## 3 Olfactory Determinations for Food Shelf Life Assessment: Principles and Main Issues  
In sensory studies, human panelists work together as an instrument to measure, analyze and interpret the data collected by the five senses (sight, touch, smell, taste and hearing), and to characterize a food product during the whole production process (i.e., development of new product, quality control, consumer acceptability, flavor and taste characterization) [15]. Depending on the food analyzed, official methods for sensory analysis are defined and validated to minimize biases derived by external conditioning, such as branding and other information that can influence consumer expectation [16]. As widely reported in the literature [4; 5; 17; 18], the quality decay of many foodstuffs, deriving from significant changes in their sensory features during storage, represents the main issue to limit SL, even before any risk to the safety of consumers occurs. Indeed, manufacturers can profitably use the information derived from sensory studies to select the best formulation or processing conditions to slow down the quality decay during storage. For these reasons, sensory evaluations appear of utmost importance in SL determination for a lot of food categories, even in combination with instrumental or chemical analysis [5]. Among all the organoleptic parameters measured during panel tests, smell plays a fundamental role in the evaluation of food quality and SL through the perception of some specific VOCs that can be directly linked to food spoilage. Humans and animals have all used the olfactory system to detect external risks and take countermeasures [19; 20; 21] (i.e., identifying food safety), based on personal experience and memory, as well as interpersonal characteristics, making the process strongly dependent upon one's cultural background, genetics, associative learning and physiological conditions [19; 22]. Actually, the underlying mechanism of smell perception appears still far from fully understood (see Bierling et al., 2021 [23] for a recent review). Indeed, smell appears as a complex phenomenon involving the intrinsic properties of a volatile, but also depending on the perceiving biological organism [24], making the investigation around it feasible through a systems approach, with the relationships between the physical space, genetic makeup of the organism, physiological activities and smell perception to be considered as a whole. In this context, some widely accepted main points related to the olfactory perception are, until now, well defined, and can be listed below:  
1. At least two conditions have to be addressed for a molecule to be perceived as a smell: the molecule should be volatile enough to evaporate; and the concentration of volatile compounds must exceed the threshold of perception specific for each molecule as a function of operating conditions adopted during tasting. Moreover, depending on its chemical structure, a molecule must show specific solubility to pass through the nasal mucosa (hydrophilic) as well as to bind to the olfactory receptors (hydrophobic) [23]. 2. Intensity is correlated positively with vapor pressure (i.e., the concentration of volatile compounds in the head space), but negatively with hydrophilicity (water solubility) [25]. Interestingly, odor discrimination seems to work independently of measuring intensity; people who cannot characterize odorant qualities as a consequence of brain lesions maintain their ability to determine odor intensity [23; 26; 27]. 3. Limited changes in the chemical structure or functional group of a volatile compound can significantly affect its smell. Thus, a model to predict odor expression from the chemical structure actually cannot be completely defined, even if odorants with the same functional group can often have similar odors [28]. 4. As the aroma of a mixture is different from the simple sum of its components, the whole aroma of a complex system cannot be predicted starting from the concentrations and proportions of single specific volatile compounds [29; 30].