1.2. Threshold value.
Odour threshold is the lowest concentration of a compound that can still be directly recognized by its odour. The threshold concentrations allow comparison of the intensity of odourous substances. Some examples are given in Table 1. These examples illustrate that great differences exist between individual aroma compounds, with an odour potential range of several orders of magnitude.
| Compound | Threshold value / mgL-1 | Compound | Threshold value / mgL-1 |
| Pyrazine | 300 | Ethylbutyrate | 0.001 |
| Ethanol | 100 | (+)-Nootkatone | 0.001 |
| Maltol | 35 | (-)-Nootkatone | 1.0 |
| Hexanol | 0.7 | 2-Methylbutyric acid ethyl ester | 0.0001 |
| Butyric acid | 0.2 | 4-Hydroxy-2,5-dimethyl-3(2H9-furanone | 0.00004 |
| Vanillin | 0.02 | 4-Methoxy-2,5-dimethyl-3(2H9-furanone | 0.00003 |
| Limonene | 0.01 | Methylmercaptan | 0.00002 |
| Linalool | 0.006 | b-Damascone | 0.000009 |
| Hexanal | 0.0045 | b-Ionone | 0.000007 |
| 2-Phenylethanal | 0.004 | 2-Isobutyl-3-methylpyrazine | 0.000002 |
| a-Ionone | 0.004 | 1-p-enthen-8-thiol | 0.00000002 |
| 2- Methylpropanal | 0.001 | ||
| Adapted from Belitz and Grosch (1987) | |||
Some discrepancies can be found in literature for the threshold concentrations. These discrepancies are due to the fact that the threshold concentrations for aroma compounds are dependent on their vapour pressure, which is affected by both temperature and medium. The values are also influenced by the assay procedure and/or the performance of the sensory panel.