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Topic hub — 2026-10-20

Do oil or mayonnaise soften green pepper bitterness?

For people who dislike green pepper, the usual advice is to fry it in oil or pair it with mayonnaise or cheese. How far have studies confirmed that this softens its bitterness? We keep bitterness on the tongue apart from the pepper's characteristic smell. This page lines up the included studies together with their conditions: the form of the pepper (liquid or stir-fried), what was added and what it was compared with, and who rated it and how. 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, in the sources searched, no study had people compare the bitterness of pepper with oil alone against pepper with nothing added.
• That the only human bitterness comparisons were conference presentations by company researchers, comparing additions with each other (mayonnaise or yolk tasting less bitter than oil or egg white).
• That the peer-reviewed paper examined smell, not bitterness: oil, butter, cheese, milk and yogurt made the pepper's smell seem weaker.

What the included studies allow us to say

• In the sources searched, no study had people compare the bitterness of green pepper with oil alone against pepper with nothing added. The only human bitterness comparisons were 3 conference presentations by company researchers, and they compared additions with each other: mayonnaise, egg yolk or sesame dressing tasted less bitter than oil, egg white or salt. [rows: Nishimura 2015 mayonnaise vs oil; Kewpie 2016 yolk vs white; Tamura 2021 sesame dressing]
• The only peer-reviewed full paper (Aoki 2017) examined smell, not bitterness: mixing vegetable oil, butter, cheese, milk or yogurt into roasted green pepper made its characteristic smell seem significantly weaker (no significant difference with skim-milk solution; mayonnaise was not tested). So there is peer-reviewed evidence that these soften the smell, while for bitterness the claim still rests on company presentations. [rows: Aoki 2017 smelled odour; Aoki 2017 odour compound]

Bitterness ratings at a glance

Only the results in which people rated bitterness. Values for "With the addition" and "Comparison" are as reported. "Direction" is the larger of the two values; 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. Values are as reported; 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 pepper's form, the addition and the evaluation, and the results. Each row is one study × one measured result, so bitterness and smell are separate rows. Under each row we give where it is in the source and whether it can be used to judge whether the addition softens 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.

• When 20 people compared raw pepper liquid with different additions, yolk (about 3.6) was less bitter than white (about 4.5), fine mayonnaise (about 3.4) less than coarse (about 4.1), and acid-denatured yolk (about 3.7) less than yolk (about 4.5) (all significant at 5%; approximate readings from figures). Plain liquid was not compared. [rows: Kewpie 2016 yolk vs white; Kewpie 2016 emulsion size; Kewpie 2016 acid-denatured yolk]
• Mixed into roasted pepper, the odour compound in the headspace fell to 9 with oil, 9 with butter, 5 with cheese, 7 with milk and 4 with yogurt (pepper alone = 100), and 17 people smelled a significantly weaker odour with oil (−0.53), butter (−1.53), cheese (−1.35) and others. [rows: Aoki 2017 odour compound; Aoki 2017 smelled odour]
• The abstract reports that stir-fried pepper dressed with 7.5 g of sesame dressing was significantly less bitter than pepper seasoned with salt matched to the dressing's salt content (20 people; no values). [rows: Tamura 2021 sesame dressing]

Candidate explanations for the differences

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

• Fat holding the odour. Fat-containing additions cut the odour compound sharply, and the authors inferred fat mattered more than protein. If green pepper “bitterness” is partly experienced together with its smell, weaker smell could make bitterness less noticeable, but no study has checked this by eating. [rows: Aoki 2017 odour compound; Aoki 2017 smelled odour]
• Yolk protein and emulsion. Yolk, finer emulsions and acid-denatured yolk tasted less bitter. One presentation (2015) proposed that emulsion droplets take up bitter compounds; another (2016) that acid changes the proteins. Acid treatment and emulsion state were varied, but the mechanism has not been established. [rows: Kewpie 2016 yolk vs white; Kewpie 2016 emulsion size; Kewpie 2016 acid-denatured yolk; Nishimura 2015 mayonnaise vs oil]
• Dilution. Liquid dairy was about 2 times the pepper weight, and water alone lowered the odour compound to 42. The part due to dilution was not separated from the effect of the addition itself. [rows: Aoki 2017 odour compound; Aoki 2017 smelled odour]

What is not known — what this comparison alone cannot separate

• How much oil or mayonnaise lowers bitterness compared with pepper with nothing added: human bitterness comparisons were only between additions, or against salt or oil. [rows: Nishimura 2015 mayonnaise vs oil; Kewpie 2016 yolk vs white; Tamura 2021 sesame dressing]
• The link between bitterness and smell: weaker smell was confirmed, but no study separately tested whether it changed perceived bitterness (the sesame-dressing study reports lower bitterness and smell but did not examine their link). [rows: Aoki 2017 smelled odour; Tamura 2021 sesame dressing]
• Who rated: all the bitterness comparisons were presentations by researchers at the company that makes the additions (conference abstracts and company releases), not peer-reviewed papers. [rows: Nishimura 2015 mayonnaise vs oil; Kewpie 2016 yolk vs white; Tamura 2021 sesame dressing]

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: 4, of which 3 had people rate bitterness; the others measured something else (compounds, smell) or are used only for context.
• Rows in the table (study × measured result): 7. Rows on perceived bitterness: 5.
• Evidence rows in the lab's ledger these rows rest on: 5. 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.
• Independent research groups among the sensory studies: 1 (reports from the same lab count as 1).

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 J-STAGE, CiNii and PubMed for studies comparing green pepper bitterness or smell with oil, mayonnaise, egg, dairy, heating or cutting, and added the sources already in the lab's ledger.

Read in the encyclopedia

• Green pepper — the vegetable in this question
• Quercitrin — a compound linked to green pepper bitterness
• Research note on green pepper bitterness — following its bitter compounds and ways of eating it

Source ledger

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