The Odor detection threshold is lowest concentration of a certain odor compound that is perceivable by the human sense of smell. The thresholds of a chemical compound is determined in part by its shape, polarity, partial charges and molecular weight. The olfactory mechanisms responsible for a compound's different detection threshold is not well understood, as such, these thresholds cannot yet be accurately predicted. Rather, they must be measured through extensive tests using human subjects in laboratory settings.

Optical isomers can have different detections thresholds since their conformations may cause them to be less perceivable for the human nose. It is only in recent years that such compounds were separated on GLC columns.

Values

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• The threshold value is the concentration an aroma or taste can be detected (air, water and fat).
• The recognition threshold is the concentration at which you can identify an odour (air, water and fat).
• The odour unit is the concentration divided by the threshold.
• The flavour impact is the value the rate of change in perception with concentration.
• The flavour contribution of an aroma component in a mixture to the total profile can be calculated from the total odour units and the number contributed by that aroma chemical.

Variables

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Threshold in a food is dependent upon:

• The threshold of the aroma in air.
• Concentration in the food
• Solubility in oil and water
• Partition coefficient between the air and the food
• The pH of the food, some aroma compounds are effected by the pH - weak organic acids are protionated at low pH making them less soluble and hence more volatile.

The concentration of an odour above a food is dependent on its solubility in that food and its vapour pressure and concentration in that food.

Olfaction | Perfumery