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Topic hub — 2026-10-20

Does boiling or soaking cut citrus peel bitterness?

When making marmalade, people boil the peel and change the water, soak it, and cut away the white pith, all to take out bitterness. How far have studies confirmed that these steps lower bitterness? We keep naringin, which tastes bitter at once, apart from limonoids, which are said to turn bitter later with heating or time. This page lines up the included studies together with their conditions: the fruit and part of the peel, the number and length of treatments, and how it was evaluated. It then separates what was established within one experiment, what differences between studies might explain, and what this comparison alone cannot separate.

What this page establishes

• That boiling and soaking sharply cut naringin in the peel, levelling off after a certain time or number of boils.
• That total limonoids were found hard to remove by soaking in water.
• That, in the sources searched, the only study in which people compared bitterness against untreated peel used jam, and water-only treatment left it about as bitter.
• That the pith is high in naringin, but limonoids are higher in the outer peel and seeds; no study was found comparing marmalade with and without the pith by taste.

What the included studies allow us to say

• Boiling and soaking sharply cut naringin, a compound behind peel bitterness: soaking natsumikan outer peel for 2–3 hours removed 80–85%, and boiling banpeiyu outer peel 5 minutes with a water change, 3 times (then 1 hour of water cooling), cut it by 66.2%. But in the sources searched, only 1 study compared human-rated bitterness against untreated peel (bitter-orange jam), and it reports jam treated with water only as about as bitter as untreated jam. [rows: Morohoshi 1971 soaking, naringin; Morishita 1986 boiling, outer peel naringin; Lagha-Benamrouche 2018 water-debittered jam]
• A drop in compounds and a drop in perceived bitterness need to be checked separately. One measurement found that limonoids (total) hardly left with water soaking, and in one study bitterness rankings did not follow residual naringin. [rows: Morohoshi 1973 limonoid removal; Morishita 1986 peel bitterness ranking]

Bitterness ratings at a glance

Only the results in which people rated bitterness. Values for "Treated peel" and "Comparison" are as reported. "Direction" shows which sample was more bitter; gaps the paper does not treat as a difference are shown as "about the same". Select a study name to jump to its detailed table.

Results in which people rated bitterness only. Where each value comes from (a table or a reading from a figure) is shown in its row; scales and methods differ between studies, so compare within a row, not across rows.

Conditions and results, study by study

For each study, the conditions for the fruit, the part of the peel, the treatment and the evaluation, and the results. Each row is one study × one measured result, so naringin, limonoids and human ratings are separate rows. Under each row we give where it is in the paper and whether it can be used to judge whether pretreatment lowers peel bitterness, with the reason. Nothing is left blank: "not given", "not read" and "not applicable" are kept apart.

What was established within an experiment

Only results where the same panel compared changed conditions within one study.

• Boiling banpeiyu peel 5 minutes with a water change, 3 times, then cooling in water for 1 hour, cut naringin from 138.0 to 46.7 mg% in the outer peel and from 388.3 to 163.0 mg% in the pith, and limonin from 14.1 to 3.1 ppm in the outer peel. Pith naringin levelled off after 2 boils. [rows: Morishita 1986 boiling, outer peel naringin; Morishita 1986 boiling, pith naringin; Morishita 1986 boiling, outer peel limonin]
• In the same study, when 10 people ranked the treated peels, the enzyme-treated peel was the most bitter (rank sum 13; hot-water method 22). The authors noted that the order did not follow residual naringin. Untreated peel was not compared. [rows: Morishita 1986 peel bitterness ranking]
• Soaking natsumikan peel removed 80–85% of the naringin from the outer peel and 60–70% from the inner white part and membranes within 2–3 hours, after which it changed little. Salt water, running water and standing water differed little (no test). [rows: Morohoshi 1971 soaking, naringin]
• Total limonoids in natsumikan outer peel hardly left in 3 hours of standing water, while about 25% left in 2% salt water and about 30% with boiling (no human tasting). [rows: Morohoshi 1973 limonoid removal]
• For bitter-orange pith jams, jam debittered with water only was about as bitter as jam not debittered, while jam debittered with salt water was significantly less bitter (per the abstract and the lab ledger). [rows: Lagha-Benamrouche 2018 water-debittered jam; Lagha-Benamrouche 2018 salt-debittered jam]
• Yuzu peel naringin fell from 18.22 to 14.02 mg% after 2 hours in water and to 4.38 mg% after boiling. [rows: Morishita 1994 soaking, yuzu naringin; Morishita 1994 boiled, yuzu naringin]

Candidate explanations for the differences

Conditions that differ between studies, or between treatments, and could explain differences in bitterness. Several conditions differ at once, so which one mattered is unknown. These are candidates, not causes.

• Limonoids. Even when naringin drops, bitterness may remain if limonoids are left. One measurement found them hard to remove by soaking, another found boiling removed them well, and one report says nomilin rose in boiled yuzu peel (the comparison value is uncertain). [rows: Morohoshi 1973 limonoid removal; Morishita 1986 boiling, outer peel limonin; Morishita 1994 boiled, yuzu nomilin]
• Salt. The abstract says salt-debittered jam was less bitter while water-debittered jam was not. Whether salt drew out more bitterness or leftover saltiness masked it cannot be told from the abstract. [rows: Lagha-Benamrouche 2018 salt-debittered jam; Lagha-Benamrouche 2018 water-debittered jam]
• Removing the pith. Naringin was about 2.5 times higher in the pith than in the outer peel, so removing the pith should cut naringin (an inference from concentrations by part, not an experiment measuring before and after). But limonoids were higher in the outer peel and seeds than in the pith, and in the sources searched no study compared marmalade made with and without the pith by taste. [rows: Morohoshi 1971 naringin by part; Morohoshi 1973 limonoid removal]
• Peel firmness. In the study where bitterness rankings did not follow residual naringin, the authors inferred that firmness also mattered. Firmness itself was measured, but whether it caused the bitterness difference was not tested separately. [rows: Morishita 1986 peel bitterness ranking]

Naringin

More in the pith

Limonoids

More in the outer peel

What is not known — what this comparison alone cannot separate

• How much perceived bitterness drops compared with untreated peel: most human ratings in this table compare treatments with one another, and only 1 (of jam) compared with untreated. [rows: Morishita 1986 peel bitterness ranking; Morohoshi 1971 marmalade bitterness; Lagha-Benamrouche 2018 water-debittered jam]
• The effect of the number of boils: in compounds, the pith levelled off after 2 boils and the outer peel fell further at the 3rd, but in the sources searched no study compared bitterness across numbers of boils by taste. [rows: Morishita 1986 boiling, outer peel naringin; Morishita 1986 boiling, pith naringin]
• Differences between fruits: the studies with human ratings used different fruits (banpeiyu, natsumikan, bitter orange), so whether the effect of pretreatment on perceived bitterness differs by fruit cannot be separated (one study compared compound changes for yuzu, konatsu, kumquat and navel orange in the same table). [rows: Morishita 1986 peel bitterness ranking; Morohoshi 1971 marmalade bitterness; Lagha-Benamrouche 2018 water-debittered jam; Morishita 1994 boiled, yuzu naringin]

How studies and rows were counted

The number of evidence rows is not the number of studies. Table rows, ledger evidence rows, papers and research groups are counted separately.

• Sources in the table: 5, of which 3 have results on people's bitterness ratings in this table; for the others, only results on something else (compounds, smell) are shown, or they are used only for context.
• Rows in the table (study × measured result): 13. Rows on perceived bitterness: 4.
• Evidence rows in the lab's ledger these rows rest on: 6. One paper can supply several rows. Rows without a ledger evidence row come from other tables of the same papers, or from sources newly read for this question.
• Research groups among the studies with bitterness results in this table, as grouped in the ledger: 3 (reports from the same lab count as 1; co-authors and affiliations can overlap, so this is not a count of independent replications).

What we looked at

The sources examined for this question, why each was included or excluded, and which full texts could not be reached. We searched PubMed, J-STAGE and CiNii for studies on pretreating citrus peel for marmalade (boiling, soaking, salt water, removing the pith) in relation to bitterness, naringin and limonin, and added the sources already in the lab's ledger.

Read in the encyclopedia

• Naringin — immediately bitter, abundant in peel and pith
• Limonin — turns bitter later with heating or time
• Yuzu peel / Natsumikan / Banpeiyu — fruits studied for this question
• Citrus bitterness — the topic hub on citrus bitter compounds and processing

Source ledger

Each study's source can be reached from its section above. All of the lab's sources are listed in the source ledger.