Spinach is a leafy vegetable containing a relatively high amount of oxalic acid. Its unpleasant aftertaste has traditionally been attributed to oxalic acid (egumi), but oxalate taste and bitterness have been reported to separate into different fractions by solid-phase extraction.
Reading the bitterness
The bitterness of raw leaves when eaten differed by sample origin. When a trained panel of 6 evaluated about 10 g of unseasoned raw leaf 3 times each on a 15-point scale, the mean ± standard deviation of bitterness was 3.0±0.56 for the open-field product and 2.4±0.50 for the commercial product, with the open-field product significantly higher. The tunnel-covered product was 2.8±0.63, intermediate between the two. Because the commercial product's cultivar, growing method, and days since harvest were unknown, the difference cannot be determined to be due to growing method alone.
In another test that examined salt-boiling, spinach was boiled for 1 minute in distilled water or 1-3% saline at 10 times its weight, then soaked in changing water for a total of 5 minutes. When 10-21 women aged 19 to their 40s answered on a 5-point scale, there was a tendency for egumi to be felt less as the salt concentration increased, but there was no significant difference among the concentration conditions. What was scored here was egumi, not bitterness itself.
In a test from the same paper that varied the type of salt, samples boiled with 3% refined salt or 3% table salt were significantly less egumi than samples boiled with water alone (p<0.01). 「天塩」 (Amashio) salt, however, showed no difference from water-boiling. It cannot be said that any salt gives the same result, and egumi cannot be read as a measurement of bitterness.
Structured claims — open a section to read each claim's confidence, evidence and limits
Sensory (7)
spinach — perceived bitterness is suppressed by: Adding a small amount of sugar (0-1.8%, in 4 steps) or salt (0-0.2%) to spinach puree lowered bitterness intensity (while sweetness and saltiness rose). Other sensory attributes such as texture and aroma did not change. The abstract reports results for the 3 vegetables together and does not show the effect at each added concentration.preliminary
Scope: Layer: food. Green-vegetable purees of broccoli, spinach, and kale, evaluated by a trained panel of 9 adults (Experiment 1).
Note: Checked against the abstract (Europe PMC record); the full text has not been checked. The same trend is reported for all 3 vegetables, and figures limited to spinach alone cannot be separated out from the abstract.
9 adults who had completed more than 20 h of green-vegetable-specific training evaluated broccoli, spinach and kale purees with added sugar (0/0.6/1.2/1.8%) or salt (0/0.2%) by descriptive analysis. Summarizing Experiments 1 and 2 together, the abstract says that sugar and salt each lowered bitterness in all 3 vegetables (while sweetness and saltiness rose) without altering other sensory properties such as texture or aroma.
Limitations: The full text has not been obtained; checked against the abstract (Europe PMC record). The abstract reports results for the 3 vegetables together, so figures for spinach alone and the effect at each added concentration cannot be separated out. Test names and p values are not in the abstract.
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ingredient-spinach —has_perceived_bitterness_suppressed_by→ (the claim sentence above)claim: clm-w68a-spinach-sugar-salt-reduces-bitterness / type: sensory / updated: 2026-10-22
spinach — perceived bitterness is suppressed by: Spinach purée with 2% added sugar was chosen as less bitter in a forced-choice task against purée without additives.preliminary
Scope: Layer: Food. Vegetable purées of spinach, kale, and broccoli; a forced-choice task with 84 adults (Experiment 2).
Note: Checked against the abstract (Europe PMC record); the full text has not been checked. Summarizing Experiments 1 and 2 together, the abstract says bitterness fell in all 3 vegetables, and does not give the number of choices for spinach alone.
Each vegetable purée was prepared with 0% or 2% sugar and assessed for bitterness in a forced-choice task with 84 adults. Summarizing Experiments 1 and 2 together, the abstract says that adding sugar lowered bitterness in all 3 vegetables.
Limitations: The full text has not been obtained; checked against the abstract (Europe PMC record). The number of choices for spinach alone, the effect size and the test names are not in the abstract.
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ingredient-spinach —has_perceived_bitterness_suppressed_by→ (the claim sentence above)claim: clm-w68a-spinach-sugar-2pct-forced-choice-bitterness / type: sensory / updated: 2026-10-22
spinach — sensory acceptability was increased by: For vegetable purées of spinach, kale and broccoli with added sugar (0%, 1%, 2%), liking in a 9-point hedonic evaluation by 99 adult consumers increased. The abstract does not report results separately by vegetable or by concentration.preliminary
Scope: Layer: Food. Experiment 3 (9-point hedonic evaluation, 99 adults).
Note: Checked against the abstract (Europe PMC record); the full text has not been checked. This concerns an increase in liking scores, not a rating of bitterness intensity itself.
Each vegetable purée was prepared with 0/1/2% sugar and evaluated by 99 adults on a 9-point hedonic scale. It describes increased liking among adult consumers with added sugar.
Limitations: The full text has not been obtained; checked against the abstract (Europe PMC record). The abstract reports the result for the vegetable purées as a whole, so the result for spinach alone and the separate effects of 1% and 2% cannot be separated out. This is a liking score, not a direct rating of bitterness intensity.
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ingredient-spinach —has_sensory_acceptability_increased_by→ (the claim sentence above)claim: clm-w68a-spinach-sugar-increases-liking / type: sensory / updated: 2026-10-22
spinach — perceived bitterness is affected by: In an extract equivalent to a 20-fold dilution of spinach leaf blades, bitterness was described as markedly weaker than in fresh leaves, while oxalate taste was felt relatively strongly; the authors described the two as contrasting responses.preliminary
Scope: Layer: Food. Sensory evaluation with symbols, by a small panel including the authors, of fresh spinach leaves (cv. Parade) and an extract equivalent to a 20-fold dilution of the leaf blades. Not a test that changed the dilution factor step by step.
「20倍希釈液においても、比較的強く感じられるシュウ酸味と、希釈によって刺激が弱まる苦味は対照的であり、ホウレンソウの苦味については、濃度依存的に味細胞を刺激する苦味成分によるものと推定される。」 (in translation: "Even in the 20-fold diluted liquid, the oxalate taste, which is felt relatively strongly, contrasts with bitterness, whose stimulus weakens with dilution; spinach bitterness is presumed to come from bitter components that stimulate taste cells in a concentration-dependent way.")
Limitations: Not a test that changed the dilution factor step by step; it remains a qualitative contrast between fresh leaves and a liquid equivalent to a 20-fold dilution. The number of assessors is not given and no statistical test is reported. The bitter compound was not identified, and "stimulating taste cells in a concentration-dependent way" is the authors' presumption.
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ingredient-spinach —has_perceived_bitterness_affected_by→ (the claim sentence above)claim: clm-w72b1-spinach-bitterness-dilution-response / type: sensory / updated: 2026-10-21
spinach shows the sensory response In a comparison of boiling for 1 minute in distilled water or 1-3% saline followed by a 5-minute water soak, there was a tendency for egumi to be felt less as the salt concentration increased, but there was no significant difference among the concentration conditionssupported
Scope: The cultivar Sunlight, boiling water at 10 times the sample weight, a panel aged 19 to their 40s including female students and staff, a 5-point evaluation
Note: The item evaluated was egumi, not a direct evaluation of bitterness. The 1-3% concentration difference is not treated as a significant effect.
There was no significant difference between water-boiling and 1%, 2%, 3% saline boiling in color, flavor, texture, egumi, or overall evaluation. Egumi showed a tendency to be felt less as the salt concentration increased.
Limitations: The exact number of participants in each individual test cannot be separated out from the text. There was no significant difference among the salt concentration conditions. This is an egumi scale, and it does not independently measure bitterness intensity.
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ingredient-spinach —has_sensory_response→ (the claim sentence above)claim: clm-spinach-salt-boil-acridity-concentration / type: sensory / updated: 2026-09-16
spinach shows the sensory response Samples boiled with 3% refined salt or 3% table salt were significantly less egumi than samples boiled with distilled water alone (p<0.01)supported
Scope: The cultivar Tonic, boiling water at 10 times the sample weight, boiled for 1 minute and soaked in water for 5 minutes, a 5-point sensory evaluation of 4 salt types and distilled water
Note: The 「天塩」 (Amashio) condition showed no significant difference from the distilled-water condition, and this is not generalized to all salts. The item evaluated is egumi.
Boiling with 3% refined salt and boiling with 3% table salt produced significantly less egumi than boiling with water (p<0.01). Boiling with 「天塩」 (Amashio) salt showed no significant difference from boiling with water.
Limitations: Composition differs by salt type, and this is a separate test from the concentration experiment that used the cultivar Sunlight. This is an egumi scale, and it does not independently measure bitterness intensity.
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ingredient-spinach —has_sensory_response→ (the claim sentence above)claim: clm-spinach-salt-type-acridity / type: sensory / updated: 2026-09-16
spinach — perceived bitterness is affected by: In a descriptive evaluation of 3 raw-leaf samples, the bitterness of the open-field product was significantly stronger than the commercial product, and the tunnel-covered product was intermediate, with no significant difference from eithersupported
Scope: Chapter 4 of a doctoral dissertation, a trained panel of 6, about 10 g of unseasoned raw leaf, a 15-point intensity scale, 3 replicates per sample
Note: The open-field product and the tunnel-covered product were both the cultivar Corvair, but the commercial product's cultivar, growing method, and the period from harvest to testing were unknown, so this is not treated as an effect of growing method alone.
On the 15-point scale, bitterness as mean ± standard deviation was 2.8±0.63 for the tunnel-covered product, 3.0±0.56 for the open-field product, and 2.4±0.50 for the commercial product, with an overall test across the 3 samples of p=0.010. The letter groupings from Fisher's protected LSD (p≤0.05) were a for the open-field product, b for the commercial product, and ab for the tunnel-covered product.
Limitations: p=0.010 is the overall test across the 3 samples, not a pairwise p-value for open-field versus commercial. This is a repeated evaluation by a 6-person panel, not treated as a number of independent growing samples. The commercial product's cultivar, growing method, harvest date, and distribution period are unknown. The dissertation chapter was checked in full, but the published paper PDF with the same title was not obtained, so an exact match between versions has not been confirmed.
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ingredient-spinach —has_perceived_bitterness_affected_by→ (the claim sentence above)claim: clm-spinach-growing-source-bitterness / type: sensory / updated: 2026-09-16