日本語版

Topic hub — 2026-10-21

Does natsumikan bitterness go down with enzymes (naringinase), freezing or canning?

A caveat first: few studies found in our search had people compare natsumikan bitterness across treatments, and all are Japanese studies from about half a century ago, with few assessors or none stated. Many studies measured that treatments reduce the bitter compound naringin, but this page does not present less of the compound as less bitterness; it keeps human bitterness ratings apart. Natsumikan bitterness has long been studied as something canneries and juice makers remove with enzymes, and whether freezing lowers it is also checked here. What do studies that changed enzymes, freezing or canning and had people compare natsumikan bitterness say? This page lines up the included studies together with their conditions: sample, treatment, how it was eaten, and who rated it on what scale. It then separates what was established within one experiment, what differences might explain, and what this comparison alone cannot separate. It is about perceived taste, not eating or health advice. Boiling or soaking the peel is covered on the citrus peel bitterness page.

What this page establishes

• That under the conditions of the two included studies, frozen pulp was more bitter than fresh, and no result showed freezing lowering bitterness.
• That segments frozen with the cover on were more bitter, and freezing in liquid nitrogen raised bitterness less.
• That adding enzyme to cans tended to weaken bitterness, but of the pairs comparing enzyme with no enzyme within the same peeling method, only one was significant in the ranking by people; and that the effect of canning itself was not compared.
• That the explanation that the measured total naringin stays close after freezing while the amount released when eaten changes is a candidate, and it has not been shown to explain the difference in bitterness on its own.

What the included studies allow us to say

• Under the conditions of the 2 included studies, freezing did not lower bitterness: frozen natsumikan pulp was more bitter than fresh (Iwata 1976: −20 °C for 4 days, frozen judged more bitter in all 28 paired comparisons, score +2.9 vs −2.0; Maotani 1979: all 7 fruits frozen at −10 °C "extremely bitter", all 7 fresh "usual"). Pulp frozen in liquid nitrogen was judged less bitter than pulp frozen slowly at −20 °C, significantly after 1, 100 and 200 days of storage (not significant at 10 days). [rows: Iwata 1976 Frozen vs fresh fruit; Maotani 1979 Frozen vs fresh fruit; Iwata 1976 Slow freezing at −20 °C vs liquid nitrogen (1 days stored); Iwata 1976 Slow freezing at −20 °C vs liquid nitrogen (10 days stored); Iwata 1976 Slow freezing at −20 °C vs liquid nitrogen (100 days stored); Iwata 1976 Slow freezing at −20 °C vs liquid nitrogen (200 days stored)]
• Adding enzyme (naringinase) to the cans tended to weaken bitterness. But of the 4 pairs on this page that compare enzyme with no enzyme within the same peeling method, only 1 was significant in the ranking by 7 judges, Sankyo naringinase vs no enzyme (−0.55 vs +0.29, 5% level); the other 3 were not significant. The paper reports 7 significant pairs in all, including pairs across peeling methods. [rows: Tsusaka 1965 Sankyo naringinase added to the can (B) vs none (A); Tsusaka 1965 Kumitanase added to the can (C) vs none (A); Tsusaka 1965 Cold-acid peeled, Sankyo naringinase (E) vs none (D); Tsusaka 1965 Cold-acid peeled, Kumitanase (F) vs none (D)]
• The studies included in this table in which people compared natsumikan bitterness across treatments are four, all Japanese studies from about half a century ago, with few assessors or none stated. Other studies have only tasting remarks without the number of people or the method. The effect of canning itself was not compared; what was compared is whether enzyme acted inside the can. Many studies measured that treatments reduce the bitter compound naringin, but less of the compound is not the same as less bitterness.

Bitterness ratings at a glance

Human bitterness ratings only. Values for "Treated sample" and "Comparison" are as reported in the tables. The studies use different scales (counts, scores, ranks, categories and dilutions), so values are not compared across studies. "Direction" shows which sample was more bitter; whether the difference is significant is shown by the test in the same "Direction / test" column. Select a study name to jump to its detailed table. Rows on naringin content are in the detailed tables by study.

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 samples, treatment, how it was eaten and who rated it, and the results. Each row is one study × one measured result, so different comparisons in the same study are separate rows. Under each row we give where it is in the paper and whether it can be used to judge whether treatments such as enzymes, freezing or canning change the bitterness people perceive in natsumikan, with the reason. Composition-only rows are kept so that it is clear they cannot be used for bitterness. Nothing is left blank: "not given", "not read" and "not applicable" are kept apart.

What was established within an experiment

Only results observed in comparisons within one study. Human ratings and compound measurements are kept apart.

• When only freezing varied, frozen pulp was more bitter in both of the 2 included studies (Iwata et al.: fruit of the same origin, −20 °C for 4 days, 28 of 28 paired comparisons; Maotani et al.: fruit from the same 1 tree, −10 °C, 7 fruits vs 7). [rows: Iwata 1976 Frozen vs fresh fruit; Maotani 1979 Frozen vs fresh fruit]
• Segments frozen with the cover on were more bitter than pulp frozen after removing the cover ("more bitter" 14 vs 0 times, 0.1% level; score +3.4 vs +1.6). Even without the cover the score was +1.6, so considerable bitterness remained. [rows: Iwata 1976 Frozen with vs without segment cover]
• Pulp frozen in liquid nitrogen was more often judged less bitter than pulp frozen slowly at −20 °C, significantly after 1, 100 and 200 days of storage (not significant at 10 days). Neither was compared with fresh fruit. [rows: Iwata 1976 Slow freezing at −20 °C vs liquid nitrogen (1 days stored); Iwata 1976 Slow freezing at −20 °C vs liquid nitrogen (10 days stored); Iwata 1976 Slow freezing at −20 °C vs liquid nitrogen (100 days stored); Iwata 1976 Slow freezing at −20 °C vs liquid nitrogen (200 days stored)]
• The measured total naringin was close between frozen and fresh fruit (69.9 vs 67.3 mg% when completely triturated; in the other study, total 84.5 vs 82.5 mg/100 g). What differed was the amount in juice pressed out with a wringer: 73.0 mg% frozen vs 23.7 mg% fresh. [rows: Maotani 1979 Frozen vs fresh (naringin in juice pressed out with a wringer); Maotani 1979 Frozen vs fresh (naringin in completely triturated pulp); Iwata 1976 Frozen vs fresh (pulp naringin)]
• Cans in which enzyme acted had less pulp naringin than cans without (15.2 vs 39.2 mg%), and 7 judges ranked them less bitter (−0.55 vs +0.29, 5% level). In a limonoid solution from peel, adding an enzyme preparation lowered the dilution at which bitterness disappeared from 6 times to 2–5 times (no number of assessors or test reported). [rows: Tsusaka 1965 Sankyo naringinase added to the can (B) vs none (A); Tsusaka 1965 Sankyo naringinase (B) vs none (A) (pulp naringin); Nomura 1963 Enzyme preparation vs none (limonoid solution from peel)]

Candidate explanations for the differences

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

• To explain why freezing raises bitterness, the authors of both studies suggest that freezing damages the tissue so naringin is released more readily (the measured totals are close, but 73.0 vs 23.7 mg% comes out when pressed with a wringer). Their accounts differ, though. Iwata et al. emphasise naringin leaching out and collecting at the pulp surface, and also write that naringin content alone cannot explain the difference in bitterness. Maotani et al. consider that unlikely to be the main cause and emphasise leaching into the juice sacs and release during chewing. The smaller rise without the segment cover is consistent with both. [rows: Maotani 1979 Frozen vs fresh (naringin in juice pressed out with a wringer); Maotani 1979 Frozen vs fresh (naringin in completely triturated pulp); Iwata 1976 Frozen with vs without segment cover]
• Differences in sourness may have mixed into the bitterness ranking. Tsusaka notes that some samples with more naringin were ranked less bitter (pre-heated and soaked G and H were judged less bitter than cold-acid E and F), suggests that because G and H had less acid, sourness may have influenced the judgement, and writes that sourness has a considerable effect on bitterness. That the enzyme difference was significant in the usual-peeling pair but not in the cold-acid pairs cannot be read apart from sourness. [rows: Tsusaka 1965 Sankyo naringinase added to the can (B) vs none (A); Tsusaka 1965 Cold-acid peeled, Sankyo naringinase (E) vs none (D); Tsusaka 1965 Cold-acid peeled, Kumitanase (F) vs none (D)]

What is not known — what this comparison alone cannot separate

• Taken together, the 8 canned lots differ in peeling, pre-heating, sterilisation and acid level as well as enzyme. Within the same peeling method the enzyme can be compared, but only by the ranking of 7 judges, with no training described. The no-enzyme controls are lots whose enzyme column is blank in the table, read as such. [rows: Tsusaka 1965 Sankyo naringinase added to the can (B) vs none (A); Tsusaka 1965 Kumitanase added to the can (C) vs none (A); Tsusaka 1965 Cold-acid peeled, Sankyo naringinase (E) vs none (D); Tsusaka 1965 Cold-acid peeled, Kumitanase (F) vs none (D)]
• The limonoid dilution result concerns a solution extracted from peel; how much the bitterness of natsumikan pulp or cans changes cannot be told from this comparison. Neither the number of assessors nor a test is reported. [rows: Nomura 1963 Enzyme preparation vs none (limonoid solution from peel)]
• The evidence is thin. The 2 freezing studies differ in freezing temperature (−20 °C and −10 °C) and in how bitterness was rated, and Iwata et al. do not give the number of assessors. Bitterness under home-freezer conditions, or when juice is frozen, has not been compared in numbers (Iwata et al. write that frozen juice did not become more bitter, but give no table). The effect of canning itself was not compared either. The enzyme tasting remark (Exp. 2) gives no number of tasters or method. [rows: Iwata 1976 Frozen vs fresh fruit; Maotani 1979 Frozen vs fresh fruit; Tsusaka 1965 Enzyme added to the can vs none (Exp. 2 tasting)]

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 4 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): 19. Rows on perceived bitterness: 14.
• 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: 4 (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 and why each was included or excluded. We searched J-STAGE and university repositories for studies comparing natsumikan (natsudaidai, amanatsu) bitterness by enzyme, freezing, heating, canning, resin, variety or harvest time, and added the sources already in the lab's ledger (the J-STAGE search screen itself was not used, and old trade-journal articles were not reached). Composition-only studies without human ratings are listed with the reason they were excluded. The search described here is a record, not a claim that nothing was missed.

Read in the encyclopedia

• Natsumikan — the fruit in this question
• Naringin — the main bitter compound in natsumikan
• Enzymatic naringinase debittering of citrus juice — removing the bitter compound with an enzyme
• Does boiling or soaking cut citrus peel bitterness? — a comparison of peel preparation

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.