Odor sensory structure

Fragrance and flavor materials of commercial interest are arranged according to the Beilstein system of functional groups, not according to their sensory properties, since relationships between odor and structure are difficult to establish. However, the Beilstein system has been abandoned in a few cases for practical reasons. 

Reviews of the association of chemical and sensory aspects were published as early as 1965 by Wick and later, for instance, by Vernin (1981), who gave examples of odor-structure relationship. Different structures can be related to similar odors, similar structures to different odors, and similar structures to similar odors. A statistical treatment of data became necessary to correlate analytical results with those obtained by sensory analysis, as stated by Adda and Jounela-Eriksson (1979). Since then, correlation between sensory analysis and instrumental analysis has been the subject of important sessions of the Weurman Flavor Research Symposia, for instance in the chapters on Sensory science in flavor research (Weurman 5th Meeting, 1987) or Correlation between sensory and instrumental analysis (Weurman 7th meeting, 1993). 

However, the relationship between the odors of single odorants and their mixtures can be investigated without regard to the molecular structures of these odorants. The sensory structures of the odors of single component odorants can be characterized, e.g., by multidimensional scaling. The sensory structure of an odorant mixture can also be characterized by some means, and then rules can be explored which tie the odor of the mixture to the odor of components. 

Different odor substances stimulate different patterns of ORCs in the olfactory epithelium, owing to the different sensitivity spectra of the ORCs (28). The pattern of activity in the epithelium evoked by a particular odor substance constitutes the first molecular image of that stimulus, which represents the determinants of the stimulating molecules (13). Thus, although olfaction is not a spatial sensory modality, in contrast, for example, to vision and somatosensation, the initial representation of an odor stimulus in the olfactory pathway does have spatial structure. 

Similarity between odors arises because dissimilar substances or mixtures of compounds may interact with receptors to create similar sensory impressions in the sensory centers of the brain. The group of musk fragrances (comprising macro-cyclic ketones and esters as well as aromatic nitro compounds and polycyclic aromatics) are, for example, compounds with similar odors but totally different structures [5, 6]. Small changes in structure (e.g., the introduction of one or more double bonds in aliphatic alcohols or aldehydes) may, however, alter a sensory... 

It is not yet possible to design a molecule with specific odor (or taste) characteristics because the relations between sensory properties of flavor compounds and their molecular properties are not well understood. As a consequence, the development of compounds with desired flavor qualities has had to rely on relatively tedious synthetic approaches. Recent advances, however, in computer-based methods developed by the pharmaceutical industry to study QSAR (quantitative structure-activity relationships) may ultimately be helpful in the rational design of new flavor-structures with predictable sensory attributes. Results from QSAR studies may also provide insight into the mechanism of the molecule-receptor interaction. 

The sensitivity and selectivity of olfaction and contact chemosensation are due (1) in the brain, to the existence of a neuronal network of neurons tuned to a specific chemical stimulus, and (2) in the periphery, to the existence of olfactory/ chemosensory receptor neurons housed in sensory microorgans called sensilla. The sensilla can best be viewed as simple cuticular porous extrusions that increase the surface that captures airborne odorants or chemicals dissolved in water droplets. They contain the receptive olfactory or chemosensory structures (Schneider, 1969). The olfactory sensilla are most numerous on the antennae and mediate the reception of sex pheromones and plant volatiles, as well as other odorants. Low volatility pheromones may also be detected by contact chemoreceptors on... 

The odorant-binding proteins (OBPs) and the chemosensory proteins (CSPs) are proteins from the lymph that are thought to accomplish these tasks, solubilizing and protecting the odorant and contact chemosensory molecules. This chapter describes the biochemical and evolutionary aspects of these two families of peripheral sensory proteins of insects. Particular attention will be paid to the subclassification of binding proteins, the diversity of gene structures and the phyletic and molecular relatedness between binding proteins from different insect species. 

Olfactory perception translates abstract chemical features of odorants into meaningful neural information to elicit appropriate behavioral responses (Shepherd, 1994 Buck, 1996). Specialized bipolar olfactory sensory neurons (OSNs) are responsible for the initial events in odor recognition. These have ciliated dendrites exposed to the environment, and a single axon that extends into the brain and forms synapses with second order projection neurons (PNs) (Shepherd, 1994 Buck, 1996). In arthropods and mammals, the first olfactory synapse is organized into glomeruli, spherical structures in which afferent olfactory neuron axons synapse with projection neuron dendrites (Hildebrand and Shepherd, 1997). 

The physico-chemical parameters of the chemical stimuli which have been shown to have relevance and to be interrelated to the sensory response it elicits as specific odor or taste, are the factors controlling concentration at the receptor areas (solubility, hydrophilicity, lipophilicity, volatility, and partition coefficients), molecular features (size, shape, stereochemical and chirality factors and functional groups), and electronic features (polarity and dipoles) controlling positioning and contact at receptor surfaces (53). Many of these physico-chemical data are not available for many of the chemical stimulants, and till they are gathered, structure-response studies will be much restricted. The effects of interactions of the above parameters appear to a larger degree in the perception of odor, the dimensions of which are many and complex viz. nuances, composite... 

Molecular structure must be implicated as odorants bind specifically with the sensory receptors called odorant receptors (ORs). The olfactory mucus has proteins called odorant binding proteins (OBPs) that dissolve the odorant molecule in the aqueous/lipid interface of the mucus. The OBPs act as binder molecules to assist the transfer of odorant to the receptor and increase its relative concentration in the mucus relative to inhaled air. They also function to remove used odorants for breakdown and free up the receptor to detect other molecules. 

In conclusion, the study of the wine aroma chemicals and the understanding of the role they play in the different wine aroma nuances have to be structured into a numbers of steps strongly constrained by the previous considerations. Such steps will be the subject of thischapter. The first step is about the screening of aroma molecules, which will be carried out by using gas chromatography-olfactometry. The second will be the isolation and identification of odorants. The third is the quantitative determination, for which only a very brief outline will be given, and the fourth is about the sensory tools used to assess the sensory role played by the different odorants. 

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