry the same value? A topic hub gathering, with sources, how dictionaries phrase their definitions, how the word extends into metaphor, its etymology, and documented confusions of taste, built from the encyclopedia's claim data.
• Bitter gourd's bitternessThe identity of the compound behind bitter gourd's bitterness has been reassigned differently by a 1982 study and a 2024 study. Salt-water soaking left bitterness unmoved, while gum arabic and cyclodextrin moved it — a topic hub gathering only the confirmed claims across compound identity, preparation, and maturation.
• Bitterness, Astringency, Egumi, and PungencyA topic hub that separates bitterness, astringency, egumi, and pungency into four layers — a basic taste, an oral touch sensation, a chemesthetic sense, and a Japanese food-sensory word — and, with evidence, extends to the time-axis recording methods huigan and the time-intensity (TI) method.
• Citrus bitternessWhere citrus bitterness comes from, why it arrives late, and how it gets removed. A topic hub gathering, using only confirmed claims, the different roles of limonin and naringin, where each compound concentrates, the ways it's removed — enzymes, resin, salt, high pressure — and how the wording splits between cultivars and product descriptions.
• Tea's bitterness and astringencyWhere is tea's bitterness and astringency decided — in the leaf, the water, or the fire? A topic hub gathering only the claims confirmed at each stage: the carriers catechins and caffeine, shading and picking, water temperature and steeping, and firing and post-fermentation.
• Coffee bitternessWhat coffee's bitterness owes to roasting, and what else moves it. Chlorogenic acid and quinides in the green bean, caffeine's share of the contribution, cultivar and defect beans, brewing and additives — a topic hub gathering only the claims that could be confirmed.
• Herbal Bitters and Amari: BitternessWhere does the bitterness of herbal bitters and amari come from, and how do pharmacopoeias, dictionaries and standards write about it differently? A topic hub gathering the compounds and bitterness values of gentian, cinchona and wormwood, the definitions in EU regulation and food codes, and how differently individual brands talk about their own bitterness.
• Bitter Taste Receptors (TAS2R)Bitter taste receptors (TAS2R) differ compound by compound in the range of substances they respond to, and genotypes such as TAS2R38 explain part, but not all, of the individual variation in bitterness. A topic hub that separates cell-level activation from bitterness as tasted in the mouth, gathering only claims that have been confirmed.
• Sansai and seasonal bitternessOf Japan's sansai (wild spring greens), only fukinoto and yomogi have isolated, named bitter compounds — and one study found those compounds barely transfer into hot water. Whether household aku removal actually moves bitterness has not been measured, and even the origin of the saying 'pile bitterness onto the spring plate' cannot be pinned down in the original text. Ginkgo nuts, akebi peel, edible chrysanthemum and the spring greens of Korea are placed on the same scale. A topic hub that keeps what has been confirmed separate from what is still unmeasured.
• Fermentation and bitter peptidesWhere does fermentation create bitter peptides, and where does it break them down? A topic hub gathering the Q-rule and its counterexamples, cheese's strain-driven creation and removal of bitterness, sake's peptide and amino-acid accounts, and the difference between receptor activation and bitterness felt on the tongue.
• Chicory and radicchio bitterness — what changes it?Treviso's specification pairs a degree word ('slightly') for precoce with an evaluation word ('pleasantly') for tardivo, which cannot be ranked — a topic hub of confirmed claims only.
• Lupin bitterness — what makes lupins bitter or sweet?Lupin bitterness comes from quinolizidine alkaloids (QA). The threshold separating 'bitter' from 'sweet' cultivars differs between EFSA and a 2025 review — a topic hub of confirmed claims only.
• Fenugreek bitterness — same source in seed and leaf?Fenugreek seed bitterness is attributed to saponins like protodioscin as background text; leaf bitterness correlates with diosgenin (r=0.92) in a direct sensory measurement — confirmed claims only.
• Cacao bitterness and astringency — where does it change?Cacao bitterness is not explained by theobromine and caffeine alone. Fermentation raises it at some facilities and lowers it at others, roasting lowers perceived bitterness as temperature rises, and adding fat lowers it without changing the compounds — a topic hub that keeps sample, compound, process and formulation apart.
• When bitterness and harm are the same compoundAmygdalin in almonds and apricot kernels, steroidal glycoalkaloids in potato and tomato. When the compound that carries the bitterness is the compound that matters at higher amounts, what do domestication, ripening and processing actually remove? A topic hub built only from claims that still hold their measurements.
• Getting used to bitternessRepeated exposure raises liking for vegetables and raises adults' pleasantness ratings for a bittersweet drink in seven days. Yet frequent coffee drinkers rate bitterness more intensely. Does getting used to it mean perceiving less, or minding less? A topic hub built only from claims that hold their measurements.
• Handling the bitterness of medicinesClarithromycin dry syrup is estimated to fall to about 1% of the bitterness intensity of a powder suspension, and coated quinine pellets release only 2.1% in five minutes. Yet adding 5% citric acid lowers overall palatability even as it suppresses bitterness. Measuring, masking, and what masking costs.
• Is scorched rice bitter?The crust of rice at the bottom of the pot has a name in many places, and in every language the word comes from scorching. Yet almost every source gathered here speaks of fragrance, not bitterness. Neither the trained panel's scale nor the consumer test's attributes included bitterness at all.
• Mediterranean bitter vegetablesMost of the wild greens recorded on Ikaria are described as bitter, and the production standard for the Sardinian artichoke writes a balance of bitter and sweet into the specification itself. In dandelion, olive leaf, lampascione and white asparagus the carrying compounds have been identified.
• African bitter leavesVernonioside A1, the bitter principle of bitterleaf, produces bitterness from 7 µg and is nearly as strong as quinine sulfate. Yet the endpoint of the washing that removes that bitterness is written only as a desirable level. A hub that places an identified molecule beside a process with no stated endpoint.
• Bitterness in the Andes and MesoamericaCassava is split into sweet and bitter varieties at 100 mg/kg of HCN in the root, and Andean potatoes into papa amarga and papa dulce. Where bitterness names a variety class, the process that removes it grows to 21–30 days in a river. This hub follows that through to what is left.
• Bitterness in Southeast AsiaThai public vegetable guides carry a category called rot khom, bitter taste, with khilek flowers and pheka pods in it. The instructions for removing that bitterness are written not in temperature but in counts: boil and discard at least twice, three to five minutes depending on how bitter you want it.
• Bitterness in offalFoie gras has its veins drawn, liverwurst its bile ducts removed, Tuscan liver paste its gall bladder lifted out unbroken. Meanwhile nam pia uses digestive fluid, niubie adds cattle bile, and the recipe for sa says: if you like it bitter, add bile.
• Arabica or robusta: which is more bitter?Which is more bitter, arabica or robusta? A study-by-study comparison of beans, roasting, brewing, panels and results, separating what an experiment established, candidate explanations, and what this comparison alone cannot separate.
• Does salting or boiling bitter melon cut its bitterness?Do salt rubbing, boiling, soaking in water or brining lower the bitterness of bitter melon? A study-by-study comparison of cut, salt concentration and time, the form tasted and the panel, separating what an experiment established, candidate explanations, and what this comparison alone cannot separate.
• Is green tea less bitter brewed cooler or cold?Is green tea less bitter brewed with cooler water or cold? A study-by-study comparison of tea, water temperature, time and panel, keeping bitterness and astringency apart, separating what an experiment established, candidate explanations, and what this comparison alone cannot separate.
• Does boiling or soaking cut citrus peel bitterness?Do boiling, soaking, brining or removing the pith take the bitterness out of citrus peel for marmalade? A study-by-study comparison of fruit, treatment and evaluation, keeping naringin (immediate) and limonoids (delayed) apart, separating what was established, candidate explanations, and what this comparison alone cannot separate.
• Do oil or mayonnaise soften green pepper bitterness?Do oil, mayonnaise, egg yolk, dressing or dairy soften the bitterness of green pepper? A study-by-study comparison of sample, addition and evaluation, keeping bitterness and smell apart, separating what was established, candidate explanations, and what this comparison alone cannot separate.
• Does onion turn bitter when sautéed for a long time?Does onion turn bitter when sautéed for a long time? A study-by-study comparison of cut, pan and heat, sautéing time, the form tasted and the panel, separating what an experiment established, candidate explanations, and what this comparison alone cannot separate.
• Is darker-roast coffee more bitter?Is darker-roast coffee more bitter? A study-by-study comparison of beans, how roast degree was measured, brewing and strength, and the panel, separating what an experiment established, candidate explanations, and what this comparison alone cannot separate.
• Do beers with higher IBU taste more bitter?Do beers with higher IBU taste more bitter? A study-by-study comparison of the beers, how bitter compounds were measured, and the panel, separating what an experiment established, candidate explanations, and what this comparison alone cannot separate.
• Is chocolate with a higher cocoa percentage more bitter?Is chocolate with a higher cocoa percentage more bitter? A study-by-study comparison of the samples (cocoa, sugar, fat), processing and the panel, separating what an experiment established, candidate explanations, and what this comparison alone cannot separate.
• Is brassica bitterness down to the TAS2R38 gene?Is finding brassica vegetables bitter down to the bitter-receptor gene TAS2R38? A study-by-study comparison of the vegetables, cooking, panels and genotype grouping, separating what an experiment established, candidate explanations, and what this comparison alone cannot separate.
• Does starter or ripening change cheese bitterness?Does cheese bitterness change with ripening time and the starter strain? A study-by-study comparison of the cheese type, strains, ripening and panels, separating what an experiment established, candidate explanations, and what this comparison alone cannot separate.
• Does adding sugar or salt suppress bitterness?Does adding sugar or salt suppress bitterness? A study-by-study comparison of the bitterants, amounts added and panels, keeping taste suppression apart from comparisons where the composition itself changes, and separating what an experiment established, candidate explanations, and what this comparison alone cannot separate.
• Which compounds set olive oil bitterness?Which compounds set the bitterness of extra virgin olive oil? A study-by-study comparison of oils, growing and milling, panels and phenolic compounds measured in the same oils, keeping bitterness apart from throat pungency and separating what an experiment established, candidate explanations, and what this comparison alone cannot separate.
• Do cooking and additions reduce spinach bitterness?Do cooking, growing or additions reduce the bitterness and egumi of spinach? A study-by-study comparison of samples, preparation and panels, keeping bitterness apart from egumi and separating what an experiment established, candidate explanations, and what this comparison alone cannot separate.
• Does repetition end dislike of bitter vegetables?Does eating bitter vegetables repeatedly change how much people like them, how much they eat, or how bitter they find them? A study-by-study comparison of participants, number of exposures, foods and ratings, keeping liking apart from perceived bitterness and separating what an experiment established, candidate explanations, and what this comparison alone cannot separate.
• Does grated daikon bitterness vary by part or grating?Does the bitterness of grated daikon change with the part of the root, the variety, the grating or the time after grating? A study-by-study comparison of samples, preparation and panels, keeping bitterness apart from pungency and stating first that only one study had people compare bitterness.
• Does freezing or enzyme cut natsumikan bitterness?Does the bitterness of natsumikan change with enzymes (naringinase), freezing or canning? A study-by-study comparison of samples, treatments and panels, stating first that freezing made it more bitter and that the studies in which people compared bitterness are few and old.
• Does roasting tea make it less bitter?Does roasting tea leaves or firing them harder change bitterness? A study-by-study comparison of teas, roasting and panels, stating first that no study was found comparing bitterness for Japanese hojicha and that among oolong studies one detected no significant difference while others report an increase.
• Does processing cacao change its bitterness?Does fermenting, drying or roasting cacao beans change bitterness? A study-by-study comparison of samples, processing and panels, stating first that a model predicts less bitterness with stronger roasting in unsweetened chocolate, while for fermentation days the direction differs by study and plant, only some drying-temperature comparisons were significant, and in some studies the steps cannot be separated.
• Does beer's bitterness change as it ages?Does storage time or temperature change how bitter bottled beer tastes? A study-by-study comparison of beers, storage conditions, panels and scales, stating first that some studies found weaker bitterness after longer storage while one found equal median bitterness scores at the start and end of refrigeration with no significant difference detected, that conditions and scales differ too much to say how much changes in how many months, that perceived bitterness and BU are read apart, and that table and discussion disagree on whether the bitter aftertaste change was significant.
• Does brewing method change coffee's bitterness?When only the brewing of the same beans changes (cold or hot water, water temperature, equipment, water composition, extraction pressure or time, strength), does perceived bitterness change? A study-by-study comparison of beans, brewing, serving, panels and scales, stating first that hot brews were more bitter than cold brews in most results while some pairs showed no significant difference, that studies varying temperature within the hot range detected no significant difference, that equipment comparisons also differ in grind and amounts, and that water results depend on what was added and how much.
• Does fermentation or ageing change dark tea's bitterness?For dark teas such as pu-erh and Liupao and other post-fermented teas, does perceived bitterness change when the length or method of post-fermentation (pile fermentation, added microbes, pickling) or storage of the same raw tea changes? A study-by-study comparison of raw tea, fermentation or storage conditions, brewing, panels and scales, stating first that some studies found bitterness weakened during post-fermentation of the same raw tea, while one found it rose before falling and one reported it rising in pickled tea, and that comparisons of stored teas from different production years cannot isolate the effect of storage time.
• Does enzyme hydrolysis make protein more or less bitter?When proteins of the kind used in protein powders are broken down by enzymes into hydrolysates (peptides), do they become more bitter or less? A study-by-study comparison of raw materials, enzymes, degree of hydrolysis, after-treatments, panels and scales, stating first that depending on the enzyme and degree of hydrolysis, bitterness rose in some cases and, at the end points shown, did not differ significantly from the unhydrolysed material in others, and that some studies lowered bitterness by treating the hydrolysate afterwards while others saw it rise after treatment.
• Does extraction or storage change olive oil bitterness?Does the bitterness of olive oil change with how it is extracted (malaxation, oxygen, pulsed electric fields, ultrasound, destoning, extraction system), filtered or racked, how the fruit is handled before milling, or how the oil is stored? A study-by-study comparison of studies that changed only these conditions for the same fruit or oil, with who rated it on what scale, stating first that the direction differs by process and splits by study or cultivar even within one process, with examples of rises, falls and no significant difference.
• Does mixing a medicine with food soften its bitterness?Does mixing a bitter medicine with food or drink soften its bitterness? A study-by-study comparison of studies that changed only what the same drug (or the same bitter sample) was mixed with (food, drink, swallowing-aid jelly, dissolving liquid) or the temperature of the water, with who rated it on what scale, stating first that for each drug and study there are falls, rises and comparisons with no significant difference, so the direction differs by drug and study. Not advice on taking medicines.
• Does skin and seed contact change how bitter wine is?Is wine bitter because of contact with the grape skins and seeds? Does the bitterness of wine change with contact with the grape skins and seeds (skin maceration, extraction time, fermentation on the skins)? A study-by-study comparison of studies that changed only the length or method of contact for the same grapes and had people rate it, with who rated it on what scale, stating first that few studies had people rate bitterness, that values can be read in only some, and that bitterness scores rose with long contact in some studies, showed no significant difference with short contact before fermentation in others and fell when seeds were removed, so the direction differs by study. Not advice on making or drinking wine.
• Does swapping salt for potassium chloride add bitterness?Does replacing part of the table salt (NaCl) with potassium chloride (KCl) in salt reduction make food bitter, and is potassium chloride bitter? A study-by-study comparison of studies that had people rate bitterness for the same food with salt partly replaced and with salt only, with the basis of the proportion replaced and who rated it on what scale, stating first that within the same study some proportions were separated as more bitter and others were not, and that in some foods no significant difference was shown or no test result was given, so the direction splits by proportion and by food. Not advice on salt reduction or health.
• Do added ingredients make a bitter sample less bitter?Does adding an ingredient such as an amino acid, an acid or a polyphenol to a bitter sample such as quinine or amino acids lower the bitterness people perceive? A study-by-study comparison of studies that had people rate bitterness for the same sample with and without the ingredient, with the amount added and who rated it on what scale, stating first that the literature leans towards reports from people looking for ways to suppress bitterness and towards one laboratory, and that even so the published means fell in some pairs, rose in others, and showed no significant difference in others, so the direction splits. Not advice on taking or mixing medicines.
• Supertasters, PROP/PTC and TAS2R38 genotypeA study-by-study comparison of studies in which groups of people with different TAS2R38 genotypes rated standard PROP or PTC solutions, impregnated paper or dissolvable films, with presentation, concentration, and who rated on what scale. It shows that supertaster groups defined by PROP ratings overlap with genotype groups but do not match them, that the AVI/AVI groups consistently had lower bitterness ratings and higher thresholds expressed as concentrations, and that whether PAV/PAV and PAV/AVI differed depended on the study and the measure. Not a way to sort people by genotype to decide food preferences or health.

The full list of hubs is on the Japanese page. See also the research notes and the bitter crossings in English.