👃 Odor and Taste of Water
Sensory determination of threshold odor number (TON) and threshold flavor number (TFN) of water using dilution method with panel assessment. Organoleptic parameter of drinking water.
Overview
Odor and taste of drinking water are among the most important organoleptic parameters evaluated by consumers. Although the presence of odor or taste does not always indicate a health hazard, it is a warning signal that may indicate chemical or microbiological contamination or metabolic products of algae and cyanobacteria (e.g., geosmin, 2-methylisoborneol — MIB).
The PN-EN 1622 standard defines two quantitative determination methods: TON (Threshold Odour Number) and TFN (Threshold Flavour Number). The method involves successive dilution of the test sample with odorless and tasteless water until the odor or taste becomes undetectable. The dilution ratio corresponds to the TON/TFN value.
The test is performed by a three-person assessment team (sensory panel), trained in recognizing typical water odors (chlorine, earthy, fishy, musty, hydrogen sulfide, phenolic). The sample is tested at 25±1°C. The standard also provides for a qualitative assessment method — determination of the type of odor (descriptors) and its intensity on a 0–4 scale.
Determination of odor and taste is required by the Regulation of the Minister of Health on the quality of water intended for human consumption. The permissible value is "acceptable to consumers and without abnormal changes". In laboratory practice, TON ≤ 2 at 25°C is considered acceptable.
Method principle
The method is based on diluting the water sample with odorless reference water (filtered through activated carbon) in defined geometric proportions. A series of dilutions is presented to a sensory panel consisting of at least 3 assessors. Each dilution is evaluated using the triangle method — the assessor receives 3 vessels: two with reference water and one with diluted sample, the task is to identify the vessel with the sample. TON is calculated as the ratio of final volume to sample volume at the highest dilution at which odor is still detectable by 50% of the panel.
Applications
- Control of drinking water quality (MoH Regulation — mandatory parameter)
- Monitoring of surface water intakes — detection of algal blooms
- Control of water treatment process (filtration on activated carbon, ozonation)
- Testing of bottled and spring water
- Testing of water in swimming pools
- Consumer complaints regarding water quality
- Identification of organoleptic contamination sources in water supply network
Key parameters
| Parameter | Value |
|---|---|
| Result — TON | 1 (no odor) to >64 (strong odor) |
| Result — TFN | 1 (no taste) to >16 (strong taste) |
| Test temperature | 25 ± 1°C (odor), 25 ± 1°C (taste) |
| Number of assessors | Minimum 3 persons (sensory panel) |
| Sample volume | Minimum 2 liters |
| Acceptability criterion | TON ≤ 2 at 25°C (drinking water) |
Standard
- Standard number
- PN-EN 1622:2006
- Title (PL)
- Jakość wody — Oznaczanie progu wyczuwalności zapachu (TON) i progu wyczuwalności smaku (TFN)
- Title (EN)
- Water quality — Determination of the threshold odour number (TON) and threshold flavour number (TFN)
Step-by-step procedure
1. Preparation of odorless water
Filter distilled water through activated carbon column. Check organoleptically for absence of odor. Prepare min. 10 L.
2. Sample preparation
Bring water sample to temperature 25±1°C in water bath. If contains chlorine, add sodium thiosulfate (10 mg/L).
3. Preliminary qualitative assessment
Sensory panel (3 persons) evaluates undiluted sample. Determines odor type (descriptors: chlorine, earthy, musty, fishy, etc.) and intensity (0–4).
4. Preparation of dilution series
Prepare geometric dilutions: 1:1, 1:2, 1:4, 1:8, 1:16, 1:32, 1:64 (sample:odorless water). Final volume 200 mL.
5. Triangle method testing
For each dilution prepare 3 vessels: 2 with odorless water and 1 with diluted sample. Assessor identifies the different sample.
6. Results recording
Record for each assessor: last dilution at which they correctly identified the sample. Work from highest dilution to lowest.
7. TON calculation
TON = ratio of final volume to sample volume at detection threshold. Result as median from panel assessments. E.g., dilution 1:4 → TON = 4.
8. Taste determination (TFN) — optional
Repeat procedure with taste assessment instead of odor. Assessor tastes sample and spits out. Only for water of confirmed microbiological quality!
9. Quality control
Check that odorless water is truly odorless (negative control). Introduce control sample with geosmin addition (10 ng/L).
10. Documentation and reporting
Record: TON, TFN, odor/taste descriptors, intensity, temperature, panel composition, date and time of test.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Erlenmeyer flasks 500 mL with ground stopper | DURAN, Simax — borosilicate glass | 30–60 PLN/pc |
| Thermostatic water bath | Memmert WNB 14, Julabo CORIO C-B5 | 5,000–15,000 PLN |
| Carbon filter for odorless water preparation | Column with granular activated carbon GAC | 500–2,000 PLN |
| Measuring cylinders 100–500 mL | Brand class A, DURAN certified | 80–200 PLN/pc |
| Laboratory thermometer | Dostmann electronic TFA, resolution 0.1°C | 200–600 PLN |
| Odorless room | Dedicated sensory assessment room with ventilation | Equipment 10,000–30,000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Granular activated carbon (GAC) | 7440-44-0 | For preparing odorless reference water, replacement every 2–4 weeks |
| Distilled / deionized water | — | Water source for filtration through activated carbon, chlorine-free |
| Sodium thiosulfate (Na₂S₂O₃) | 7772-98-7 | For neutralizing residual chlorine in tap water samples, 10 mg/L |
Health and safety (OHS)
- Taste only water of confirmed microbiological quality (no E. coli, enterococci)
- Sodium thiosulfate — harmless, use protective gloves when preparing solutions
- Activated carbon — dust may irritate respiratory tract, use mask when replacing bed
- Sensory assessment room must be free from odors (cosmetics, food, cigarette smoke)