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Topic hub — 2026-10-21

Does cheese bitterness change with ripening and the choice of starter bacteria?

A caveat first: few studies could be read in full down to the values, and the only one comparing strains and ripening time in a ripened cheese is a study of reduced-fat Cheddar. Ripened cheese sometimes turns bitter, and a common explanation is that milk protein (casein) is broken down by the enzymes of the starter bacteria into bitter peptides. What do studies that changed the starter strain or the ripening time and had people compare bitterness say? This page lines up the included studies together with their conditions: the cheese type, the starter strains or adjuncts, ripening temperature and time, and who rated it on what scale. It then separates what was established within one experiment, what differences between studies might explain, and what this comparison alone cannot separate. It compares studies; it is not advice on choosing or making cheese.

What this page establishes

• That in one study, starters from the same parent differing only in enzyme type clearly changed the bitterness people perceived.
• That bitterness rose with ripening, but the significant comparison was of the mean over strains, and results for ripening differences within each strain are not shown.
• That strain differences were significant at some time points and not others, and that one study's conclusions and results disagree.
• That one study of adjunct bacteria conflicts between abstract and text on the direction of bitterness, studies of yeasts could be read only as abstracts, and scales and cheese types differ between studies, so scores cannot be set side by side.

What the included studies allow us to say

• Changing the starter strain changed the bitterness people perceived in cheese. In reduced-fat Cheddar made with strains from one parent differing only in their proteinase, the mean over 3 time points was 1.5 for the strain without the enzyme and 3.9 for the group h strain (0–9 scale). Bitterness also rose with ripening: the mean of the 4 strains was 2.4 at 2 months, 2.9 at 4 months and 3.6 at 6 months, with 6 months significantly higher than the other 2 time points. [rows: Broadbent 2002 group h strain vs no-proteinase strain; Broadbent 2002 6 vs 2 months]
• But the only ripened-cheese study whose bitterness values could be checked in full text was the reduced-fat Cheddar one. In unripened fresh cheese too, strains with the proteinase gene prtP (3.9, 4.1) were more bitter than strains without it (1.7, 2.8) on a 0–5 scale. A study of adjunct bacteria could be read in full but gives no values, and studies of yeasts and other starters could be read only as abstracts. [rows: Forler 2021 prtP-positive vs prtP-negative strains; Benech 2003 Nisin-producing strain vs without nisin; Ferreira 2003 1 yeast alone vs both; Midje 2000 SK11 as main starter vs commercial starter]

Bitterness ratings at a glance

Human bitterness ratings. Values for "Group compared" and "Comparison" are as reported in the tables or text (values shown only in figures are marked as such). Besides comparisons between strains, rows on ripening time are included. For studies read only as abstracts, only the direction stated in the abstract is shown. "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.

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 cheese and strains, ripening, serving 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 the starter strain or the length of ripening changes the bitterness people perceive in cheese, 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.

• In reduced-fat Cheddar both strain and ripening time had significant effects on bitterness (P<0.0001). Over 3 time points the group a (3.1), group e (3.2) and group h (3.9) strains were significantly more bitter than the strain without the enzyme (1.5), but at 2 months only group h (3.5) was significantly more bitter, while group a (2.5) and group e (2.2) were not significantly different from the strain without the enzyme (1.4). [rows: Broadbent 2002 group h strain vs no-proteinase strain; Broadbent 2002 At 2 months]
• In unripened fresh cheese, the prtP-positive strains (3.9, 4.1) were significantly more bitter than the negative strains (1.7, 2.8). The conclusions say the negative strains did not introduce bitterness, but the Discussion reports LTH 7112 (2.8) as significantly more bitter than a control acidified without fermentation (1.0). [rows: Forler 2021 prtP-positive vs prtP-negative strains; Forler 2021 prtP-negative strain vs control]

No enzyme

group a

group e

group h

Candidate explanations for the differences

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

• A candidate explanation for strain differences is the amount of bitter peptide that the bacterial proteinase lactocepin makes from casein. In the reduced-fat Cheddar, bitterness correlated at r=0.56 with αS1-CN f1-9, a peptide made by lactocepin. The other peptide, β-CN f193-209 (r=0.58), comes from the coagulant chymosin and showed no strain effect on its concentration (P>0.5). Both are correlations, so they do not show that those peptides set the bitterness. [rows: Broadbent 2002 group h strain vs no-proteinase strain]
• Some studies report that adjunct bacteria, yeasts and the choice of main starter also move bitterness. Cheese with an adjunct L. casei was less bitter (full text). For the nisin-producing strain, the abstract (more bitter) and the text (less bitter than without nisin) conflict. Abstracts also say a single yeast alone produced bitter flavours and SK11 as the main starter gave substantially lower bitterness than a commercial starter. Values and tests were not checked. [rows: Benech 2003 Nisin-producing strain vs without nisin; Ferreira 2003 1 yeast alone vs both; Midje 2000 SK11 as main starter vs commercial starter]

What is not known — what this comparison alone cannot separate

• Between studies the cheese type (ripened reduced-fat Cheddar vs unripened fresh cheese), the scale (0–9 vs 0–5) and the panels differ, so the bitterness scores cannot be set side by side. [rows: Broadbent 2002 group h strain vs no-proteinase strain; Forler 2021 prtP-positive vs prtP-negative strains]
• Whether the same direction holds for full-fat cheese or other types such as Gouda, and how ripening time and strain combine, cannot be separated by the included studies. Only the reduced-fat Cheddar study let us check both strain and ripening time in full text. [rows: Broadbent 2002 group h strain vs no-proteinase strain; Broadbent 2002 6 vs 2 months]

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 5 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): 8. Rows on perceived bitterness: 8.
• Evidence rows in the lab's ledger these rows rest on: 0. 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: 5 (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 Europe PMC for studies in which people rated cheese bitterness while starter strains, adjunct cultures or ripening time were varied, and added the sources already in the lab's ledger (J-STAGE and CiNii were not searched). Where the full-text page stopped at a verification screen or was paywalled, the study is marked as read from the abstract only.

Read in the encyclopedia

• Cheese — the food in this question
• Bitter peptides — the candidate compounds
• Fermentation and bitter peptides — the fermentation topic hub
• Is finding broccoli, kale and other brassicas bitter down to a gene (TAS2R38)? — a comparison page in the same format

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.