Revisiting psychophysical work on the quantitative and qualitative odour properties of simple odour mixtures: a flavour chemistry view. Part 1: intensity and detectability. A review.

The different psychophysical studies carried out on the quantitative properties of binary mixtures are reviewed. Results from 520 mixtures have been reprocessed and plotted together. The most likely outcome of a binary mixture of two different odorants at supra-threshold levels is a significant reduction of its odour intensity not only with respect to the total addition of intensities, but also to what could be expected from an auto-addition following Stevens's law. Half of the mixtures behave asymmetrically, so that one of the odorants can reduce the overall odour intensity of the mixture more efficiently than the other. This is most frequent when there is a marked difference in the hedonic character of the odorants, and the least pleasant odorant plays the role of suppressor. Such suppression can be quantitatively relevant. The addition of the third, fourth or fifth isointense constituent do not cause any further intensity increase, which indicates the existence of strong signal compression mechanisms at supra-receptor level. Synergism is highly infrequent and may be linked to the appearance of new odour concepts. In the case of mixtures of sub-threshold odorants, the oldest reports are based on the odour activity value concept, and, in general, show the existence of a certain degree of cooperation between the components of the mixture. The most recent research makes use of detectability functions and probabilistic considerations and has demonstrated that cooperation can be categorized as hypoaddition, perfect addition (independence of response) or even synergy. Hypoaddition is most frequent at higher concentrations. Some reports suggest that hypoaddition is related to the structural similarity but results are not conclusive. Other reports demonstrate that under or peri-threshold odorants are able to introduce significant qualitative and quantitative changes on supra-threshold odour solutions. So far it is not feasible to assess when and why this enhancing effect takes place. It can be hypothesized that the answer will require an integral treatment of quality and intensity. Copyright © 2011 John Wiley & Sons, Ltd.

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