Research note 013 — 2026-08-31
How Leaf, Root, Peel, and Seed Become Bitter Seasonings
Search for a bitter seasoning, and before the name of the ingredient comes up, it is theplant part and the process that come into view. For citrus, it is not the juice but the white mesocarp; for artichoke, not the edible receptacle but the leaf; for sesame, not the seed itself but the sequence of soaking, germinating, roasting, and grinding. In this round of collection, 15 entries from the candidate encyclopedia were connected to their sources, while the broader story of their culinary use is left unverified.
What this page establishes
- How much the naringin concentration in citrus's white peel falls under alkaline treatment or adsorbent treatment has been measured individually in several experiments.
- Within a single yuzu-marmalade process, an opposite-direction change has been confirmed: naringin decreases while nomilin increases.
- Fenugreek leaf's bitter value has been reported to differ by population, and a correlation with diosgenin content is also shown in one study.
- The bitter constituents of bitter medicinal herbs such as centaury and marrubiin are organized in EMA assessment reports, but this has not yet reached a sensory reconstitution test of them as culinary herbs.
1. Marmalade Is a Vessel That Can Hold Bitterness
Under the Codex standard, citrus marmalade is a product that may use whole fruit, pulp, juice, water extract, peel, and the like. The standard neither praises nor dislikes bitterness. But the structure that lets peel remain inside the product creates room to design sweetness and bitterness together within the same jar.
Scope: Codex CXS 296-2009 (revised 2025)
Note: A product definition under the standard. It does not indicate the presence or desirability of bitterness.
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seasoning-citrus-marmalade —is_defined_in_codex_as→ "A product that brings citrus fruit to a suitable viscosity and that may use whole fruit, pulp, purée, juice, water extract, peel, and the like"2. "Debittering" Is Not a Single Operation
In peel from Vietnam's cam sành, dilute alkaline treatment lowered naringin. In a separate study on bitter-orange peel, water alone did not lower the bitterness ranking, but a repeated process that included brine did, and a jam made under one condition was the most preferred. Both, however, are controlled food-processing trials. This page does not rewrite them into a household procedure that uses chemical treatment, and it keeps the drop in compound levels, perception, and preference recorded separately.
Scope: An experiment in which peel was blanched for 2 minutes in 100 ppm NaOH, then soaked in the same solution for 1 hour
Note: A measured decrease in naringin. This is not a recommendation for household alkaline treatment.
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ingredient-sanh-orange-peel —is_reduced_by→ process-citrus-alkaline-debitteringScope: A jam comparison of a water-only condition against several NaCl conditions, using bitter-orange peel
Note: Water alone matched the bitter control; the NaCl conditions lowered it. The result of a whole process including repeated soaking and heating.
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seasoning-bitter-orange-marmalade —has_perceived_bitterness_suppressed_by→ process-citrus-salt-debitteringScope: A bitterness ranking by a trained panel of 30, and a separate liking evaluation by another 30
Note: The most preferred condition, but the drop in bitterness cannot be fully separated from compound loss, saltiness, and process differences.
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seasoning-bitter-orange-marmalade —has_sensory_acceptability_increased_by→ "The 0.625% NaCl peel treatment among the tested conditions"3. In Yuzu, the Candidate Bitterness Moved in Opposite Directions
In yuzu peel that had gone through water soaking and heating, naringin fell while nomilin rose. Compounds alike called 「苦味成分」 (in translation: "bitter compounds") do not necessarily move in the same direction. Within this set of marmalades, it was nomilin that corresponded most strongly with perceived bitterness.
Scope: A 1994 marmalade process using yuzu peel
Note: The candidate bitter compounds do not move in the same direction. Naringin was reported to be below threshold in this sample.
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seasoning-yuzu-peel-marmalade —was_observed_with→ "A decrease in naringin and an increase in nomilin after water soaking and heating"Scope: Single-fruit and blended marmalades of yuzu, konatsu, kumquat, and navel orange; sensory evaluation by 13 panelists
Note: Bitterness and nomilin were correlated in this sample set. This is not asserted as the sole cause of bitterness in marmalade in general.
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seasoning-yuzu-peel-marmalade —has_perceived_bitterness_associated_with→ compound-nomilinIn a doctoral dissertation's high-pressure method, bitterness was rated weaker than with the heat method. At the same time, sweetness, sourness, color, and yuzu-like aroma also changed. It is more accurate to read this not as a 「苦味だけを抜く技術」 (in translation: "technique that removes only bitterness"), but as a process that rearranges the whole configuration of flavor.
Scope: The same formulation compared under high-pressure processing and heat processing by 27 students and staff
Note: The high-pressure method scored lower on bitterness, and higher on liking of the bitterness and other measures. Not reducible to a high-pressure effect alone.
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seasoning-yuzu-peel-marmalade —has_perceived_bitterness_suppressed_by→ process-high-pressure-marmaladeScope: High-pressure processing versus heat processing of yuzu marmalade
Note: Multiple attributes changed at the same time; this is not simply a technology for 『脱苦味だけ』 (in translation: "debittering alone").
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process-high-pressure-marmalade —was_observed_with→ "Stronger sweetness and sourness, weaker bitterness, and better-preserved color and yuzu-like aroma than the heat method"4. The Albedo Returns, from Discarded Part to Research Subject
Grapefruit-peel jam is recorded as a sample that people actually ate and rated for acceptability. Bitterness intensity itself, however, was not measured directly. In pomelo's white albedo (the mesocarp), naringin and limonin were quantified and a debittering trial was carried out, and a downward direction was also reported on the sensory side.
Scope: The existence of grapefruit peel jam (GPJ) and its consumer acceptability
Note: Bitterness intensity itself was not directly measured, so this is not stated as 『苦味が好まれた』 (in translation: "the bitterness was preferred").
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seasoning-grapefruit-marmalade —is_documented_in→ "A 5-point liking evaluation by 31 people comparing 6 types of citrus-peel jam"Scope: Kao Nampung pomelo albedo, 0.7% β-CD combined with ultrasound processing
Note: The chemical quantification of naringin and limonin is consistent with the methods and tables. Health-effect claims are not adopted.
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ingredient-pomelo-pith —is_reduced_by→ process-pomelo-cyclodextrin-ultrasoundScope: A comparison reported as a rank-rating by a 15-member trained panel
Note: The methods describe a 0-5 scale, but the untreated value in Table 3 is 7.20. The direction is reported, but there is an internal inconsistency in the numeric scale.
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ingredient-pomelo-pith —has_perceived_bitterness_suppressed_by→ process-pomelo-cyclodextrin-ultrasoundThat pomelo paper, though, has an important flaw. The untreated value in its table exceeds the upper limit of the scale described in its methods. So the chemical quantification is kept as 「支持あり」 (in translation: "supported"), the sensory direction as 「予備的」 (in translation: "preliminary"), and the absolute score as unresolved. Adopting a paper and trusting it unconditionally are not the same thing.
Scope: An internal cross-check between the text and tables of Jaroennon et al. 2025
Note: The inconsistency itself is recorded here. No correction has been confirmed.
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ingredient-pomelo-pith —has_methodological_limitation→ "The 0-5 scale stated in the sensory-evaluation methods is inconsistent with the untreated bitterness value of 7.20 in Table 3"5. In Sesame Paste, What's Missing from the Experiment Is More Interesting Than the Candidate Compound
Under the Codex Near East Regional Standard, Tehena is a ground product of mature, roasted, dehulled sesame. In a germinated-sesame-paste study, by contrast, sesame was soaked and germinated before being roasted and ground, and arginine's taste-activity value exceeded one. As a candidate for bitterness, this is interesting. But the present paper's method has no independent sensory test. That is a shortcoming, and at the same time a fine gap open for a reconstitution or removal experiment to move into.
Scope: Codex Near East Regional Standard CXS 259R-2007 (revised 2025)
Note: A regional-standard definition of Tehena. It is not applied uniformly to every commercial tahini or sesame paste.
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seasoning-tahini —is_defined_in_codex_as→ "A product made by grinding mature, roasted, and dehulled sesame (Sesamum indicum L.)"Scope: Paste SG2-36, after 2 hours of soaking, 36 hours of germination, and roasting at about 130°C for 40 minutes
Note: Arg's TAV was above 1, but the present paper does not present a sensory-test design, and 『最も苦い』 (in translation: 'the most bitter') depends on the description in an earlier experiment.
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seasoning-germinated-sesame-paste —has_candidate_bitterness_contributors→ compound-arginineScope: An interpretation of the conclusion of Hou et al. 2021
Note: An audit claim that points out a missing method. It does not dismiss arginine as a candidate.
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seasoning-germinated-sesame-paste —has_methodological_limitation→ "The present paper has no independent sensory-evaluation method, so the chemical TAV alone cannot establish the cause of bitterness in the paste as a whole"6. Fenugreek Leaf's Bitterness Was Not a Fixed Value
Even within the same fenugreek, the leaf's bitter value differed from population to population. A strong correlation with diosgenin content has also been reported, but correlation is not proof of cause. There is room to separate out species, population, growing environment, and co-varying saponins.
Scope: The WHO method, using a panel of 10 adults, on 30 Iran-origin populations grown under the same conditions
Note: Population differences are preserved; this is not treated as a fixed value for fenugreek leaves in general.
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ingredient-fenugreek-leaves —has_reported_bitter_value_of→ "9.43, 10.32, and 9.64 ×10³ units/g across 3 populations of T. foenum-graecum"Scope: A correlation analysis across 30 populations of 10 species
Note: A correlation within the same sample set; it is not proof that diosgenin is a direct cause of bitterness.
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compound-diosgenin —is_correlated_with→ "The leaf bitter value of the same 30 populations (r=0.92)"7. Drawing a Line Between Bitter Medicinal Herbs and Culinary Herbs
EMA assessment reports precisely organize the plant parts and candidate bitter constituents of centaury, horehound, blessed thistle, dandelion (the whole plant with root), artichoke leaf, and yarrow. That is a valuable entry point, but it does not directly prove culinary use or suitability as a food ingredient. Here, all of them are kept as 「文脈資料」 (in translation: "contextual material") and are not placed on the same footing as a direct experiment's「支持あり」 (in translation: "supported").
Scope: A literature synthesis of the dried flowering aerial parts in the EMA assessment report
Note: A compositional summary from a public assessment report. It is not a recommendation for food use or a safety endorsement.
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spice-centaury —has_key_bitter_organoleptics→ "Secoiridoid glycosides, mainly swertiamarin, and the very bitter centapicrins"Scope: The dried flowering aerial parts of Marrubium vulgare
Note: The EMA assessment report records its role in bitterness and a standard of 0.7% or more in the dried herbal drug. This is not a recommendation for culinary use.
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spice-horehound —has_major_bitter_compound→ compound-marrubiinScope: The dried flowering aerial parts of Cnicus benedictus
Note: The EMA assessment report records 0.2-0.7% and a bitter index of 1:1800. Food use and safety require separate verification.
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spice-blessed-thistle —has_major_bitter_compound→ compound-cnicinScope: A literature synthesis of the leaves and root, within the dried whole plant with root, in the EMA assessment report
Note: Not a direct sensory test of the root alone; a compositional summary from a medicinal-plant assessment report.
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ingredient-dandelion-root —has_key_bitter_organoleptics→ "Taraxinic acid β-D-glucopyranosyl ester and 11β,13-dihydrolactucin"Scope: The EMA assessment report on the dried leaf of Cynara cardunculus
Note: The assessment report identifies 47-83% of the leaf's 0.4% bitter sesquiterpene lactones as cynaropicrin. A secondary record states it is higher in young leaves.
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ingredient-artichoke-leaf —has_major_bitter_compound→ compound-cynaropicrinScope: The EMA assessment report on the aerial parts of Achillea millefolium
Note: Not a value measured by EMA itself. It is not treated as a representative bitterness-intensity value or a recommendation for food use.
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spice-yarrow —has_reported_bitter_value_of→ "A record that the German Pharmacopoeia sets an upper-limit standard of 5,000"The Unverified Romance Stays on the Record
How far does chinotto marmalade keep the fruit's bitterness as a character trait, rather than removing it? Through what record does a medicinal-herb specimen cross over into a kitchen spice? Behind the oil and roasted aroma of germinated sesame, can its bitterness really be reconstituted as arginine? There is no answer yet. That is exactly why this is kept, not as a settled claim, but asa question carrying its source and a note of how far it has been confirmed.
Honest notes
- The Codex and EMA documents are standards, identification, and literature-synthesis materials; they are not sensory experiments.
- Listing in a medicinal-plant record is not read here as a recommendation for household foraging, eating, or use as a seasoning.
- Alkaline, high-pressure, and ultrasound conditions are kept here as research processes, not turned into household recipes.
- Papers with internal inconsistencies are not excluded either; the claims that can be salvaged are separated from the figures that cannot.
Frequently asked
How can the bitterness of citrus's white peel (the pith) be reduced?
Several laboratory treatment methods have been reported. One report states that blanching peel in a 100 ppm sodium hydroxide solution for 2 minutes, then soaking it for 1 hour, lowered the peel's naringin content from 16.58 to 3.70 mg/100g. In another experiment, a treatment combining cyclodextrin and ultrasound is confirmed to have substantially lowered both naringin and limonin from their pre-treatment levels. Both, however, are laboratory-controlled conditions, and neither is a description that recommends household reproduction.
Does adding salt reduce marmalade's bitterness?
In a comparative jam experiment using bigarade-orange peel, the water-only treatment was reported to be about the same as the bitter control, while the group treated with added sodium chloride showed a lower bitterness ranking. In a liking evaluation from the same experiment, the 0.625% sodium chloride treatment was most preferred by 30 evaluators. However, the authors note that the salt treatment also involves the loss of other compounds such as phenolics and vitamin C, so the change in bitterness alone cannot be taken as the reason it was preferred.
Why is sesame paste (tahini) bitter?
This has not yet been fully established. One study reports the arginine content and TAV (taste activity value) of a paste made from roasted, germinated sesame, listing arginine as a candidate contributor to bitterness. However, the same study has no independent sensory-evaluation method, and it is presented, as the study's own interpretation, that chemical figures alone cannot establish the cause of bitterness in the paste as a whole.
What is not known
- No study has yet been found that connects sesame paste's bitterness, as a chemical candidate compound, to an actual sensory evaluation.
- The correlation between fenugreek leaf's bitter value and diosgenin content remains a correlation from a single study; a causal relationship separating out other compounds or cultivar differences has not yet been confirmed.
- Which reference scale marrubiin's bitter value of 3.0 was calculated on cannot be identified from the review's description alone.
- The upper limit of bitter value attributed to the German Pharmacopoeia has not yet been reached in the original text itself, and its relationship to a measured average or to culinary intensity has also not been confirmed.
Related: Cooking Does Not Erase Bitterness — It Edits It ／Hypothesis Box ／Seasoning encyclopedia ／Spice and herb encyclopedia
How to cite this page
Aoki, M. (NIGAMI LAB) (2026). Leaf, Root, Peel, Seed, and Bitter Seasoning. https://nigamilab.com/en/notes/plant-parts-to-bitter-seasonings/
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