Topic hub — 2026-10-22
Does post-fermentation or ageing change the bitterness of dark tea?
This page collects studies on dark teas such as pu-erh and Liupao and other post-fermented teas in which people rated bitterness: studies comparing the post-fermentation process of the same raw tea, and also studies comparing stored samples from different production years. Differences between kinds of tea, roasting and water temperature are not covered here (roasting is on the tea roasting page and water temperature on the green tea temperature page). In the included studies, there are results in which perceived bitterness weakened during pile fermentation of the same raw tea. However, one study found bitterness rose partway through fermentation before falling, and one study of pickled tea reports bitterness rising as fermentation went on. Studies comparing stored teas from different production years report lower bitterness in the longer-stored samples, but production year and manufacturer differ together, so the effect of storage time alone cannot be isolated. Scales, raters and brewing differ by study, so it cannot be said across studies how much bitterness falls in how many years, and in many studies the values are only in figures that we could not obtain. In the list, rows following fermentation of the same raw tea come first, and rows of a different nature are marked with a tag at the start of the label (storage, heat pretreatment and fermentation, added microbe, pickled tea) and grouped after them. Bitterness and astringency are read as separate items, and composite words such as mellow or mellow-and-thick are not read as low bitterness. This page lines up the included studies together with their conditions: the raw tea, the fermentation or storage, the brewing, and who rated them 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 advice on how to drink or store tea. Tea bitterness as a whole is covered on the tea bitterness and astringency page, and Japanese post-fermented teas in the research note on Japanese post-fermented teas.
On this page (10 sections)
- What this page establishes
- What the included studies allow us to say
- Bitterness ratings at a glance
- Conditions and results, study by study
- What was established within an experiment
- Candidate explanations for the differences
- What is not known — what this comparison alone cannot separate
- How studies and rows were counted
- What we looked at
- Read in the encyclopedia
What this page establishes
- That some studies found perceived bitterness weakened during pile fermentation of the same raw tea, while in many of them the values are only in figures and unconfirmed.
- That one study found bitterness first rose and then fell during fermentation, so bitterness cannot be said to always fall in one direction.
- That one study of pickled tea (fermented sealed underground in bamboo tubes) reports bitterness rising as fermentation went on, with a process different from dark-tea pile fermentation.
- That comparisons of stored teas from different production years report lower bitterness in the longer-stored samples, but in all of them production year varies together with storage time (and in some, manufacturer or blend also differs), so the effect of storage time alone cannot be isolated.
- That in a study fermenting with a fungus after heat pretreatment the pretreatment also differs, and in a study adding microbes it cannot be confirmed whether the control without an added microbe was held under the same conditions, so in neither can the effect of the microbe alone be stated.
- That for Japanese post-fermented teas, this search found no study in which people rated bitterness.
What the included studies allow us to say
- In studies that followed post-fermentation of the same raw tea with people rating bitterness, there are results in which bitterness weakened. In a study of 36-day pile fermentation of ancient-tree raw tea (Wang T et al. 2023), the end-point bitterness of 3.73 was significantly lower than the raw material, and in a study comparing raw tea with the ripe tea made from the same batch (Deng et al. 2022), the ripe tea was also significantly less bitter. However, in a study of 60-day pile fermentation of Yinghong No. 9 raw tea (Yang W et al. 2025), bitterness first rose from 3 at day 0 to 5 at day 20, then fell to "−" (no noticeable taste) at 60 days (no sensory test is shown). For Bulang pickled tea (Zhou J et al. 2025), bitterness is reported higher after 3 months of fermentation. [rows: Wang T 2023 Raw sun-dried green tea (RM) vs end of 36-day pile fermentation (F4); Deng 2022 Sun-dried green tea (SGT) vs the ripe tea pile-fermented from it (PRT); Yang W 2025 Pile fermentation day 0 vs day 20; Yang W 2025 Pile fermentation day 0 vs day 60; Zhou J 2025 Pickled tea: Bulang pickled tea at 0 months (B0) vs 3 months (B3) of fermentation]
- Studies comparing stored teas from different production years (Bo et al. 2026, Zheng X et al. 2026, Ma B et al. 2025) report lower bitterness in the longer-stored samples; Zheng X et al. give 8.60 at 2 years and 2.95 for the 1980s. However, in every one of these studies the samples compared differ in production year. In Zheng X et al. manufacturer and blend also differ, and Ma B et al. include the same 6 series in each group but with separate samples per group, and the raw material and processing within each series were not checked. The effect of storage time alone cannot be isolated. A study adding microbes (Wang X et al. 2026) and a study fermenting with a fungus after heat pretreatment (Xu et al. 2025) also report lower bitterness values or descriptions. In Xu et al. the pretreatment also differs, and in Wang X et al. it cannot be confirmed whether the control without an added microbe was held under the same 7-day conditions. [rows: Bo 2026 Storage (samples from different production years): text description over 5 storage stages (1–9 years) of ripened tea "Laotongzhi 9978"; Zheng X 2026 Storage (samples from different production years): ripened tea stored 2 years vs the 1980s (80ND); Ma B 2025 Storage (samples from different production years): raw pu-erh new tea (up to 2 years) vs aged tea (over 8 years); Wang X 2026 Added microbe: Liupao tea without added microbe (N/A) vs 7 days with E. cristatum (8730); Xu 2025 Heat pretreatment and fermentation: raw summer–autumn green tea (CK) vs 8 days of E. cristatum fermentation after pretreatment]
- Scales, raters and brewing differ by study, so it cannot be said across studies how much bitterness falls in how many years. Bitterness and astringency are read as separate items, and composite words such as mellow or mellow-and-thick, or bitter-astringent (bitterness plus astringency), are not read as low bitterness. For Japanese post-fermented teas (Goishicha, Awa-bancha, Ishizuchi-kurocha and others), this search found no study in which people rated bitterness. This page lines up studies of perceived taste and is not advice on how to drink or store tea.
Bitterness ratings at a glance
Human bitterness ratings only. Raw tea, brewing and scales differ by study, so values are not compared across studies. The two samples compared are set out as before or shorter (or without an added microbe) and after or longer (or with an added microbe). The "Direction" column shows which sample was more bitter ("... more bitter"); in every row on this page the larger value is the more bitter one. The direction shows which published value is larger, or what the text states, and is not necessarily the result of an individual test between the two conditions shown. Whether the difference is significant is shown by the test in the same "Direction / test" column. Rows with no test shown and rows whose values are only in figures we did not obtain are kept as they are. Labels begin with a tag for rows of a different nature; rows without a tag compare fermentation of the same raw tea and come first in the list. "Storage (samples from different production years)" marks comparisons that line up samples made in different years rather than following the same tea for years; "Heat pretreatment and fermentation" marks a comparison in which steaming, piling and sterilisation differ together with fungal fermentation; "Added microbe" marks comparisons that add a microbe to an already fermented tea; and "Pickled tea" marks a comparison of fermentation sealed underground in bamboo tubes. 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.
| Study | Comparison and condition | Measured as | Before, shorter or without added microbe | After, longer or with added microbe | Direction / test |
|---|---|---|---|---|---|
| Wang T 2023 | Raw sun-dried green tea (RM) vs end of 36-day pile fermentation (F4) Sun-dried green tea (RM) made from 1 bud and 3 leaves of ancient tea plants in Laobanzhang village, piled unsterilised and without a starter at about 40% moisture and about 1 m high, turned 4 times and ended at 36 days (no pile replication described); 3 g in 150 mL boiling water for 5 min | intensity from 0 (none) to 10 (very strong) (9 trained panellists) | Figure not obtained, so values unconfirmed | 3.73 | Before, shorter or without added microbe more bitter Tested, significant |
| Yang W 2025 | Pile fermentation day 0 vs day 20 500 kg of Yinghong No. 9 sun-dried green tea piled, with water added and the pile turned every 10 days, moisture 20–25% and pile temperature 50–55 °C, up to 60 days (a single pile); Table 1 bitterness at 0, 10, 20, 30, 40, 50 and 60 days was 3, 4, 5, 4, 2, 1 and "−"; taste rated on the second infusion (3 g, 150 mL boiling water, 5 min) | 10-point scale (10 trained experts); integers in Table 1, not stated whether means or consensus values; "−" means no noticeable taste | 3 | 5 | After, longer or with added microbe more bitter No test reported |
| Yang W 2025 | Pile fermentation day 0 vs day 60 500 kg of Yinghong No. 9 sun-dried green tea piled, with water added and the pile turned every 10 days, moisture 20–25% and pile temperature 50–55 °C, up to 60 days (a single pile); Table 1 bitterness at 0, 10, 20, 30, 40, 50 and 60 days was 3, 4, 5, 4, 2, 1 and "−"; taste rated on the second infusion (3 g, 150 mL boiling water, 5 min) | 10-point scale (10 trained experts); integers in Table 1, not stated whether means or consensus values; "−" means no noticeable taste | 3 | "−" (no noticeable taste; not given as 0) | Before, shorter or without added microbe more bitter No test reported |
| Deng 2022 | Sun-dried green tea (SGT) vs the ripe tea pile-fermented from it (PRT) Sun-dried green tea from 2 piles of 2020 (Rong's No. 1 and No. 2) and the ripe tea made from each, stated to be from the same batch; fermentation days and temperature not given; 3 g in 150 mL boiling water, infusions of 2 and 5 min mixed and rated at 50 °C | 0–10 (5 trained panellists) | Figure not obtained, so values unconfirmed | Figure not obtained, so values unconfirmed | Before, shorter or without added microbe more bitter Tested, significant |
| Bo 2026 | Storage (samples from different production years): text description over 5 storage stages (1–9 years) of ripened tea "Laotongzhi 9978" Cakes of one product with the same recipe and protocol from 5 different production years (stored 1, 3, 5, 7 and 9 years, 3 cakes per year), stored in a Kunming warehouse; 3 g in 150 mL boiling water for 5 min, rated at 55 ± 2 °C | intensity from 0 (none) to 10 (very strong) (9 trained panellists, mean of 3 cakes per year) | Figure not obtained, so values unconfirmed | Figure not obtained, so values unconfirmed | Before, shorter or without added microbe more bitter Tested; significance reported, but which pair differs is unclear |
| Zheng X 2026 | Storage (samples from different production years): ripened tea stored 2 years vs the 1980s (80ND) A cross-sectional comparison of 23 ripened teas from several manufacturers, blends and warehouses, grouped into 9 stages from 2 years to the 1980s; bitterness at 15 years was 7.20; 3.0 g in 150 mL boiling water for 5 min | structured 10-point scale, 0 none, 5 moderate, 10 very strong (5 trained assessors, 3 replicates on separate days) | 8.60 | 2.95 | Before, shorter or without added microbe more bitter Tested; significance reported, but which pair differs is unclear |
| Ma B 2025 | Storage (samples from different production years): raw pu-erh new tea (up to 2 years) vs aged tea (over 8 years) 18 raw teas with samples of new, aging (3–7 years) and aged tea for each of the same 6 series (6 series in every group; separate samples from different years in each group); warehouses in Kunming and Guangzhou; 3.0 g in 150 mL boiling water for 2 and 5 min (emphasis on the second infusion) | 0–10 (6 trained tasters, 3 times at different tasting temperatures) | Figure not obtained, so values unconfirmed | Figure not obtained, so values unconfirmed | Before, shorter or without added microbe more bitter Tested; significance reported, but which pair differs is unclear |
| Xu 2025 | Heat pretreatment and fermentation: raw summer–autumn green tea (CK) vs 8 days of E. cristatum fermentation after pretreatment 10 g summer–autumn green tea at 40% moisture, steamed (100 °C, 10 min), piled (70 °C, 1.5 h) and sterilised (121 °C, 20 min), then inoculated with E. cristatum and incubated at 28 °C for 8 days; 3 g in 150 mL boiling water for 4 min | intensity from 0 (none) to 10 (very strong) (9 trained panellists, 3 replicates) | Figure not obtained, so values unconfirmed | Figure not obtained, so values unconfirmed | Before, shorter or without added microbe more bitter Tested, significant |
| Wang X 2026 | Added microbe: Liupao tea without added microbe (N/A) vs 7 days with B. coagulans (No. 10059) Liupao tea after traditional fermentation (N/A) used as the starting material and further fermented for 7 days with one strain each (50 °C in the methods, 40–60 °C in the results); 3.0 g in 150 mL boiling water for 8 min | scale of 0 (none), 5 (moderate) and 10 (high) (6 trained panellists, mean of 3 sessions) | 4.25 | 3.00 | Before, shorter or without added microbe more bitter Test method described; result for this comparison not reported |
| Wang X 2026 | Added microbe: Liupao tea without added microbe (N/A) vs 7 days with B. coagulans (JZ-1) Liupao tea after traditional fermentation (N/A) used as the starting material and further fermented for 7 days with one strain each (50 °C in the methods, 40–60 °C in the results); 3.0 g in 150 mL boiling water for 8 min | scale of 0 (none), 5 (moderate) and 10 (high) (6 trained panellists, mean of 3 sessions) | 4.25 | 3.50 | Before, shorter or without added microbe more bitter Test method described; result for this comparison not reported |
| Wang X 2026 | Added microbe: Liupao tea without added microbe (N/A) vs 7 days with E. cristatum (8730) Liupao tea after traditional fermentation (N/A) used as the starting material and further fermented for 7 days with one strain each (50 °C in the methods, 40–60 °C in the results); 3.0 g in 150 mL boiling water for 8 min | scale of 0 (none), 5 (moderate) and 10 (high) (6 trained panellists, mean of 3 sessions) | 4.25 | 2.75 | Before, shorter or without added microbe more bitter Test method described; result for this comparison not reported |
| Zhou J 2025 | Pickled tea: Bulang pickled tea at 0 months (B0) vs 3 months (B3) of fermentation 1 bud and 2 leaves withered, boiled for 20 min, packed into bamboo tubes, sealed and fermented underground; sampled at 0–3 months and sun-dried; 3 g sun-dried leaves in 150 mL boiling pure water for 5 min | a 9-point scale from 0 to 9 in the methods but a 6-point intensity scale in the results text, inconsistent within the source (10 tea evaluation experts) | Figure not obtained, so values unconfirmed | 3.6 | After, longer or with added microbe more bitter Test method described; result for this comparison not reported |
Conditions and results, study by study
For each study, the conditions for the raw tea, the fermentation or storage, the brewing and who rated them, 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 changing the length or method of post-fermentation or storage of the same raw tea changes the intensity of bitterness people perceive, with the reason. Nothing is left blank: "not given", "not read" and "not applicable" are kept apart, and values from figures we did not obtain are marked as such.
Wang T 2023 sensory only
Changes in sensory characteristics, chemical composition and microbial succession during fermentation of ancient plants Pu-erh tea. Food Chemistry: X — doi:10.1016/j.fochx.2023.101003
- How deeply checked
- Checked in full text — Full text via Europe PMC (methods "Fermentation of pu-erh tea and sample collection", "Sensory evaluation of tea leaves" and "Statistical analysis"; results "Changes in sensory characteristics"); Fig. 1B with the bitterness values and Table S1 not obtained Checked by: LLM (one extracted, another checked against the source; no human check), 2026-10-22.
- Samples
- 1 bud and 3 leaves from ancient tea plants in Laobanzhang village, Xishuangbanna; spread, fixed, rolled and sun-dried into sun-dried green tea (raw material, RM)
- Processing and cooking
- Pile-fermented by the traditional method in June–July 2022; raw material, water and containers not sterilised, no starter; about 40% moisture, about 1 m high, turned 4 times according to pile temperature and ended at 36 days; sampled at each turning (F1–F4, F4 the end point); no pile replication described
- How it was served or eaten
- 3 g dry tea in 150 mL boiling water for 5 min; sensory room at 25 °C
- Who rated, on what scale
- 9 trained panellists (5 men and 4 women, aged 24–48); QDA (bitterness, astringency, thickness, sweetness, sourness, umami) from 0 (undetectable) to 10 (very strong); one-way ANOVA with Dunnett multiple comparisons (p<0.05)
| Measured | Comparison and condition | Before, shorter or without added microbe | After, longer or with added microbe | Unit | Direction / test |
|---|---|---|---|---|---|
| Bitterness intensity (sensory) | Raw sun-dried green tea (RM) vs end of 36-day pile fermentation (F4) Sun-dried green tea (RM) made from 1 bud and 3 leaves of ancient tea plants in Laobanzhang village, piled unsterilised and without a starter at about 40% moisture and about 1 m high, turned 4 times and ended at 36 days (no pile replication described); 3 g in 150 mL boiling water for 5 min | Figure not obtained, so values unconfirmed | 3.73 | intensity from 0 (none) to 10 (very strong) (9 trained panellists) | Before, shorter or without added microbe more bitter Tested, significant |
| From the text: F4 was significantly less bitter than RM (one-way ANOVA with Dunnett multiple comparisons, p<0.05). The values for RM and the intermediate F1–F3 are only in Fig. 1B. Figure not obtained, so values unconfirmed, so whether the decline was steady cannot be shown. Astringency was also lower at F4 (3.57), while sweetness (6.97) and thickness (7.23) were higher. Bitterness and astringency were rated as separate items Where: Methods, "Fermentation of pu-erh tea and sample collection", "Sensory evaluation of tea leaves" and "Statistical analysis"; Results, "Changes in sensory characteristics"; Fig. 1B (not obtained) For whether changing the length or method of post-fermentation (pile fermentation, added microbes, pickling) or storage for the same raw tea changes the intensity of bitterness people perceive: Usable — The same raw tea compared before and after pile fermentation, brewed the same way, rated by 9 trained panellists and tested; however no pile replication is described and the raw-material value is only in a figure and unconfirmed | |||||
Yang W 2025 sensory only
Effects of Pile-Fermentation Duration on the Taste Quality of Single-Cultivar Large-Leaf Dark Tea: Insights from Metabolomics and Microbiomics. Foods 14(4):670 — doi:10.3390/foods14040670
- How deeply checked
- Checked in full text — Full text via Europe PMC (sections 2.2, 2.3, 2.6, 3.1 and 3.5; Table 1) Checked by: LLM (one extracted, another checked against the source; no human check), 2026-10-22.
- Samples
- Sun-dried green tea of 1 bud and 2–3 leaves from the single large-leaf cultivar Yinghong No. 9 (July–September 2019)
- Processing and cooking
- 500 kg of sun-dried green tea piled and sprayed with water at 20% of its weight; water added and the pile turned every 10 days, moisture 20–25%, pile temperature 50–55 °C (following a Guangdong standard); sampled at 0, 10, 20, 30, 40, 50 and 60 days (5 replicates each)
- How it was served or eaten
- 3 g in 150 mL freshly boiled distilled water for 2 min (first infusion, for colour), then another 150 mL for 5 min (second infusion, for taste and aroma)
- Who rated, on what scale
- 10 trained experts (4 men and 6 women, aged 25–55); GB/T 23776-2018; bitter, astringent, sweet, sour and fresh tastes on a 10-point scale; whether Table 1 values are means or consensus values is not stated; no test method for sensory comparisons is given
| Measured | Comparison and condition | Before, shorter or without added microbe | After, longer or with added microbe | Unit | Direction / test |
|---|---|---|---|---|---|
| Bitterness intensity (sensory) | Pile fermentation day 0 vs day 20 500 kg of Yinghong No. 9 sun-dried green tea piled, with water added and the pile turned every 10 days, moisture 20–25% and pile temperature 50–55 °C, up to 60 days (a single pile); Table 1 bitterness at 0, 10, 20, 30, 40, 50 and 60 days was 3, 4, 5, 4, 2, 1 and "−"; taste rated on the second infusion (3 g, 150 mL boiling water, 5 min) | 3 | 5 | 10-point scale (10 trained experts); integers in Table 1, not stated whether means or consensus values; "−" means no noticeable taste | After, longer or with added microbe more bitter No test reported |
| The direction shows which value in Table 1 is larger. The methods give no test for the sensory comparisons (the statistics section covers only metabolites), and Table 1 shows no errors or significance marks. The text also says "the bitterness slightly increased in the early stage, then consistently decreased". However, the Fig. 1A description calls day 10 "slightly reduced bitter, astringent", and the mechanism in section 3.5 says bitterness and astringency fell in the early stage, which does not match Table 1 for bitterness. This page follows Table 1 Where: Sections 2.2 and 2.3; section 3.1; Table 1; section 3.5 For whether changing the length or method of post-fermentation (pile fermentation, added microbes, pickling) or storage for the same raw tea changes the intensity of bitterness people perceive: Limited — Time points of the same pile of the same raw tea, but there is no test for the sensory comparison, and the table values are integers whose nature (mean or consensus) is unknown; a single pile | |||||
| Bitterness intensity (sensory) | Pile fermentation day 0 vs day 60 500 kg of Yinghong No. 9 sun-dried green tea piled, with water added and the pile turned every 10 days, moisture 20–25% and pile temperature 50–55 °C, up to 60 days (a single pile); Table 1 bitterness at 0, 10, 20, 30, 40, 50 and 60 days was 3, 4, 5, 4, 2, 1 and "−"; taste rated on the second infusion (3 g, 150 mL boiling water, 5 min) | 3 | "−" (no noticeable taste; not given as 0) | 10-point scale (10 trained experts); integers in Table 1, not stated whether means or consensus values; "−" means no noticeable taste | Before, shorter or without added microbe more bitter No test reported |
| The direction is from Table 1. The methods give no test for the sensory comparisons (the statistics section covers only metabolites), and Table 1 shows no errors or significance marks. The abstract and text say bitterness and astringency were "significantly" reduced at 60 days, but no supporting test is shown. Astringency fell steadily from 8 at day 0, and sweetness appeared from day 30. The overall score was highest at 60 days (96) Where: Section 2.3; section 3.1; Table 1; abstract For whether changing the length or method of post-fermentation (pile fermentation, added microbes, pickling) or storage for the same raw tea changes the intensity of bitterness people perceive: Limited — Time points of the same pile of the same raw tea, but there is no test for the sensory comparison, and the table values are integers whose nature (mean or consensus) is unknown; a single pile | |||||
Deng 2022 sensory only
Mellow and Thick Taste of Pu−Erh Ripe Tea Based on Chemical Properties by Sensory−Directed Flavor Analysis. Foods 11(15):2285 — doi:10.3390/foods11152285
- How deeply checked
- Checked in full text — Full text via Europe PMC (sections 2.1–2.3, 2.9 and 3.1; Table 1; the Fig. 1 legend); Fig. 1 with the bitterness values and Table S1 not obtained Checked by: LLM (one extracted, another checked against the source; no human check), 2026-10-22.
- Samples
- Yunnan sun-dried green tea (SGT; Rong's No. 1 and No. 2 piles of 2020) and the pu-erh ripe tea (PRT) pile-fermented from each; all stated to be from the same batch; supplied by a tea company in Lincang
- Processing and cooking
- Pile fermentation (days, temperature and other conditions not given)
- How it was served or eaten
- 3 g in 150 mL boiling water for 2 min, then the same leaves in another 150 mL for 5 min; the two infusions mixed and cooled to 50 °C for rating
- Who rated, on what scale
- 5 trained panellists; bitterness, astringency, mellow-and-thick taste, sweetness and acceptability on 0–10; one-way ANOVA and Duncan test (p<0.05)
| Measured | Comparison and condition | Before, shorter or without added microbe | After, longer or with added microbe | Unit | Direction / test |
|---|---|---|---|---|---|
| Bitterness intensity (sensory) | Sun-dried green tea (SGT) vs the ripe tea pile-fermented from it (PRT) Sun-dried green tea from 2 piles of 2020 (Rong's No. 1 and No. 2) and the ripe tea made from each, stated to be from the same batch; fermentation days and temperature not given; 3 g in 150 mL boiling water, infusions of 2 and 5 min mixed and rated at 50 °C | Figure not obtained, so values unconfirmed | Figure not obtained, so values unconfirmed | 0–10 (5 trained panellists) | Before, shorter or without added microbe more bitter Tested, significant |
| From the text: bitterness differed significantly between the two infusions (p<0.01, one-way ANOVA and Duncan test), with bitterness attenuated in the ripe tea. Values are in Fig. 1 and Table S1. Figure not obtained, so values unconfirmed. Astringency was also weaker and mellow-and-thick taste and sweetness stronger in the ripe tea; mellow-and-thick is a separate item and is not read as low bitterness Where: Sections 2.1 (Table 1), 2.2, 2.3 and 2.9; section 3.1; Fig. 1 (not obtained) For whether changing the length or method of post-fermentation (pile fermentation, added microbes, pickling) or storage for the same raw tea changes the intensity of bitterness people perceive: Limited — Raw tea and the ripe tea made from the same batch, with a test, but fermentation days and conditions are not given and values are only in a figure and unconfirmed | |||||
Bo 2026 sensory only
Metabolomic and Sensory Insights into the Aging Mechanism of Ripened Pu-Erh Tea over Nine Years. Molecules 31(11):1937 — doi:10.3390/molecules31111937
- How deeply checked
- Checked in full text — Full text via Europe PMC (sections 2.1, 3.2, 3.3 and 3.7; the Fig. 1 legend); Fig. 1B with the bitterness values by year and Table S1 not obtained Checked by: LLM (one extracted, another checked against the source; no human check), 2026-10-22.
- Samples
- Ripened pu-erh cakes "Laotongzhi 9978" (357 g each, a Kunming tea company); same recipe and same fermentation and pressing protocol, raw material from Menghai; samples from 5 different production years, stored 1, 3, 5, 7 and 9 years, 3 cakes per year (15 in total)
- Processing and cooking
- Stored in original packaging in a dedicated warehouse in Kunming, annual mean 15–25 °C and 50–70% relative humidity
- How it was served or eaten
- 3 g in 150 mL boiling water for 5 min; taste rated at 55 ± 2 °C
- Who rated, on what scale
- 9 trained panellists (5 men and 4 women, aged 24–48) rated each of the 3 cakes per year; QDA from 0 (undetectable) to 10 (very strong); one-way ANOVA and Duncan test (p<0.05)
| Measured | Comparison and condition | Before, shorter or without added microbe | After, longer or with added microbe | Unit | Direction / test |
|---|---|---|---|---|---|
| Bitterness intensity (sensory) | Storage (samples from different production years): text description over 5 storage stages (1–9 years) of ripened tea "Laotongzhi 9978" Cakes of one product with the same recipe and protocol from 5 different production years (stored 1, 3, 5, 7 and 9 years, 3 cakes per year), stored in a Kunming warehouse; 3 g in 150 mL boiling water for 5 min, rated at 55 ± 2 °C | Figure not obtained, so values unconfirmed | Figure not obtained, so values unconfirmed | intensity from 0 (none) to 10 (very strong) (9 trained panellists, mean of 3 cakes per year) | Before, shorter or without added microbe more bitter Tested; significance reported, but which pair differs is unclear |
| The direction follows the text description across the 5 storage stages ("bitterness (2.5~1.17) significantly decreased (p<0.05)" as storage progressed), not a check of the 1-year and 9-year means. The text refers to Fig. 1B and Table S1, neither of which was obtained, so which value belongs to which year and any pairwise results are unconfirmed. Production year differs together with storage time (these are samples of the same product made in different years, not the same tea followed for 9 years). Astringency also fell (3.17~1.67), while sweetness and thickness rose Where: Section 2.1; sections 3.2 (samples), 3.3 (sensory) and 3.7 (statistics); Fig. 1B (not obtained) For whether changing the length or method of post-fermentation (pile fermentation, added microbes, pickling) or storage for the same raw tea changes the intensity of bitterness people perceive: Limited — Same product and recipe, but each storage time is a different production year, so the effect of storage time alone cannot be isolated; which value belongs to which year is also unconfirmed | |||||
Zheng X 2026 sensory only
Decoding long-term aging of ripened Pu-erh tea: sensory equilibrium, metabolic pathways, and chemical markers. Food Chemistry: X 35:103755 — doi:10.1016/j.fochx.2026.103755
- How deeply checked
- Checked in full text — Full text via Europe PMC (sample, sensory and statistics sections; results "Sensory profiles and their physicochemical basis across storage duration"; the Fig. 1 legend); Fig. 1A and Table S1 not obtained Checked by: LLM (one extracted, another checked against the source; no human check), 2026-10-22.
- Samples
- 23 ripened pu-erh teas (loose and compressed) made from assamica sun-dried green tea under the national standard, mostly benchmark blends from several well-known manufacturers; 9 stages of 2, 5, 10, 15, 20, 25, 30 and 35 years and the 1980s (80ND), with separate samples per stage (3 at 15 years)
- Processing and cooking
- Naturally stored in warehouses in Guangdong (Dongguan, Foshan) and Yunnan (Xishuangbanna) (annual mean about 21–22 °C, humidity around 70–75%)
- How it was served or eaten
- 3.0 g in 150 mL boiling water for 5 min; about 5 mL tasted; room at 21 ± 1 °C
- Who rated, on what scale
- 5 trained assessors (3 men and 2 women, aged 23–58, over 90 h of training); 6 QDA descriptors on a structured 10-point scale (0 none, 5 moderate, 10 very strong), 3 replicates on separate days; one-way ANOVA with Student t-tests for pairwise comparisons
| Measured | Comparison and condition | Before, shorter or without added microbe | After, longer or with added microbe | Unit | Direction / test |
|---|---|---|---|---|---|
| Bitterness intensity (sensory) | Storage (samples from different production years): ripened tea stored 2 years vs the 1980s (80ND) A cross-sectional comparison of 23 ripened teas from several manufacturers, blends and warehouses, grouped into 9 stages from 2 years to the 1980s; bitterness at 15 years was 7.20; 3.0 g in 150 mL boiling water for 5 min | 8.60 | 2.95 | structured 10-point scale, 0 none, 5 moderate, 10 very strong (5 trained assessors, 3 replicates on separate days) | Before, shorter or without added microbe more bitter Tested; significance reported, but which pair differs is unclear |
| The direction shows which value given in the text is larger. The text says QDA differed significantly among samples overall (p<0.001), The values 8.60 and 2.95 were confirmed in the text, but how the significance marks in Fig. 1A apply to this pair is unconfirmed because the figure was not obtained. Statistics were one-way ANOVA and pairwise Student t-tests. The abstract calls the 15-year sample "mild bitterness" and the text "moderate bitterness (7.20)", while 5 is moderate on the scale in the methods, so the words and the value do not match; the 15-year sample is not read as weakly bitter. The electronic tongue showed a bitterness peak at 25 years, separate from the human ratings Where: Methods, "Tea samples", "Quantitative descriptive analysis (QDA)" and "Statistical analyses"; Results, "Sensory profiles and their physicochemical basis across storage duration"; abstract; Fig. 1A (not obtained) For whether changing the length or method of post-fermentation (pile fermentation, added microbes, pickling) or storage for the same raw tea changes the intensity of bitterness people perceive: Limited — A cross-sectional comparison of ripened teas from different manufacturers, blends and warehouses arranged by storage time, so the effect of storage time alone cannot be isolated; the test for this pair of stages is unconfirmed | |||||
Ma B 2025 sensory only
Quantitative descriptive analysis, non-targeted metabolomics and molecular docking reveal the dynamic aging and taste formation mechanism in raw Pu-erh tea during the storage. Food Chemistry: X 25:102234 — doi:10.1016/j.fochx.2025.102234
- How deeply checked
- Checked in full text — Full text via Europe PMC (sample, sensory, QDA and statistics sections; results "Sensory evaluation result and taste evolution during the storage"; the Fig. 1 legend); Fig. 1B and Tables S1 and S4 not obtained Checked by: LLM (one extracted, another checked against the source; no human check), 2026-10-22.
- Samples
- 18 compressed raw pu-erh teas: 6 series (DZ, LC, FP, YW, MZ, SY) × 3 stages, new tea (up to 2 years), aging tea (3–7 years) and aged tea (over 8 years); the raw material and origin of each series are in Table S1, not read
- Processing and cooking
- Naturally stored in dedicated warehouses in Kunming (annual mean 15 °C, 60% humidity) and Guangzhou (annual mean 22 °C, 77% humidity)
- How it was served or eaten
- 3.0 g in 150 mL boiling water for 2 min, then another 150 mL for 5 min (emphasis on the second infusion); about 5 mL tasted for QDA; room at 21 ± 1 °C
- Who rated, on what scale
- 6 trained tasters (3 men and 3 women, aged 22–45, over 20 h of training); 6 tastes on 0–10; each sample rated 3 times at different tasting temperatures; statistics in the methods include t-tests and one-way ANOVA with Tukey HSD
| Measured | Comparison and condition | Before, shorter or without added microbe | After, longer or with added microbe | Unit | Direction / test |
|---|---|---|---|---|---|
| Bitterness intensity (sensory) | Storage (samples from different production years): raw pu-erh new tea (up to 2 years) vs aged tea (over 8 years) 18 raw teas with samples of new, aging (3–7 years) and aged tea for each of the same 6 series (6 series in every group; separate samples from different years in each group); warehouses in Kunming and Guangzhou; 3.0 g in 150 mL boiling water for 2 and 5 min (emphasis on the second infusion) | Figure not obtained, so values unconfirmed | Figure not obtained, so values unconfirmed | 0–10 (6 trained tasters, 3 times at different tasting temperatures) | Before, shorter or without added microbe more bitter Tested; significance reported, but which pair differs is unclear |
| From the text: a one-way ANOVA over the 3 groups found a significant difference in bitterness (P<0.01), with new tea the most bitter and bitterness declining gradually with storage. The text refers to Table S4 (the one-way ANOVA over 3 groups) and Fig. 1B, neither of which was obtained, so the values and any pairwise result for new versus aged tea are unconfirmed. The P<0.05 in the abstract does not contradict P<0.01. The 6 are "series", not confirmed to be the same brand or raw material Where: Methods, "Materials and reagents", "QDA of tea tastes" and "Statistical analyses"; Results, "Sensory evaluation result and taste evolution during the storage"; Fig. 1B (not obtained) For whether changing the length or method of post-fermentation (pile fermentation, added microbes, pickling) or storage for the same raw tea changes the intensity of bitterness people perceive: Limited — Each group contains the same 6 series, but each group has separate samples from different years, and the raw material and processing within each series were not checked (supplementary table); values and pairwise tests are also unconfirmed, so this cannot be confirmed as the effect of storage time alone | |||||
Xu 2025 sensory only
Effects of Solid-State Fermentation with Eurotium cristatum on the Physicochemical, Sensory, and Volatile Profiles of Summer–Autumn Green Tea. Foods 14(21):3681 — doi:10.3390/foods14213681
- How deeply checked
- Checked in full text — Full text via Europe PMC (sections 2.1, 2.2, 2.4, 2.6 and 3.1); Fig. 1K with the bitterness values not obtained Checked by: LLM (one extracted, another checked against the source; no human check), 2026-10-22.
- Samples
- Summer–autumn green tea (cultivar Fuding white tea, from a tea company in Guiyang)
- Processing and cooking
- 10 g leaves brought to 40% moisture, pretreated by steaming (100 °C, 10 min), piling (70 °C, 1.5 h) and sterilisation (121 °C, 20 min), then inoculated with E. cristatum at 1‰ and incubated at 28 °C for 8 days; raw tea (CK), day 0 (pretreated only) and days 2, 4, 6 and 8 sampled (3 replicates)
- How it was served or eaten
- 3 g in 150 mL boiling water for 4 min
- Who rated, on what scale
- 9 trained panellists (5 men and 4 women, aged 20–40), calibrated with reference solutions including quinine hydrochloride; 0 (absent) to 10 (very strong), 3 replicates; one-way ANOVA and Tukey HSD (p<0.05)
| Measured | Comparison and condition | Before, shorter or without added microbe | After, longer or with added microbe | Unit | Direction / test |
|---|---|---|---|---|---|
| Bitterness intensity (sensory) | Heat pretreatment and fermentation: raw summer–autumn green tea (CK) vs 8 days of E. cristatum fermentation after pretreatment 10 g summer–autumn green tea at 40% moisture, steamed (100 °C, 10 min), piled (70 °C, 1.5 h) and sterilised (121 °C, 20 min), then inoculated with E. cristatum and incubated at 28 °C for 8 days; 3 g in 150 mL boiling water for 4 min | Figure not obtained, so values unconfirmed | Figure not obtained, so values unconfirmed | intensity from 0 (none) to 10 (very strong) (9 trained panellists, 3 replicates) | Before, shorter or without added microbe more bitter Tested, significant |
| From the text: the 8-day sample was significantly less bitter and astringent than the unfermented summer–autumn green tea (CK) (p<0.05, one-way ANOVA and Tukey HSD); no statistically significant difference was found for sweetness, umami or sourness (p>0.05). Values are in Fig. 1K. Figure not obtained, so values unconfirmed. No bitterness comparison between day 0 (pretreated only) and day 8 is given, so the effect of steaming, piling and sterilisation cannot be separated from that of the fungus Where: Sections 2.2, 2.4 and 2.6; section 3.1; Fig. 1K (not obtained) For whether changing the length or method of post-fermentation (pile fermentation, added microbes, pickling) or storage for the same raw tea changes the intensity of bitterness people perceive: Limited — Same raw tea and tested, but the 2 samples also differ in the steaming, piling and sterilisation pretreatment, so this is not the effect of fungal fermentation alone; values are only in a figure and unconfirmed | |||||
Wang X 2026 sensory only
Integrated sensory evaluation and feature-based molecular networking elucidate flavor improvement and associated metabolites in dark tea fermented by Eurotium cristatum and Bacillus coagulans. Food Chemistry: X 39:104544 — doi:10.1016/j.fochx.2026.104544
- How deeply checked
- Checked in full text — Full text via Europe PMC (methods "Materials" and "Human sensory evaluation"; results "Preparation and features of novel LPTs"; the Fig. 2 legend); Fig. 2B and Table S1 not obtained Checked by: LLM (one extracted, another checked against the source; no human check), 2026-10-22.
- Samples
- Liupao tea after traditional fermentation (N/A, supplied by a China Tea company), used as the starting material for the other samples
- Processing and cooking
- N/A further fermented for 7 days with B. coagulans (No. 10059), B. coagulans (JZ-1) or E. cristatum (8730), one strain each; 50 °C in the methods and 40–60 °C in the results; pile height 80–100 cm, moisture 10–25%
- How it was served or eaten
- 3.0 g in 150 mL boiling water for 8 min
- Who rated, on what scale
- 6 trained panellists (3 men and 3 women, aged 18–55), blinded, on 3 separate days; 6 attributes on a scale of 0 (none), 5 (moderate) and 10 (high); the mean of the 6 panellists per session was analysed by one-way ANOVA
| Measured | Comparison and condition | Before, shorter or without added microbe | After, longer or with added microbe | Unit | Direction / test |
|---|---|---|---|---|---|
| Bitterness intensity (sensory) | Added microbe: Liupao tea without added microbe (N/A) vs 7 days with B. coagulans (No. 10059) Liupao tea after traditional fermentation (N/A) used as the starting material and further fermented for 7 days with one strain each (50 °C in the methods, 40–60 °C in the results); 3.0 g in 150 mL boiling water for 8 min | 4.25 | 3.00 | scale of 0 (none), 5 (moderate) and 10 (high) (6 trained panellists, mean of 3 sessions) | Before, shorter or without added microbe more bitter Test method described; result for this comparison not reported |
| The direction shows which mean given in the text is larger. The methods state one-way ANOVA, but the text gives no pairwise result (p values or marks) for bitterness, and the marks in Fig. 2B are unconfirmed (figure not obtained). Whether N/A was held under the same 7-day temperature conditions is not stated, so this cannot be called the effect of the microbe alone. Astringency was 4.50 for N/A, 3.75 for No. 10059 and 5.00 for E. cristatum, moving differently from bitterness Where: Methods, "Materials" and "Human sensory evaluation"; Results, "Preparation and features of novel LPTs"; Fig. 2B (not obtained) For whether changing the length or method of post-fermentation (pile fermentation, added microbes, pickling) or storage for the same raw tea changes the intensity of bitterness people perceive: Limited — Same Liupao tea with or without an added microbe, but whether the control was held under the same 7-day conditions is not stated, and the text gives no pairwise test result for bitterness | |||||
| Bitterness intensity (sensory) | Added microbe: Liupao tea without added microbe (N/A) vs 7 days with B. coagulans (JZ-1) Liupao tea after traditional fermentation (N/A) used as the starting material and further fermented for 7 days with one strain each (50 °C in the methods, 40–60 °C in the results); 3.0 g in 150 mL boiling water for 8 min | 4.25 | 3.50 | scale of 0 (none), 5 (moderate) and 10 (high) (6 trained panellists, mean of 3 sessions) | Before, shorter or without added microbe more bitter Test method described; result for this comparison not reported |
| The direction shows which mean given in the text is larger. The methods state one-way ANOVA, but the text gives no pairwise result (p values or marks) for bitterness, and the marks in Fig. 2B are unconfirmed (figure not obtained). Whether N/A was held under the same 7-day temperature conditions is not stated, so this cannot be called the effect of the microbe alone. Astringency was 4.50 for N/A, 3.75 for No. 10059 and 5.00 for E. cristatum, moving differently from bitterness Where: Methods, "Materials" and "Human sensory evaluation"; Results, "Preparation and features of novel LPTs"; Fig. 2B (not obtained) For whether changing the length or method of post-fermentation (pile fermentation, added microbes, pickling) or storage for the same raw tea changes the intensity of bitterness people perceive: Limited — Same Liupao tea with or without an added microbe, but whether the control was held under the same 7-day conditions is not stated, and the text gives no pairwise test result for bitterness | |||||
| Bitterness intensity (sensory) | Added microbe: Liupao tea without added microbe (N/A) vs 7 days with E. cristatum (8730) Liupao tea after traditional fermentation (N/A) used as the starting material and further fermented for 7 days with one strain each (50 °C in the methods, 40–60 °C in the results); 3.0 g in 150 mL boiling water for 8 min | 4.25 | 2.75 | scale of 0 (none), 5 (moderate) and 10 (high) (6 trained panellists, mean of 3 sessions) | Before, shorter or without added microbe more bitter Test method described; result for this comparison not reported |
| The direction shows which mean given in the text is larger. The methods state one-way ANOVA, but the text gives no pairwise result (p values or marks) for bitterness, and the marks in Fig. 2B are unconfirmed (figure not obtained). Whether N/A was held under the same 7-day temperature conditions is not stated, so this cannot be called the effect of the microbe alone. Astringency was 4.50 for N/A, 3.75 for No. 10059 and 5.00 for E. cristatum, moving differently from bitterness; the lowest bitterness of the 4 and the highest sweetness, 7.50 Where: Methods, "Materials" and "Human sensory evaluation"; Results, "Preparation and features of novel LPTs"; Fig. 2B (not obtained) For whether changing the length or method of post-fermentation (pile fermentation, added microbes, pickling) or storage for the same raw tea changes the intensity of bitterness people perceive: Limited — Same Liupao tea with or without an added microbe, but whether the control was held under the same 7-day conditions is not stated, and the text gives no pairwise test result for bitterness | |||||
Zhou J 2025 sensory only
Dynamics Changes in Physicochemical Properties, Antioxidant Activity, and Non-Volatile Metabolites During Bulang Pickled Tea Fermentation. Foods 14(5):878 — doi:10.3390/foods14050878
- How deeply checked
- Checked in full text — Full text via Europe PMC (sections 2.2, 2.3, 2.9 and 3.1; the Fig. 1 legend); Fig. 1B with the B0–B2 values not obtained Checked by: LLM (one extracted, another checked against the source; no human check), 2026-10-22.
- Samples
- 1 bud and 2 leaves from Banzhang village, Bulangshan township, Menghai county, Yunnan (2021)
- Processing and cooking
- Withered, boiled for 20 min, cooled and drained, packed into bamboo tubes and sealed for fermentation underground (pickled tea); sampled at 0, 1, 2 and 3 months (B0–B3) and sun-dried for rating
- How it was served or eaten
- 3 g sun-dried leaves in 150 mL boiling pure water for 5 min
- Who rated, on what scale
- 10 tea evaluation experts (4 men and 6 women, aged 23–50); bitterness, astringency, sweet aftertaste, sourness and fermentation taste; a 9-point scale from 0 to 9 in the methods, but a 6-point intensity scale in the results text; the statistics section gives one-way ANOVA and Duncan test (p<0.05)
| Measured | Comparison and condition | Before, shorter or without added microbe | After, longer or with added microbe | Unit | Direction / test |
|---|---|---|---|---|---|
| Bitterness intensity (sensory) | Pickled tea: Bulang pickled tea at 0 months (B0) vs 3 months (B3) of fermentation 1 bud and 2 leaves withered, boiled for 20 min, packed into bamboo tubes, sealed and fermented underground; sampled at 0–3 months and sun-dried; 3 g sun-dried leaves in 150 mL boiling pure water for 5 min | Figure not obtained, so values unconfirmed | 3.6 | a 9-point scale from 0 to 9 in the methods but a 6-point intensity scale in the results text, inconsistent within the source (10 tea evaluation experts) | After, longer or with added microbe more bitter Test method described; result for this comparison not reported |
| From the text: the unfermented tea (B0) was lowest for all 5 tastes, intensities rose with fermentation, and B3 is given as bitterness 3.6 and astringency 3.6. The B0–B2 values are in Fig. 1B. Figure not obtained, so values unconfirmed. The statistics section states one-way ANOVA and Duncan test in general, but the text gives no result (p values or letters) for the bitterness comparison. Pickled tea is an anaerobic fermentation, a different process from the pile fermentation of dark tea Where: Sections 2.2, 2.3 and 2.9; section 3.1; abstract; Fig. 1B (not obtained) For whether changing the length or method of post-fermentation (pile fermentation, added microbes, pickling) or storage for the same raw tea changes the intensity of bitterness people perceive: Limited — Time points of pickling the same raw tea, but the scale is described inconsistently within the source and there is no test result for the bitterness comparison; pickled tea is an anaerobic fermentation, a different process from dark-tea pile fermentation | |||||
What was established within an experiment
Only results observed in comparisons within one study. Results with no test shown are stated as such.
- In the same pile in Wang T et al., bitterness at the end of 36-day pile fermentation (F4) was 3.73, significantly lower than the raw tea (Dunnett, p<0.05). Astringency was also lower at 3.57, while sweetness (6.97) and thickness (7.23) were higher. The raw-material and intermediate values are only in a figure and unconfirmed. [rows: Wang T 2023 Raw sun-dried green tea (RM) vs end of 36-day pile fermentation (F4)]
- In Table 1 of Yang W et al. for the same pile, bitterness was 3 at day 0, 4 at day 10, 5 at day 20, 4 at day 30, 2 at day 40, 1 at day 50 and "−" at day 60. Astringency fell steadily from 8 at day 0. Bitterness and astringency moved differently. No test of the sensory comparisons is shown. [rows: Yang W 2025 Pile fermentation day 0 vs day 20; Yang W 2025 Pile fermentation day 0 vs day 60]
- In the same batch in Deng et al., the ripe tea was significantly less bitter than the raw tea (p<0.01). Astringency was also weaker and mellow-and-thick taste and sweetness stronger in the ripe tea. Values are only in a figure and unconfirmed, and fermentation days are not given. [rows: Deng 2022 Sun-dried green tea (SGT) vs the ripe tea pile-fermented from it (PRT)]
- In Xu et al., the sample fermented with E. cristatum for 8 days after pretreatment was significantly less bitter and astringent than the raw summer–autumn green tea (p<0.05). No statistically significant difference was found for sweetness, umami or sourness. No bitterness comparison with day 0 (pretreated only) is given. [rows: Xu 2025 Heat pretreatment and fermentation: raw summer–autumn green tea (CK) vs 8 days of E. cristatum fermentation after pretreatment]
- In Wang X et al., bitterness was 4.25 for Liupao tea without an added microbe and 3.00, 3.50 and 2.75 for the 3 samples fermented 7 days with a microbe added. The text gives no pairwise test result for bitterness. Astringency for the E. cristatum sample was 5.00, higher than 4.50 without a microbe, moving differently from bitterness. [rows: Wang X 2026 Added microbe: Liupao tea without added microbe (N/A) vs 7 days with B. coagulans (No. 10059); Wang X 2026 Added microbe: Liupao tea without added microbe (N/A) vs 7 days with B. coagulans (JZ-1); Wang X 2026 Added microbe: Liupao tea without added microbe (N/A) vs 7 days with E. cristatum (8730)]
- In comparisons of stored teas from different production years, Bo et al. write that bitterness fell significantly to 2.5~1.17 as storage progressed, Zheng X et al. give 8.60 at 2 years and 2.95 for the 1980s, and Ma B et al. report a significant difference in bitterness across 3 groups (P<0.01) with new tea the most bitter. In none of them were the pairwise tests, or the match between values and years in Bo et al., confirmed. [rows: Bo 2026 Storage (samples from different production years): text description over 5 storage stages (1–9 years) of ripened tea "Laotongzhi 9978"; Zheng X 2026 Storage (samples from different production years): ripened tea stored 2 years vs the 1980s (80ND); Ma B 2025 Storage (samples from different production years): raw pu-erh new tea (up to 2 years) vs aged tea (over 8 years)]
- In Bulang pickled tea in Zhou J et al., the intensities of 5 tastes rose with fermentation, and bitterness at 3 months is given as 3.6, with the unfermented tea lowest. The scale is described inconsistently within the source, and the text gives no test result for the bitterness comparison. [rows: Zhou J 2025 Pickled tea: Bulang pickled tea at 0 months (B0) vs 3 months (B3) of fermentation]
Candidate explanations for the differences
Conditions that differ between studies and could explain the differences in bitterness results, and the authors' interpretations. Several conditions differ at once, so which one mattered is unknown. These are candidates, not causes.
- The authors attribute weaker bitterness to the breakdown or transformation of esterified catechins, theaflavins and similar compounds during fermentation and the build-up of polymers such as theabrownins (Wang T et al., Deng et al., Xu et al.). This is an interpretation from compositional changes and does not establish the cause of lower perceived bitterness. [rows: Wang T 2023 Raw sun-dried green tea (RM) vs end of 36-day pile fermentation (F4); Deng 2022 Sun-dried green tea (SGT) vs the ripe tea pile-fermented from it (PRT); Xu 2025 Heat pretreatment and fermentation: raw summer–autumn green tea (CK) vs 8 days of E. cristatum fermentation after pretreatment]
- Why bitterness first rose in Yang W et al. cannot be told from Table 1. The authors' mechanism (section 3.5) says bitterness and astringency fell early, which does not match Table 1 for bitterness, and the Fig. 1A description also calls day 10 "slightly reduced". This page uses the Table 1 values. [rows: Yang W 2025 Pile fermentation day 0 vs day 20]
- A candidate explanation for the difference between the report of rising bitterness in pickled tea and the reports of weakening bitterness with pile fermentation or storage is the difference in process (anaerobic fermentation of boiled leaves sealed underground in bamboo tubes, versus moistened piles or dry storage). However, there is only 1 pickled-tea study, with an inconsistent scale, so the process difference cannot be called the cause. [rows: Zhou J 2025 Pickled tea: Bulang pickled tea at 0 months (B0) vs 3 months (B3) of fermentation; Wang T 2023 Raw sun-dried green tea (RM) vs end of 36-day pile fermentation (F4)]
What is not known — what this comparison alone cannot separate
- All the storage comparisons line up samples from different production years rather than following the same tea for years. Bo et al. describe one product with the same recipe and protocol, but production year varies together with storage time. In Zheng X et al. manufacturer, blend and warehouse also differ. Ma B et al. include the same 6 series in each group, but with separate samples per group, and the raw material and processing within each series were not checked. These comparisons alone cannot isolate the effect of storage time. [rows: Bo 2026 Storage (samples from different production years): text description over 5 storage stages (1–9 years) of ripened tea "Laotongzhi 9978"; Zheng X 2026 Storage (samples from different production years): ripened tea stored 2 years vs the 1980s (80ND); Ma B 2025 Storage (samples from different production years): raw pu-erh new tea (up to 2 years) vs aged tea (over 8 years)]
- In Xu et al., the steaming, piling and sterilisation pretreatment and the fungal fermentation differ together, with no bitterness comparison against the pretreated-only sample. In Wang X et al., whether the control without an added microbe was held under the same 7-day conditions is not stated. In neither can the effect of the microbe alone be separated. [rows: Xu 2025 Heat pretreatment and fermentation: raw summer–autumn green tea (CK) vs 8 days of E. cristatum fermentation after pretreatment; Wang X 2026 Added microbe: Liupao tea without added microbe (N/A) vs 7 days with E. cristatum (8730)]
- Many mean values are only in figures or supplementary tables, which were not obtained, so the numbers are unconfirmed (the raw material in Wang T et al., Deng et al., the per-year values in Bo et al., Ma B et al. and Xu et al.). On the other hand, the significance of the comparisons reported by Wang T et al., Deng et al. and Xu et al. was confirmed in the text, while pairwise results in Bo et al., Zheng X et al. and Ma B et al. are unconfirmed. The size of the declines cannot be compared on this page. [rows: Wang T 2023 Raw sun-dried green tea (RM) vs end of 36-day pile fermentation (F4); Deng 2022 Sun-dried green tea (SGT) vs the ripe tea pile-fermented from it (PRT); Bo 2026 Storage (samples from different production years): text description over 5 storage stages (1–9 years) of ripened tea "Laotongzhi 9978"; Ma B 2025 Storage (samples from different production years): raw pu-erh new tea (up to 2 years) vs aged tea (over 8 years); Xu 2025 Heat pretreatment and fermentation: raw summer–autumn green tea (CK) vs 8 days of E. cristatum fermentation after pretreatment]
- Wang T et al. (pile fermentation) and Bo et al. (storage) are from the same research group, with overlapping authors and the same panel description (9 trained panellists, 5 men and 4 women, aged 24–48), but whether they were the same people is unconfirmed. They are not counted as independent replications. [rows: Wang T 2023 Raw sun-dried green tea (RM) vs end of 36-day pile fermentation (F4); Bo 2026 Storage (samples from different production years): text description over 5 storage stages (1–9 years) of ripened tea "Laotongzhi 9978"]
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: 9, of which 9 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): 12. Rows on perceived bitterness: 12.
- Evidence rows in the lab's ledger these rows rest on: 1. 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: 8 (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 Europe PMC for studies combining dark teas and other post-fermented teas such as pu-erh, Liupao and Fu brick tea, or tea storage, ageing and solid-state fermentation, with human ratings of bitterness, searched J-STAGE for Japanese post-fermented teas, and added the sources already in the lab's ledger. Teas that are not dark teas (black and white tea) and leaves of plants other than tea were kept as references outside the main table. Studies with no human bitterness rating, no item for bitterness intensity alone, fermentation of tea infusion rather than leaves, or no comparison of post-fermentation or storage were excluded. Studies whose values are only in figures or supplementary tables we could not obtain are marked as such. The search described here is a record, not a claim that nothing was missed.
| Source | Decision | Why |
|---|---|---|
| Changes in sensory characteristics, chemical composition and microbial succession during fermentation of ancient plants Pu-erh tea (2023) doi | Included in the table | Ancient-tree raw tea pile-fermented for 36 days, with the raw material and end point rated by 9 trained panellists; a source already in the lab's ledger, whose author field was corrected against the original on 2026-10-22; read in full |
| Effects of Pile-Fermentation Duration on the Taste Quality of Single-Cultivar Large-Leaf Dark Tea: Insights from Metabolomics and Microbiomics (2025) doi | Included in the table | A single pile of single-cultivar raw tea rated every 10 days up to 60 days; its table shows bitterness first rising and then falling; no sensory test is shown; read in full |
| Mellow and Thick Taste of Pu−Erh Ripe Tea Based on Chemical Properties by Sensory−Directed Flavor Analysis (2022) doi | Included in the table | Compares raw tea with the ripe tea from the same batch; the main topic is the compounds behind mellow-and-thick taste, but bitterness is tested as a separate item; read in full |
| Metabolomic and Sensory Insights into the Aging Mechanism of Ripened Pu-Erh Tea over Nine Years (2026) doi | Included in the table | A storage study comparing samples of one product from 5 production years; its label in the list begins with "Storage (samples from different production years)"; a source already in the lab's ledger, where the wording "single lot" was corrected on 2026-10-22; read in full |
| Decoding long-term aging of ripened Pu-erh tea: sensory equilibrium, metabolic pathways, and chemical markers (2026) doi | Included in the table | A cross-sectional comparison of 23 ripened teas from different manufacturers, blends and warehouses arranged by storage time; its label in the list begins with "Storage (samples from different production years)"; read in full |
| Quantitative descriptive analysis, non-targeted metabolomics and molecular docking reveal the dynamic aging and taste formation mechanism in raw Pu-erh tea during the storage (2025) doi | Included in the table | A cross-sectional comparison of 18 raw teas from 6 series grouped into 3 storage stages; its label in the list begins with "Storage (samples from different production years)"; read in full |
| Effects of Solid-State Fermentation with Eurotium cristatum on the Physicochemical, Sensory, and Volatile Profiles of Summer–Autumn Green Tea (2025) doi | Included in the table | Summer–autumn green tea fermented with E. cristatum after heat pretreatment; because pretreatment and fungus differ together, its label in the list begins with "Heat pretreatment and fermentation"; read in full |
| Integrated sensory evaluation and feature-based molecular networking elucidate flavor improvement and associated metabolites in dark tea fermented by Eurotium cristatum and Bacillus coagulans (2026) doi | Included in the table | Fermented Liupao tea further fermented with added microbes; its label in the list begins with "Added microbe"; read in full |
| Dynamics Changes in Physicochemical Properties, Antioxidant Activity, and Non-Volatile Metabolites During Bulang Pickled Tea Fermentation (2025) doi | Included in the table | Bulang pickled tea rated at 0–3 months; because the process differs from dark-tea pile fermentation, its label in the list begins with "Pickled tea"; read in full |
| Reshaped local microbiology metabolism by raw tea according to pile fermentation in the dark tea (2025) doi | Read as context (not in the table) | Three raw teas dried in different ways from the same fresh leaves were each pile-fermented for 0, 4, 8, 12, 16 and 20 days, and 6 trained panellists rated 4 tastes including bitterness on a 5-point scale (0.5 steps); the time series per raw tea is in scope, but the text does not state the direction of bitterness and the values are only in Fig. 5A. Figure not obtained, so values unconfirmed; read in full |
| Non-Volatile Metabolic Reprogramming and Sensory Evolution of Anhua Qianliang Tea During Long-Term Storage (2026) doi | Read as context (not in the table) | Anhua Qianliang tea from the same producer and method, from 9 production years (stored 2–25 years), rated by 12 trained panellists with 0–5 QDA; the text says young teas were bitter-astringent and old teas sweet and round; values are only in Fig. 6A. Figure not obtained, so values unconfirmed; the p values in Fig. 6B are correlations between attributes, not differences between storage groups; read in full |
| Metabolome and Microbiome Analysis to Study the Flavor of Summer Black Tea Improved by Stuck Fermentation (2023) doi | Read as context (not in the table) | Summer black tea of Yinghong No. 9 given stuck (moist piled) fermentation for 35 days and rated by 8 people with 0–10 QDA; the text says bitterness and astringency weakened, and Fig. 1D shows sensory results with significance marks; the values and marks are in the figure. Figure not obtained, so values unconfirmed; the overall scores of 87 and 93 are not copied because the order given in the text is inconsistent; read in full |
| Changes in flavor quality of raw Liupao tea during the piling process (2024) doi | Read as context (not in the table) | Raw Liupao tea piled for 0–30 h in the same process, with bitterness scored separately by 10 people; the methods give 0–10 but the Fig. 3 legend 0–8; the text says "bitter, astringent" at 0–6 h and that "bitter" disappears from 9 h, which is not read as zero bitterness; values are only in Fig. 3. Figure not obtained, so values unconfirmed; read in full |
| Determination of the variations in the metabolic profiles and bacterial communities during traditional craftsmanship Liupao tea processing (2024) doi | Read as context (not in the table) | Traditional Liupao processing steps and 2–12 h of piling described and scored by 3 panellists; the text says bitterness and astringency weakened with piling, and the quality score fell at 12 h; no separate numerical bitterness value could be obtained; read in full |
| Unraveling the flavor formation process of mellow and thick-type ripened Pu-erh tea through non-targeted metabolomics and metagenomics (2025) doi | Read as context (not in the table) | The same raw material piled for 0–30 days, evaluated by 5 people with a 100-point quality score and descriptions; "slight bitterness" is noted at S5; the only separate bitterness values are from an electronic tongue (instrument); read in full |
| Multi-omics insights into floral–fruity aroma formation during Pu’er tea fermentation inoculated with a synthetic fungal community (2026) doi | Read as context (not in the table) | Raw tea inoculated with several fungi and fermented for 0–32 days, evaluated by consensus descriptions from 5 assessors; it says the taste changed from bitter-astringent to smooth and mellow-thick, but there is no bitterness value; read in full |
| Quality Evolution of Pu-Erh Raw Tea over 1–22 Years of Aging (2026) doi | Read as context (not in the table) | Yiwu raw tea from the same producer, 18 production years (stored 1–22 years), described by 5 assessors; bitterness is only described, with no bitterness-intensity score or pairwise test; astringency is said to rise again at 9–10 years; read in full |
| Integrated sensory, chemical, metabolomic, and antioxidant profiling reveals coordinated quality transformation of Lapsang Souchong black tea during decade-long storage (2026) doi | Read as context (not in the table) | Storage of a black tea (Lapsang Souchong), not a dark tea, so not in the main table; 8 production years from the same garden (3 batches each) rated 0–7 by 15 trained panellists; bitterness is described as "relatively stable", with no correlation or p value given, and it is not among the 6 attributes reported with a significant monotonic relationship; not counted as an example of "no change" |
| Evaluation of taste quality of Keemun congou black tea during ripening and the effect of this quality on antioxidant capacity and in vitro inhibition of α-amylase and α-glucosidase (2025) doi | Read as context (not in the table) | The same crude Keemun black tea taken through 2 rounds of resting and re-firing; not a dark tea and involving heating, so not in the main table; rated 1–5 by 12 trained panellists, and the text says bitterness weakened significantly, but the Duncan test and p<0.05 in the statistics section refer to compound concentrations, and the sensory test and pairwise p values are unconfirmed |
| Integrated metabolomics and molecular docking reveal fermentation-dependent flavonoid transformation and bitterness attenuation in Eucommia ulmoides dark tea (2026) doi | Read as context (not in the table) | The raw material is Eucommia ulmoides leaf, not tea (Camellia sinensis), so not in the main table; fermented by the dark-tea method, and the text says bitterness after 2 days of natural piling (SFT) was significantly lower than unfermented leaves (NFT) (p<0.001), and bitterness after golden-flower treatment (FFT) was 42.00% lower than unfermented leaves (NFT); the Student t-test is stated for metabolite comparisons, and its link to the sensory test is unconfirmed; a reference from a non-tea leaf |
| Flavor changes of Yunnan large-leaf cultivar white tea during different aging periods (2025) doi | Read as context (not in the table) | White tea (not post-fermented) made in 2017, 2019, 2021 and 2023, evaluated by 12 people; a decline in bitterness is described, but there is no separate bitterness score |
| 麹菌による茶葉の呈味改善 (2008) | Excluded | A Japanese study fermenting second-flush tea with koji mould for 3 days; people rated astringency, umami, body and sweetness (paired comparison, 10 people), with no bitterness item; "bitterness" in the English abstract corresponds to astringency in the sensory items |
| 酵母発酵茶の製造条件の検討及びラットにおける血糖値上昇抑制作用 (2009) | Excluded | A Japanese study fermenting unfermented tea with sugar and yeast for 5 days; human ratings (10 people) covered astringency, umami, body and sweetness, with no bitterness item |
| Sensory and Metabolomic Analysis Reveal the Quality Evolution of Liupao Tea During Long-Term Aging (2026) doi | Excluded | Liupao tea stored 8, 13 and 20 years; bitterness was measured by an electronic tongue (instrument), with no human bitterness rating |
| Study on bitterness reduction and aroma enrichment of Kejia green tea based on storage duration through UPLC-MS/GC–MS technology (2026) doi | Excluded | Kejia green tea stored 0, 10 and 20 years; human evaluation was a 100-point quality score, and the only separate bitterness values are from an electronic tongue (instrument) |
| Effects of storage time on the quality and microbial community of ripe Pu-erh tea (2025) doi | Excluded | A study of storage time for ripe pu-erh; bitterness was assessed by an electronic tongue (instrument), with no human bitterness rating |
| Impact of co-fermentation with Debaryomyces hansenii and Bacillus subtilis on the flavor characteristic of dark tea infusion (2026) doi | Excluded | Fermentation of extracted tea infusion, not leaves, with yeast and bacteria, outside the scope of leaf post-fermentation; the taste score is a composite quality score from "strong bitter-astringent" to "mellow with sweet aftertaste", not a bitterness score |
| Characterizing and decoding the effects of co-culture fermentation on dark tea infusion flavor development: elucidating microbial succession and metabolic dynamics mediated by Eurotium cristatum and water kefir (2025) doi | Excluded | Fermentation of extracted tea infusion, not leaves, with E. cristatum and water kefir, outside the scope of leaf post-fermentation |
| Flavor Quality and Lipid-Lowering Function of Mixed Fermented Pu-erh Tea with Various Monascus Species (2025) doi | Excluded | Sterilised raw pu-erh fermented with Monascus; the table includes "slightly bitter", but there is no separate numerical bitterness intensity |
| Enhancing the Flavor Profile of Summer Green Tea via Fermentation with Aspergillus niger RAF106 (2023) doi | Excluded | Summer green tea fermented with Aspergillus niger; sensory evaluation gave quality scores for colour, aroma and taste, with no separate bitterness score |
| Process Optimization and Quality Components Analysis of γ-Aminobutyric Acid Pickled Tea (2024) doi | Excluded | GABA pickled tea; human evaluation is a quality assessment, with no separate bitterness score |
| Optimizing Lactic Acid Bacteria Fermentation for Enhanced Summer and Autumn Tea Quality (2024) doi | Excluded | Summer–autumn tea fermented with lactic acid bacteria; methods and results cover microbial counts, composition and aroma analysis, and no human sensory method could be confirmed |
| An Efficient and Safe Rapid Aging Technology for Tea: UV-C Irradiation Enhances the Taste and Aroma of Fresh Pu’er Raw Tea Toward a Naturally Aged Profile (2025) doi | Excluded | A study of accelerating ageing with UV-C irradiation; its sensory attributes include no separate bitterness item |
| Influences of Fermentation Temperature on Volatile and Non-Volatile Compound Formation in Dark Tea: Mechanistic Insights Using Aspergillus niger as a Model Organism (2026) doi | Excluded | A study of fermentation temperature with Aspergillus niger; there is a human quality evaluation but no separate bitterness score |
| Temperature regulates flavor and bioactivity in dark tea fermentation: Insights from an Aspergillus tubingensis single-strain model (2026) doi | Excluded | A study of fermentation temperature with A. tubingensis; human sensory confirmation is left for future work, so there is no human bitterness rating |
| New insights into optimal storage conditions for raw Pu-erh tea: modulating post-fermentation flavor via microbial metabolism under controlled water activity (2026) doi | Excluded | The same raw pu-erh stored at different humidity and water activity, close to the scope, but no human bitterness rating could be confirmed |
| Impact of processing techniques on chemical compositions and sensory attributes across five types of Yunnan large-leaf teas (2026) doi | Excluded | A comparison of processing methods across 5 types of tea, not of the length or method of post-fermentation or storage |
| Effects of Different Citrus Varieties and Harvesting Time on the Quality of Citrus Dark Tea (2025) doi | Excluded | A comparison of citrus varieties and harvest times; there are human bitterness scores, but it is not a comparison of post-fermentation or storage |
| Substrate-Mediated Raw Material Grade Affects Sensory Quality, Chemical Composition, and Fungal Community of Fu Brick Tea (2025) doi | Excluded | Mainly a comparison of raw-material grades for Fu brick tea; there is also bitterness QDA before and after fermentation for each grade, but extraction time differs (5 and 8 min) between raw tea and Fu brick tea, so brewing is mixed into the comparison |
| Effect of Pressing Process on Metabolomics Profiling and Sensory Properties: A Comparative Study of Fu Brick Tea Versus Fu Loose Tea from Identical Raw Dark Tea (2025) doi | Excluded | A comparison of pressed and loose tea from the same raw material; no separate comparison of bitterness intensity could be confirmed |
| Comparison of the Fungal Community, Chemical Composition, Antioxidant Activity, and Taste Characteristics of Fu Brick Tea in Different Regions of China (2022) doi | Excluded | A comparison of Fu brick teas from different regions, not of post-fermentation or storage |
| Study on the Functional, Physiological, and Sensory Properties of Coffee Leaf Dark Tea Processed Using the Pile Fermentation Method of Pu-Erh Tea (2026) doi | Excluded | Coffee leaves, not tea |
| Enhancing the flavor quality of Penthorum chinense Pursh through Aspergillus fermentation (2026) doi | Excluded | A Penthorum plant, not tea |
| Mechanistic insights into taste modulation of Xanthoceras sorbifolium leaf tea fermented by Aspergillus cristatus and Aspergillus neoniger: an integrated component–taste relationship analysis (2026) doi | Excluded | Xanthoceras sorbifolium leaves, not tea |
| Untargeted Metabolomics Combined with Bioassay Reveals the Change in Critical Bioactive Compounds during the Processing of Qingzhuan Tea (2021) doi | Excluded | A study of Qingzhuan tea processing; people scored astringency, not bitterness |
| Effects of the flowering process on the metabolite profiles and bioactivities of Shaanxi Fu brick tea (2026) doi | Excluded | No human bitterness rating could be confirmed from the methods and results |
| Biochemical Components Associated With Microbial Community Shift During the Pile-Fermentation of Primary Dark Tea (2018) doi | Excluded | No human bitterness rating could be confirmed from the methods and results |
| Multidimensional Analysis Reveals the Flavor Quality Formation Mechanism During the Primary Pile Fermentation of Dark Tea (2026) doi | Excluded | No human bitterness rating could be confirmed from the methods and results; direct sensory evaluation is described as future work |
| The Longer the Storage Time, the Higher the Price, the Better the Quality? A 1H-NMR Based Metabolomic Investigation of Aged Ya’an Tibetan Tea (Camellia sinensis) (2022) doi | Excluded | A study of storage time for Ya'an Tibetan tea, close to the scope, but with no human bitterness rating |
| Polysaccharide Conjugates' contribution to mellow and thick taste of Pu-erh ripe tea, besides Theabrownin (2024) doi | Excluded | There is sensory evaluation, but it is a study of mellow-and-thick taste by fractionating and reconstituting infusions, not a comparison of post-fermentation or storage conditions |
| An integrated flavoromics and chemometric analysis of the characteristic flavor, chemical basis and flavor wheel of ancient plant ripened pu-erh tea (2025) doi | Excluded | There are bitterness scores, but it compares products from 5 regions, not post-fermentation or storage |
| Metabolic and Microbial Community Profiles of Century-Old Pu-Erh Tea: An Integrative Metabolomic and Microbiomic Analysis (2026) doi | Excluded | Expert sensory assessment was used to confirm authenticity, but there is no bitterness comparison, and raw and ripe teas are mixed |
| Integrated Microbiome–Metabolome Analysis and Functional Strain Validation Reveal Key Biochemical Transformations During Pu-erh Tea Pile Fermentation (2025) doi | Excluded | Methods and results cover compositional, microbial and strain-function analyses; no human bitterness comparison could be confirmed |
Read in the encyclopedia
- Tea pile fermentation — moistening and piling tea leaves, a step that can change astringency, sweetness, colour and aroma along with bitterness
- Ripe pu-erh tea — tea post-fermented in moist piles with microorganisms involved
- Tea's bitterness and astringency — the topic hub on tea bitterness
- Does roasting tea make it less bitter? — a comparison of roasting and bitterness
- Awa-bancha, Goishicha, Ishizuchi-kurocha — a research note on Japanese post-fermented teas
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.
How to cite this page
Aoki, M. (NIGAMI LAB) (2026). Does fermentation or ageing change dark tea's bitterness?. https://nigamilab.com/en/topics/dark-tea-fermentation-bitterness/
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