## 3 Olfactory Determinations for Food Shelf Life Assessment: Principles and Main Issues
In this context, the interactions among aroma compounds in a mixture can be classified into four types: masking effect, synergistic effect, no effect and additive action [30; 31]. Even if compounds showing different structures generally demonstrate masking actions, while molecules with similar aroma and structure appear to be prone to present additive action or synergistic effect, these behaviors cannot be generalized. Starting from the above-mentioned and widely accepted evidence for the olfactory perception mechanism, it is necessary to highlight that the complete olfactory chain is complex and far from fully understood, possibly leading to several biases, as discussed in the following section:  
1. Final odor perception be can directly influenced by a lot of external variables other than the chemical composition of molecules, that can affect the relationship between smell and chemical structure; during sniffing, the odor concentration that effectively reaches the nostrils can be affected by nasal flow characteristics [28; 32; 33]; odor perception can also be affected by pre-receptor events, such as the bond to odorant-binding proteins or the enzymatic conversion of odorants in the nasal mucus (e.g., conversion to acids and alcohols of aldehydes and esters) [28; 34; 35]. Further, one main issue in the prediction of the final smell expression of a specific olfactory stimulus is that the human olfactory perception is part of a multisensory integration among all the sensorial and social information gathered by our environment, and not a linear analytical process of molecule detection [23]. 2. Odorants can pass through the nasal passages (so-called orthonasal stimulation) and via the mouth (retronasal stimulation) [24]; nasopharyngeal or nasal mucus differ in composition; thus, aroma perception can significantly differ in orthonasal and retronasal olfaction, because of the different solubility of volatile compounds in the two media [36]. The perception of taste appears affected by odors over the retronasal pathway and vice versa; thus, gustatory and olfactory experiences are generally blended [24; 37]. 3. As the number of odorous molecules that humans are able to distinguish appear to be dramatically higher than the number of olfactory receptors identified till now (from 400,000 to 1 million estimated possible odorant molecules [38] vs. only 396 unique olfactory receptors [39]), the most promising theory to represent the method of odor identification is that a small number of olfactory receptors respond to a great number of odorants in a combinatorial way. According to this view, the receptors can be broadly tuned and respond to many different odorants, being most responsive to structurally similar odorants, or narrowly tuned, responding to a small group of odorants [28; 40]. 4. The polymorphism of olfactory receptors represents the molecular basis of the extreme variability widely detected at genetical and physiological levels in the human olfaction perception for both specific sensitivity and general olfactory acuity, with sensitivity varying by several orders of magnitude between individuals [28; 41]. 5. The difficulty called the \"tip-of-the-nose phenomenon\" [24] suggests that olfaction is often an unconscious process [32], during which humans are able to recognize smells, but they often have problems labeling them linguistically. This is likely the reason why defining smell based on olfactory perception is not intuitive and needs the recall to a visual or tactile aspect [42; 43]. 6. The close anatomic relationships between the systems deployed for olfaction and for emotion [44] account for the important links found between these two functions [45; 46; 47; 48; 49]. More than any other sensory modality, olfaction is like emotion in attributing positive (appetitive) or negative (aversive) valence to the environment.