Odor perception, mechanism

The properties of perfume materials are intimately related to their chemical constitution, but the mechanisms whereby chemical structure leads to odor perception involve, in crucial ways, a physical phenomenon the mutual attraction forces between molecules. These forces determine the rate of evaporation of odor materials from solutions or surfaces, they are the basis of fixation and substantivity, they explain why the odor quality of mixtures varies depending upon the solvent or base in which they are incorporated. They are also involved in the very process of odor perception, in the contact between the odorant molecule and the receptor cell. Moreover they are at the heart of distillation, extraction, solubility, and the mechanism of chromatography. In this chapter, we will briefly discuss the physical basis of some of these phenomena, showing also how the attraction forces between molecules are related to their chemical structure. 

Figure 7-1 Schematic Representation of the Taste Process. Source From L.M. Beidler, Facts and Theory on the Mechanism of Taste and Odor Perception, in Chemistry of Natural Food Flavors, 1957, Quartermaster Food and Container Institute for the Armed Forces.

Development history of F F industry represents the eagerness of mankind toward discovering the enigma shrouding the science of odor. This was, and will continue to be, one of the major driving forces in this field of chemistry and biology. Nowadays, aroma science, like the mechanism of odor perception (discussed in Section 4.15.7) and fine aroma chemistry taking into account the concept of chirality, for instance, continues... 

The mechanism of odor perception is very complicated and the least understood of all our senses. It is well accepted that the perception of an odor, meaning the actual recognition by the brain, goes through a cascade of events. 

Most olfaction models in the literature are far too simplistic and too mechanical in nature, and none of them have succeeded in accounting for all of the observations about olfaction. As described, recent advances in our understanding have confirmed that odor perception, as predicted by Polak (19), starts with a combinatorial mechanism at the receptor level (1) and involves pattern recognition in the higher brain (4). No single odorant-receptor interaction will be the sole determinant of odor percept, and even knowledge of the pattern elicited at the olfactory bulb is insufficient to enable prediction of the cortical image of odor. Therefore, structure/odor models are and, for the foreseeable future, will remain statistical tools rather than mechanistic indicators. 

The background of the evaluation of air quahty using human subjects and the currently available methodologies are presented as well as the sources and chemical compounds measured. Section 2 focuses on the perception mechanisms for odour and irritants. Techniques to evaluate air quahty with human panels are described and discussed in Sect. 3. And in Sect. 4 some of the indoor air pollutants are discussed with respect to their odor characteristics and their possible indoor-related sources using the results of several European projects. 

It is helpful to understand how odorants reach the olfactory neurons in humans since this mechanism may be partially responsible for determining aspects of odor perception. The olfactory neurons form a neuroepithelium that line protrusions (turbinates) in the lateral walls of the nasal cavity. The turbinates are a series of folds made of bony lateral extensions. In humans, the majority of olfactory epithelium is located on the olfactory cleft which is directly linked to the olfactory bulb in the brain by the cribiform plate (Figure 1.4). Approximately 6 million neurons pass through the cribiform plate to about 8,0(X) glomeruli in each olfactory bulb. This forms a direct connection between the olfactory receptors and the brain. 

Wilcox R. and Johnson R.E. (1995). Scent counter-marks specialised mechanisms of perception and response to odors in hamsters. J Comp Psychol 109, 349-356. 

Perception of odor is therefore a physical mechanism by which information is processed in the brain. Day by day new odors are appearing, all of which are immediately accepted and sorted within the seemingly unlimited categories of the olfactory brain. The brain not only recognizes this information, but evaluates it, sorts it. and associates it with experiences, events, likes, and dislikes. 

In contrast to visual perception, where the sole stimulus is a photon, and even in contrast to olfaction with its structurally much more diversified stimuli, taste perception is exceptional, because the taste stimuli differ even more than odorants in size and chemical complexity, ranging from H+ ions to carbohydrates, amino adds, and proteins. Consequently, taste transduction mi t involve different mechanisms for different stimuli. In the ihesus monkey, taste reception has been located anatomically to defined loci at either the anterior or the posterior part of the tongue. 

Odor, however, is a very obvious property of any chemical compound, and thus, speculation about the mechanism of perception is very tempting. Experienced fragrance chemists can predict odor type with much better than random accuracy, and a commercial driver exists in terms of design of novel materials for the fragrance industry. Therefore, it is not surprising that many structure/odor correlations and olfaction models have been reported and debated, often very hotly, in the literature. 

Chemical understanding interprets phenomena in terms of the positions and movements of atoms and the forces acting upon them. The Oxford English Dictionary defines phenomenon as "that of which the senses or mind directly takes note an immediate object of perception." This chapter has argued that behavioral performance permits an assessment of what another creature can detect by means of olfaction alone, thus extending the notion of "senses or mind" beyond the realm of the human. The belief that all terrestrial vertebrates share similar olfactory mechanisms implies that these will ultimately prove susceptible to chemical understanding. The confirmation of such understanding will be the ability to archive odors. 

Odor and taste perception permits recognition and discrimination between a large number of different molecules. The detection mechanism is based on the processing of signals from several neurons in an array processing system, and does not require the presence of specific receptor proteins. The same mechanism has been proposed for the action of eye irritant molecules and studies have been made to correlate the absorption behavior of eye irritant compounds in the lipid matrix not only by hydrophobicity, but through a more complicated procedure related to phase transition phenomena in the lipid matrix. 

It has to be remarked that in spite of the widely accepted term electronic nose, current devices are still far from the structure and functions of natural olfaction sense. The unique common feature between artificial and natural system is that both are largely based on arrays of nonselective sensors. The concept underlying electronic nose systems has been demonstrated to be independent on the particular sensor mechanism indeed during the last two decades almost all the available sensor technologies have been utilized as electronic noses. Clearly, all these sensors are very different from the natural receptors. These dissimilarities make the perception of electronic nose very different from that of natural olfaction, so that the instrumental perception of the composition of air cannot be called odor measurement because odor is the sensation of smell as perceived by human olfaction. Nonetheless, the term odor analysis with electronic noses is now largely adopted, but it is important to keep in mind, especially in medical applications, that the electronic nose measurement may be very distant from the human perception. 

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