Flavour E-Book
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- Herausgeber: John Wiley & Sons
- Kategorie: Fachliteratur
- Sprache: Englisch
This book will cover all aspects of flavour perception, including aroma, taste and the role of the trigeminal nerve, from the general composition of food to the perception at the peri-receptor and central level. This book will answer to a growing need for multidisciplinary approaches to better understand the mechanisms involved in flavour perception.
The book presents the bases of anatomy of sensory perception. It will provide the requisite basic knowledge on the molecules responsible for flavour perception, on their release from the food matrix during the eating process in order to reach the chemosensory receptors, and on their retention and release from and transformation by bodily fluids of the oral and nasal cavities. It will also bring current knowledge on the multimodal interactions.
This book will also cover the recent evolution in flavour science: characterisation of molecules, interaction with food matrix and more recently, physic-chemical and physiological and events during oral processing increasingly considered.
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Veröffentlichungsjahr: 2016
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Table of Contents
Cover
Title Page
Copyright
List of Contributors
Preface
List of Abbreviations
Chapter 1: Olfactory system in mammals: structural and functional anatomy
1.1 Introduction
1.2 Organization and function of the peripheral olfactory system
1.3 Anatomical and functional organization of the main olfactory bulb
1.4 Central odour processing
1.5 Conclusion
References
Chapter 2: Odorant metabolizing enzymes in the peripheral olfactory process
2.1 Introduction
2.2 Odorant metabolizing enzymes
2.3 Functional role of OMEs in the olfactory process
2.4 OME characteristics potentially influencing their function in olfaction
2.5 Conclusion and perspectives
Acknowledgements
References
Chapter 3: The vertebrate gustatory system
3.1 Introduction
3.2 Taste anatomy
3.3 Tastants
3.4 Taste receptors
3.5 Taste signal transduction
3.6 Conclusions
Acknowledgements
References
Chapter 4: Bioadhesion and oral fluids—perireceptor modulators of taste perception?
4.1 Introduction
4.2 Saliva: a complex, dynamic and multifunctional fluid
4.3 Interaction of the oral fluids with taste receptors
4.4 Bioadhesion—A ubiquitous phenomenon in the oral cavity
4.5 The impact of bacteria on taste and function of the taste receptors
4.6 Discussion and ideas for future research
4.7 Conclusion
Acknowledgement
References
Chapter 5: Basic physiology of the intranasal trigeminal system
5.1 Introduction
5.2 Neuroanatomy of the trigeminal system
5.3 Physiology of the trigeminal system
5.4 Assessment of the trigeminal system
5.5 Interactions between trigeminal system and olfaction
5.6 Conclusion
Acknowledgement
References
Chapter 6: Characterization of aroma compounds: structure, physico-chemical and sensory properties
6.1 Introduction: volatile vs. aroma compounds in food
6.2 Food odorants: structure, aroma and physicochemical properties
6.3 Main classes of food odorants
6.4 Conclusion
References
Chapter 7: Characterization of taste compounds: chemical structures and sensory properties
7.1 Introduction
7.2 Sweet-tasting compounds
7.3 Bitter tastants
7.4 Umami and kokumi compounds
7.5 Salty and sour compounds
7.6 Development of new tasting compounds
Acknowledgements
References
Chapter 8: Sensory characterization of compounds with a trigeminal effect for taste modulation purposes
8.1 Introduction
8.2 Trigeminal perception
8.3 Interactions between taste and trigeminal perception
8.4 Taste compounds with trigeminal sensory qualities
8.5 Conclusion
Acknowledgements
References
Chapter 9: Interactions between aroma compounds and food matrix
9.1 Introduction
9.2 Physico-chemical characteristics of aroma compounds influencing binding and release
9.3 Physico-chemical interactions between aroma compounds and different classes of non-volatile compounds
9.4 Simultaneous effects of different components in complex matrices
9.5 Modelling binding and release of flavour compounds
9.6 Conclusion
References
Chapter 10: Aroma release during in-mouth process
10.1 Instrumental techniques for monitoring in-mouth aroma release
10.2 Model and real systems of aroma release
10.3 Aroma release and matrix interactions
10.4 Sensory analysis and
in vivo
aroma release
10.5 Conclusion and perspectives
References
Chapter 11: Release of tastants during in-mouth processing
11.1 Introduction
11.2 Instrumental techniques to measure in-mouth release of tastants
11.3 Influence of food composition on the release of tastants at the macromolecular level
11.4 Influence of composition on the release of tastants at the microscopic and molecular levels
11.5 Influence of individual oral physiology on the release of tastants
11.6 Attempts to model the release of tastants
11.7 Conclusion
References
Chapter 12: Interactions between saliva and flavour compounds
12.1 Introduction
12.2 Influence of flavour compounds on salivation
12.3 Flavour compounds transport to sensory receptors
12.4 Binding of flavour compounds by saliva
12.5 Modification of flavour compounds by saliva
12.6 Conclusion
References
Chapter 13: Orthonasal and retronasal perception
13.1 Significance of ortho- and retronasal perception
13.2 Methods to investigate ortho- and retronasal olfaction
13.3 Differences between orthonasal and retronasal olfaction
13.4 Retronasal olfaction and gustatory or mechanosensory influences
13.5 Clinical aspects of retronasal olfaction
13.6 Conclusion
References
Chapter 14: Perception of mixtures of odorants and tastants: sensory and analytical points of view
14.1 Introduction
14.2 Perceptual aspects of odour mixtures
14.3 Recombination strategies
14.4 Mixture of tastants
14.5 Odour-taste interactions in real foods
14.6 Conclusion
References
Chapter 15: Odour mixture coding from the neuronal point of view
15.1 Introduction
15.2 Peripheral neural coding of mixtures
15.3 Olfactory bulb coding of mixtures
15.4 What remains to be achieved by the olfactory cortex for giving birth to the perceptual object?
15.5 Conclusion on mixture coding performances over the olfactory system
References
Chapter 16: Multisensory flavour perception
16.1 Introduction
16.2 Olfactory-gustatory interactions in multisensory flavour perception
16.3 Oral-somatosensory contributions to multisensory flavour perception
16.4 Eating with our eyes: on the visual modulation of multisensory flavour perception
16.5 Auditory contributions to multisensory flavour perception
16.6 The cognitive neuroscience of multisensory flavour perception
16.7 Conclusions
References
Index
Food Science and Technology Books
End User License Agreement
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Guide
Cover
Table of Contents
Preface
Begin Reading
List of Illustrations
Chapter 1: Olfactory system in mammals: structural and functional anatomy
Figure 1.1 Schematic drawing of the rodent olfactory system (sagittal cross section through the nasal region of the head, lower jaw is not shown). Inset
A
shows the different cell layers observed in the olfactory bulb and the neuronal connections. Inset
B
represents the various cell types and structures located in the olfactory mucosa. Inset
C
schematizes the connectivity of glutamatergic neurons in the PCx (Source: Adapted from Ekberg and St John 2014, Haberly 2001). Abbreviations: AOB, accessory olfactory bulb; aPCx, anterior piriform cortex; DP, deep pyramidal cells; EPL, external plexiform layer; FB, feedback interneurons; FF, feed forward interneurons; G, glomeruli; GG, Grueneberg ganglion; GL, glomerular layer; Gr, granule cells; GrL, granular cell layer; IPL, internal plexiform layer; LOT, lateral olfactory tract; M, mitral cells; MCL, mitral cell layer; Mp, multipolar cells; OB, olfactory bulb; OE, olfactory epithelium; ONL, olfactory nerve layer; pgC, periglomerular cells; pPCx, posterior piriform cortex; OSN, olfactory sensory neuron; SL, semilunar cells; SO, septal organ; SP, superficial pyramidal cells; T, tufted cells. A colored version of this Figure can be found in the online version of this chapter.
Figure 1.2 Schematic illustration of the main anatomical efferents arising from the main olfactory bulb in rodents. The primary cortex consists in regions receiving bulbar outputs conveyed by the LOT. The piriform cortex relays the olfactory information to several neocortical areas involved in complex processes such as multisensory integration, flavour perception and decision-making. It is also connected with the lateral hypothalamus that plays a role in feeding behaviour. Within the amygdala, bulbar efferents primarily target the superficial cortical nuclei (Aco, PLco) which are connected to deep nuclei such as BLA. The NLOT is located in the rostral part of the amygdala. Unlike other parts of olfactory amygdala, it does not project directly to the hypothalamus. Olfactory information is also transmitted to reward circuit through the olfactory tubercle efferents reaching the ventral striatum. The entorhinal cortex, as a gateway to hippocampus, allows olfactory-related mnesic processes. Abbreviations: Aco, anterior cortical nucleus of amygdala; AI, agranular insular cortex; AON: anterior olfactory nucleus; BLA, basolateral amygdala; IL, infralimbic cortex; LEC, lateral entorhinal cortex; LH, lateral hypothalamus; LOT, lateral olfactory tract; NLOT, nucleus of the lateral olfactory tract; OFC, orbitofrontal cortex; OT, olfactory tubercle; PCx, piriform cortex; PLco, posterolateralcortical nucleus of amygdala; SON, supraoptic nucleus; TT, tenia tecta. A colored version of this Figure can be found in the online version of this chapter.
Chapter 2: Odorant metabolizing enzymes in the peripheral olfactory process
Figure 2.1 Schematic representation of the OE structure. The right part of the Figure presents the different phases of the metabolism towards an odorant xenobiotic. Phase III presents the hypothetical excretion of phase I and phase II metabolites in the olfactory mucus. CYP, cytochrome P450; GST, glutathione-S-transferase, UGT, UDP-glucuronosyltransferase; MRP, multidrug resistance-associated protein; MDR, multidrug resistance protein.
Chapter 3: The vertebrate gustatory system
Figure 3.1 Taste buds of the mouse vallate papillae express bitter taste receptor genes in a subset of type II taste receptor cells. Cross-section of a taste bud from the vallate papillae of a knock-in mouse expressing green fluorescent protein (top, left) under the control of the Tas2r131 gene promoter stained with an α-gustducin antiserum (top, right). The overlay image (bottom) shows that Tas2r131 signals are confined to a subset of α-gustducin cells. A colored version of this Figure can be found in the online version of this chapter.
Figure 3.2 Homology model of the human bitter taste receptor TAS2R38. Ribbon model of the TAS2R38 taster variant shown from the side. The extracellular side points to the top of the image, the intracellular side to the bottom. The three amino acid positions that differ between the main taster and non-taster variants are labeled. The non-taster variant would show alanine in position 49, valine in position 262, and isoleucine in position 296 instead. The image is based on a previously published in silico model (Source: Adapted from Biarnes et al. 2010).
Chapter 4: Bioadhesion and oral fluids—perireceptor modulators of taste perception?
Figure 4.1 Scheme on interactions of tastants and taste pore material (the illustration was modified according to Matsuo 2000). There are three different types of taste receptor cells: dark type cells, and light type II and III cells (Source: Adapted from Azzali 1997, Liman et al. 2014). In general, the taste pore material serves as a barrier and buffer, but due to certain enzymes and proteins tastants might be processed and modified; PRPs: proline rich proteins; CAVI: carbonic anhydrase VI.
Chapter 5: Basic physiology of the intranasal trigeminal system
Figure 5.1 Overview of the trigeminal pathway. Neurons within sensory fibers from V1 (ophthalmic nerve), V2 (maxillary nerve) and V3 (mandibular nerve) have their cell bodies within the semilunar ganglion in Meckel's cave (SL); their first relay station is in the trigeminal nucleus of the brain stem (TN). Second order neurons project to the ventro-posteriormedial nucleus of the thalamus (TH). Third order neurons project to the somatosensory cortex (SS) in the parietal cortex. Note the somatotopy which is preserved from the periphery to the central nervous system. A colored version of this Figure can be found in the online version of this chapter.
Chapter 6: Characterization of aroma compounds: structure, physico-chemical and sensory properties
Figure 6.1 Structures of alkyl substituted γ-lactones with different odour descriptors. [1] (R)-γ-octalactone (coconut notes with almond notes, spicy green); [2] (R)-γ-nonalactone (soft coconut with fatty-milky aspects, strong, sweet); [3] (R)-γ-decalactone (caramel, fatty sweet fruity note, soft coconut); [4] (R)-γ-dodecalactone (bloomy notes with aldehyde and woody aspects, strong, fruit-sweet). Source: Adapted from Cooke et al. (2009).
Figure 6.2 Odoriferous pyrazines with related structures. [1] 2-ethyl-3,5-dimethylpyrazine (OT 0.04 μg/L in water); [2] 2-ethyl-3,6-dimethylpyrazine (OT 8.6 μg/L in water); [3] 2-ethenyl-3-ethyl-5-methylpyrazine (OT 0.014 ng/L in air); [4] 3-ethenyl-2-ethyl-5-methylpyrazine (OT 109.5 ng/L in air); [5] 2-methoxy-3,5-dimethylpyrazine (OT 0.01ng/L in air); [6] 3-methoxy-2,5-dimethylpyrazine (OT 56 ng/L in air).
Figure 6.3 Whisky lactones (quercus lactones) enantiomeric structures with different odour notes.
Figure 6.4 Wine lactones enantiomers with their odour thresholds (ng/L in air). Source: Adapted from Guth (1996).
Figure 6.5 Fresh green odour components from green leaves. [1] – (3Z)-hexenol (leaf alcohol), [2] – (3Z)-hexenal, [3] - n-hexanol, [4] – (3E)-hexenol, [5] – (3E)-hexenal, [6] – n-hexanal. [7] – (2E)-hexenol. [8] – (2E)-hexenal (leaf aldehyde). Source: Adapted from Hatanaka (1996).
Figure 6.6 Basic heterocyclic structures of aroma compounds in food. [1] tiophene; [2] 1-pyrroline; [3] thiazole; [4] 2-furanone; [5] 4-pyrone; [6] pyrazine; [7] pyridine.
Figure 6.7 Carotenoid degradation products found as food odorants. [1] – β-ionone, [2] – β-damascenone, [3] – (E,E)-megastigma-4,6,8-triene, [4] – β-damascone, [5] – vitispirane, [6] – theaspirone.
Chapter 7: Characterization of taste compounds: chemical structures and sensory properties
Figure 7.1 Chemical structure of natural sweet-tasting compounds.
Figure 7.2 Chemical structure of artificial sweet-tasting compounds.
Figure 7.3 Three-dimensional structure of sweet-tasting proteins.
Figure 7.4 Chemical structure of some typical bitter-tasting molecules.
Figure 7.5 Chemical structure of some umami compounds.
Figure 7.6 Chemical structure of some kokumi compounds.
Figure 7.7 Chemical structure of some sour molecules.
Figure 7.8 Chemical structure of new tasting molecules.
Chapter 8: Sensory characterization of compounds with a trigeminal effect for taste modulation purposes
Figure 8.1 Approach to screening and characterization of samples with potential taste modulation and trigeminal effects.
Figure 8.2 Sample evaluation procedure in water and water-based model systems.
Figure 8.3 Sensory attributes and taste modulation properties of alligator pepper extract tasted in water-based model systems (Test performed by 15 trained panellists).
Figure 8.4 Sensory characterization of (R)-strombine and glycine at various concentrations: Number of subjects (Out of 30 panellists) who selected the proposed sensory attributes.
Figure 8.5 Detection thresholds for polygodial in comparison with those of capsaicin and piperine.
Figure 8.6 Intensity of cooling effect over time for menthol, dihydroumbellulol and water.
Chapter 10: Aroma release during in-mouth process
Figure 10.1 Schematic representation of an APCI source. Source: Quéré et al. (2014). Reproduced with permission of John Wiley & Sons.
Figure 10.2 Schematic representation of a PTR-MS instrument. Source: Blake et al. (2009). Reproduced with permission of American Chemical Society.
Figure 10.3 Schematic representations of three model mouths (a–c) and an artificial throat (d). Source: (a) Ruth (2000). Reproduced with permission of Elsevier; (b) Deibler et al. (2001). Reproduced with permission of American Chemical Society; (c) Salles et al. (2007). Reproduced with permission of Elsevier; (d) Weel et al. (2004). Reproduced with permission of American Chemical Society.
Figure 10.4 Typical aroma-release profile presented by two groups of panellists for three milk gels of different hardness (M0, M3, and M10). The dashed line represents the main swallow event. Source: Gierczynski et al. (2008). Reproduced with permission of American Chemical Society.
Figure 10.5 Radial plots representing aroma release profiles for three different coffees (DECAF, DARK, and MEDIUM). Profiles are portrayed in aggregate (left) or individual (right) form for two selected panellists (p1 and p2). The semi-circular bands on the outer margins represent chemical classes, based upon tentative identification of the peaks. Circles mark significant differences among coffee types (ANOVA and Tukey's test,
p
< 0.05). Source: Romano et al. (2014). Reproduced with permission of Elsevier.
Chapter 11: Release of tastants during in-mouth processing
Figure 11.1 Influence of Fat/Dry matter ratio of a model cheese on in-mouth sodium release (C max, top panel) and saltiness perception (I max, lower panel). Source: Lawrence et al. (2012a). Reproduced with permission of American Chemical Society.
Figure 11.2 Concentrations of sodium in saliva (a) and saltiness perception (b) During the eating of model cheeses over time for three subjects. Source: Phan et al. (2008). Reproduced with permission of Elsevier.
Chapter 12: Interactions between saliva and flavour compounds
Figure 12.1 Structure of the lipocalin-1 (LCN1).
A
. ribbon diagram of topview into the calyx of LCN1;
B
. surface representation of topview into the calyx of LCN1;
C
. ribbon diagram of side view of LCN1;
D
. surface representation of side view of LCN1.
Figure 12.2 Theoretical mechanisms of astringency. 1-formation of soluble PRP•tannin noncovalent complexes; 2- formation of soluble PRP•tannin aggregates; 3- precipitation of PRP•tannin aggregates; 4- alteration of the properties of the salivary mucosal pellicle due to tannin binding.
Chapter 13: Orthonasal and retronasal perception
Figure 13.1 There are two pathways for odours to reach the olfactory epithelium. The orthonasal route (dark arrow) is used during sniffing, for example to identify odours like rose, smoke, or other odorants in the environment. Through the retronasal route (light arrow), the flavour of foods can be perceived (Source: Adapted from Bojanowski and Hummel 2012).
Figure 13.2 Flavour identification test (“Schmeckpulver”). Photograph of the psychophysical investigation of retronasal olfactory perception. Powders like cacao, paprika or garlic are applied to the tongue. Subjects have to select one of four items that best describes the flavour (Source: Adapted from Heilmann et al. 2002).
Chapter 14: Perception of mixtures of odorants and tastants: sensory and analytical points of view
Figure 14.1 Different percepts that can arise from an odour mixture of two odours noted A and B. C is an odour dissimilar to A and B. The size of the circles reflects perceived intensity.
Chapter 15: Odour mixture coding from the neuronal point of view
Figure 15.1 Curves of EOGs' amplitudes as a function of concentrations for OCT and CIT, singly and in mixture at different ratios. The curves show the CIT suppressive action on mixture response. This suppression is similar for 1/1 and 10/1 ratios whereas for the 1/10 ratio, EOGs reflect the dominating action of CIT. Indeed in the insert box, the two EOG curves, for CIT alone and OCT/CIT at 1/10 ratio, are similar when they are represented as a function of CIT concentrations alone. It is as if, at 1/10 mixture ratio, OCT effect was totally masked by the presence of CIT. As the mixture was designed in vapour phase, whatever the ratio and the concentrations to which compounds are delivered, CIT was always less concentrated than OCT. So the ratios 1/1 10/1 and 1/10 only proportionally modulated this difference but never resulted in equalizing or getting CIT more concentrated than OCT. These experimental conditions perfectly respected ecological conditions.
Figure 15.2 Single ORN recording
in vivo
in rats stimulated with OCT, CIT and OCT/CIT mixtures. The rough spike recordings as well as the concentration/response frequency curves show that, even though a silent actor when delivered alone, CIT suppressed in a ratio-dependent manner the OCT response in mixture. The grey horizontal bars under the single recordings indicate the 2-second stimulation delivery.
Figure 15.3 Hypothetical isosteric-syntopic (competitive) or allosteric interactions of two odorant molecules (A and B) at single ORN level. The two types of interactions can result in mixture suppression: the AB mixture response frequency is lower than that of the best compound (A). Here, when delivered alone, B is silent and thus unable to induce an observable ORN excitation. When delivered simultaneously to A, the silent actor B actively concurs in reducing the mixture ORN response.
Figure 15.4 Hypothetical engagement of PI3K and cAMP signaling pathways in ORN response to odorants (Source: Adapted from Ukhanov et al. 2011). On the left, schematic representation of OR and signaling pathways. On the right, calcium excitatory responses of one isolated ORN: the left trace was obtained in control condition and the right one in presence of PI3K pharmacological blocker.
A
: OCT delivered alone is assumed to bind and excite the OR/ORN exclusively throughout the cAMP pathway. So the blocking of PI3 kinase (PI3K) pathway (symbolized by the rectangular grey mask) does not impact OCT response.
B
: OCT/ CIT mixture is assumed to engage both cAMP by OCT and PI3K by CIT, the latter wholly or partly suppressing the OCT response. In mixture condition, the pharmacological blocking of PI3K activity restores an excitatory response similar to this induced by OCT alone.
C
: CIT delivered alone is mainly observed to be a silent actor. However PI3K blocking revealed an excitatory response mediated through cAMP, demonstrating that CIT can become an excitatory actor by switching from PI3K pathway to cAMP if the former is unavailable.
Chapter 16: Multisensory flavour perception
Figure 16.1 This Figure highlights the results of a series of experiments conducted by Dalton and colleagues, showing the integration of orthonasal olfactory and gustatory cues (Source: Adapted from Dalton et al. 2000). When a sub-threshold saccharin solution was placed on the participant's tongue, a significant increase in olfactory sensitivity (to Benzaldehyde; Benz) was observed, despite the fact that the tastant had no odour, thus demonstrating the multisensory interaction of olfaction and taste. By contrast, holding a small amount of water in the mouth had no effect on olfactory thresholds, nor did holding a solution that happened to contain monosodium glutamate (MSG). These latter results highlight the stimulus dependency (both olfactory and gustatory) of multisensory integration in human flavour perception, a result that has now been demonstrated in many other studies.
Figure 16.2 The results of Zampini et al.'s (2008) study highlighting the influence of colour on people's ability to correctly identify orange- and blackcurrant-flavoured solutions (Source: Adapted from Zampini et al., 2008). The results are separated by the taster status of the participant (non-taster, medium taster, or supertaster). Overall, non-tasters identified 19% of the solutions correctly, the medium tasters 31%, and the supertasters 67%.
Figure 16.3 Results highlighting the effect of changing the food-eating sounds heard by participants on their multisensory perception of the crispness of potato chips (Source: Zampini and Spence, 2004. Reproduced with permission of John Wiley & Sons.). The self-produced biting sounds were fedback over headphones and attenuated by 0, −20 and −40 dB. Potato chips were judged as crisper (and fresher) when the overall sound level was boosted, and/or when just the high frequency components of the sound (> 2kHz) were amplified.
List of Tables
Chapter 1: Olfactory system in mammals: structural and functional anatomy
Table 1.1 Numbers of OR genes in the genome sequence from 13 placental mammalian species (Source: Adapted from Niimura et al. 2014). An intact gene was defined as a sequence starting from an initiation codon and ending with a stop codon that did not contain any disrupting mutation. A pseudogene was defined as a sequence with a nonsense mutation, frameshift, deletion within conserved regions, or some combination thereof. A truncated gene represents a partial sequence of an intact gene
Chapter 2: Odorant metabolizing enzymes in the peripheral olfactory process
Table 2.1 Summary of the different studies investigating OME function in the olfactory process
Table 2.2 Prediction of the human OME subcellular localization. All the phase I and II OMEs detected in the human OE were analyzed. Human orthologues of the rat and mice OMEs also detected in the OE were added. The sequences were analyzed with SIGNALP 4.0. For each protein, the UniProt code/accession number, the predicted localization (S for secreted, NS for non-secreted or SA for anchor secreted), the secretion, anchoring and cleavage probabilities and the cleavage site are indicated
Chapter 3: The vertebrate gustatory system
Table 3.1 Categorization of bitter taste receptors of different species based on their tuning width
Table 3.2a Known agonists of TAS2R14
Table 3.2b Compounds that do not activate TAS2R14
Chapter 4: Bioadhesion and oral fluids—perireceptor modulators of taste perception?
Table 4.1 Functional components of oral fluids potentially relevant for taste perception in man (findings based only on investigations in animals are specified in brackets)
Chapter 6: Characterization of aroma compounds: structure, physico-chemical and sensory properties
Table 6.1 Selected heterocyclic compounds of high sensory importance in foods
Chapter 7: Characterization of taste compounds: chemical structures and sensory properties
Table 7.1 Sweet-tasting compounds, relative sweetness compared to sucrose and aftertaste
Table 7.2 Bitter-tasting compounds, chemical classes, examples of food source and detection thresholds
Table 7.3 Concentration of Free L-Glutamic Acid in Foods
Table 7.4 Concentration of 5' ribonucleotides in foods
Chapter 8: Sensory characterization of compounds with a trigeminal effect for taste modulation purposes
Table 8.1 Sensory qualities associated to different types of trigeminal stimuli. The dark grey indicates the main sensation at usual concentrations in food products
Table 8.2 Terms used in the sensory characterization of taste compounds with trigeminal sensory qualities
Table 8.3 Examples of water-based model systems to screen trigeminal and taste modulation effects
Chapter 9: Interactions between aroma compounds and food matrix
Table 9.1 Main physico-chemical characteristics of aroma compounds
Table 9.2 Air/Matrix partition coefficients (w/v) of aroma compounds in systems containing lipids
Table 9.3 Air/Matrix partition coefficients (w/v) of aroma compounds in water solutions
Chapter 10: Aroma release during in-mouth process
Table 10.1 Proton affinities of the main air constituents and some volatile organic compounds representative of different chemical classes
Chapter 12: Interactions between saliva and flavour compounds
Table 12.1 Summary of the proteins whose abundance differs after a gustatory stimulation
Flavour
From Food to Perception
Edited by
Elisabeth Guichard
Center for Taste and Feeding Behaviour, CNRS, INRA, University of Bourgogne Franche-Comté, Dijon, France
Christian Salles
Center for Taste and Feeding Behaviour, CNRS, INRA, University of Bourgogne Franche-Comté, Dijon, France
Martine Morzel
Center for Taste and Feeding Behaviour, CNRS, INRA, University of Bourgogne Franche-Comté, Dijon, France
Anne-Marie Le Bon
Center for Taste and Feeding Behaviour, CNRS, INRA, University of Bourgogne Franche-Comté, Dijon, France
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Library of Congress Cataloging-in-Publication Data
Names: Guichard, Elisabeth, 1956- editor. | Salles, Christian, editor. | Morzel, Martine, editor. | Le Bon, Anne Marie, editor.
Title: Flavour : from food to perception / edited by Elisabeth Guichard, Christian Salles, Martine Morzel, Anne Marie Le Bon.
Description: Chichester, West Sussex ; Hoboken, NJ : John Wiley & Sons Inc., 2017. | Includes bibliographical references and index.
Identifiers: LCCN 2016032586| ISBN 9781118929414 (cloth) | ISBN 9781118929407 (Adobe PDF) | ISBN 9781118929391 (epub)
Subjects: LCSH: Taste. | Flavor. | Chemical senses.
Classification: LCC QP456 .F53 2017 | DDC 612.8/7–dc23 LC record available at https://lccn.loc.gov/2016032586
A catalogue record for this book is available from the British Library.
Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books.
Cover image: Art\_of\_Sun/Getty Images
List of Contributors
Carmen Barba
Center for Taste and Feeding Behaviour, CNRS, INRA, University of Bourgogne Franche-Comté, Dijon, France
Maik Behrens
Department Molecular Genetics, German Institute of Human Nutrition Potsdam-Rehbruecke, Nuthetal, Germany
Loïc Briand
Center for Taste and Feeding Behaviour, CNRS, INRA, University of Bourgogne Franche-Comté, Dijon, France
Francis Canon
Center for Taste and
Feeding Behaviour, CNRS, INRA, University of Bourgogne Franche-Comté, Dijon, France
Isabelle Cayeux
Firmenich SA Research & Development Department, Geneva, Switzerland
Frédérique Datiche
Center for Taste and Feeding Behaviour, CNRS, INRA, University of Bourgogne Franche-Comté, Dijon, France
Patricia Duchamp-Viret
Centre de Recherche en Neurosciences de Lyon (CRNL), UMR CNRS 5292, INSERM U1020, Université Lyon1, Lyon Cedex, France
Philippe Faure
Center for Taste and Feeding Behaviour, CNRS, INRA, University of Bourgogne Franche-Comté, Dijon, France
Johannes Frasnelli
CÉAMS, Research Center, Sacré-Coeur Hospital, Montréal, Québec, Canada
Department of Anatomy, Université du Québec à Trois-Rivières, Trois-Rivières, Québec, Canada
Jean Gascuel
Center for Taste and Feeding Behaviour, CNRS, INRA, University of Bourgogne Franche-Comté, Dijon, France
Florence Gingras-Lessard
CÉAMS, Research Center, Sacré-Cœur Hospital, Montréal, Québec, Canada
Faculté de Médecine, Université de Montréal, Montréal, Québec, Canada
Anna Gracka
Faculty of Food Science and Nutrition, Poznań University of Life Sciences, Poznań, Poland
Xavier Grosmaitre
Center for Taste and Feeding Behaviour, CNRS, INRA, University of Bourgogne Franche-Comté, Dijon, France
Elisabeth Guichard
Center for Taste and Feeding Behaviour, CNRS, INRA, University of Bourgogne Franche-Comté, Dijon, France
Christian Hannig
Polyclinic of Operative Dentistry and Pediatric Dentistry, Medical Faculty Carl Gustav Carus, TU Dresden, Dresden, Germany
Matthias Hannig
Clinic of Operative Dentistry,
Periodontology and Preventive Dentistry, University Hospital, Saarland University, Homburg/Saar, Germany
Hassan-Ismail Hanser
Center for Taste and Feeding Behaviour, CNRS, INRA, University of Bourgogne Franche-Comté, Dijon, France
Jean-Marie Heydel
Center for Taste and Feeding Behaviour, CNRS, INRA, University of Bourgogne Franche-Comté, Dijon, France
Thomas Hummel
Smell & Taste Clinic, Department of Otorhinolaryngology, TU Dresden, Dresden, Germany
Henryk Jeleń
Faculty of Food Science and Nutrition, Poznań University of Life Sciences, Poznań, Poland
Anni Laffitte
Center for Taste and Feeding Behaviour, CNRS, INRA, University of Bourgogne Franche-Comté, Dijon, France
Anne-Marie Le Bon
Center for Taste and Feeding Behaviour, CNRS, INRA, University of Bourgogne Franche-Comté, Dijon, France
Wolfgang Meyerhof
Department Molecular Genetics, German Institute of Human Nutrition Potsdam-Rehbruecke, Nuthetal, Germany
Martine Morzel
Center for Taste and Feeding Behaviour, CNRS, INRA, University of Bourgogne Franche-Comté, Dijon, France
Fabrice Neiers
Center for Taste and Feeding Behaviour, CNRS, INRA, University of Bourgogne Franche-Comté, Dijon, France
Eric Neyraud
Center for Taste and Feeding Behaviour, CNRS, INRA, University of Bourgogne Franche-Comté, Dijon, France
Laurianne Paravisini
Center for Taste and Feeding Behaviour, CNRS, INRA, University of Bourgogne Franche-Comté, Dijon, France
Department of Food Science and Nutrition, University of Minnesota, Saint Paul, Minnesota, United States
Andrea Romano
Department of Food
Quality and Nutrition, Research and Innovation Centre, Fondazione Edmund Mach (FEM), San Michele all'Adige, Italy
Faculty of Science and Technology, Freie Universität Bozen – Libera Università di Bolzano, Bozen-Bolzano, Italy
Christian Salles
Center for Taste and Feeding Behaviour, CNRS, INRA, University of Bourgogne Franche-Comté, Dijon, France
Han-Seok Seo
Department of Food Science, University of Arkansas, Fayetteville, Arkansas, U.S.A
Charles Spence
Crossmodal Research Laboratory, Department of Experimental Psychology, University of Oxford, Oxford, UK
Christian Starkenmann
Firmenich SA Research & Development Department, Geneva, Switzerland
Thierry Thomas-Danguin
Center for Taste and Feeding Behaviour, CNRS, INRA, University of Bourgogne Franche-Comté, Dijon, France
Preface
Flavour perception plays a key role in the acceptability of food by consumers. Flavour science has become a broad research subject aiming to provide a comprehensive understanding of flavour from its generation in food to its perception during eating. All the aspects of flavour perception are presented, including aroma, taste and trigeminal sensation, from the general composition of food to the perception at the peri-receptor, receptor and central level taking into account the recent evolution in flavour science. The book responds to a growing need for pluridisciplinary approaches to better understand the mechanisms involved in flavour perception. It provides to scientists and industries working on flavour and food a general view of the mechanisms of flavour perception including multimodal interactions between sensory modalities. This will offer new tools and approaches for the reformulation of foods, for example increasing the organoleptic acceptability of products with low salt, fat and sugar content or elaborated with new sources of fat and proteins.
The first part brings basic knowledge on the chemosensory systems involved in the perception of flavour molecules, complemented with the latest scientific papers. This includes the structural and functional anatomy of olfactory, gustatory and trigeminal systems in vertebrates and the olfactory and gustatory peri-receptor mechanisms.
The second part, focused on flavour compounds in food, provides basic knowledge on the structural, physico-chemical and sensory properties of flavour (aroma, taste and trigeminal) compounds and their interactions with the different macromolecules present in the food matrix.
The third part describes in more details the in-mouth mechanisms leading to the release of aroma and taste compounds from the food matrix during the eating process and their transformation by bodily fluids of the oral cavity before reaching the chemosensory receptors.
The fourth part covers different aspects of flavour perception, from orthonasal and retronasal perception, biological and analytical aspects involved in the perception of mixture of odorants and tastants to multimodal sensory interactions.
List of Abbreviations
3D
3 dimensions
γ
ι
activity coefficient
ABT
1-aminobenzotriazole
Aco
anterior cortical nucleus of amygdala
ADH
alcohol dehydrogenase
AEDA
aroma extract dilution analysis
AI
agranular insular cortex
AL
antennal lobe
AMP
adenosine-5'-monophosphate
AOB
accessory olfactory bulb
AON
anterior olfactory nucleus
AOS
allene oxide synthase
APCI-MS
atmospheric pressure chemical ionization-mass spectrometry
aPCx
anterior piriform cortex
a
PRP
acidic proline-rich protein
ATP
adenosine triphosphate
AUC
area under the curve
BLA
basolateral nucleus
BNPP
bis-p-nitro-phenylphosphate
BOLD
blood-oxygen-level dependent
b
PRP
basic proline-rich protein
Ca
calcium
cAMP
cyclic adenosine monophosphate
CaSR
calcium sensing receptor
CAVI
carbonic anhydrase VI
CD
cyclodextrin
C
G
concentration of the volatile in the headspace
cGMP
cyclic guanosine monophosphate
CHARM
combined hedonic aroma response measurement
CIT
citral
C
M
concentration of the volatile compound in the matrix
CNGC
cyclic nucleotide gated channels
CNV
cranial nerve 5
CYP
cytochrome P450 enzyme
Da
dalton
DE
dextrose equivalent
DF
detection frequency
DIMS
direct injection-mass spectrometry
d
liq
liquid density (kg L
−1
)
DOSY
diffusion ordered spectroscopy investigation
DP
deep pyramidal cell
EEG
electro-encephalogram
EgCG
epigallocatechin gallate
EGF
epidermal growth factor
ENaC
epithelial sodium channel
EOG
electro-olfactogram
EPL
external plexiform layer
ER
endoplasmic reticulum
ERP
event-related potentials
ESP
exocrine-gland-secreting peptide
FB
feedback interneuron
FDA
food and drug administration
FF
feed forward interneuron
FFT
furfurylthiol
FID
flame ionisation detection
fMRI
functional magnetic resonance imaging
FPR
formyl peptide-like receptor
FSCM
finger span cross modality
GBC
globose basal cell
GC-GOOD
gas chromatography-global olfactometry omission detection
GC-MS
gas chromatography-mass spectrometry
GC-O
gas chromatography-olfactometry
GC-PO
gas chromatography-pedestal olfactometry
GC-R
gas chromatography recomposition-olfactometry
GDP
guanosine diphosphate
GG
Grueneberg ganglion
GL
glomerular layer
GMP
guanosine 5'-monophosphate
GPCR
G protein-coupled receptor
g
PRP
glycosylated proline-rich protein
Gr
granule cells
GRAS
generally regarded as safe
GrL
granular cell layer
GSH
glutathione
GST
glutathione-S-transferase
HEK
human embryonic kidney cell
HPL
hydroperoxide lyase
HPLC
high performance liquid chromatography
Hst
histatin
IAA
isoamyl acetate
IBMX
3-isobutyl-1-methylxanthine
IDP
intrinsically disordered protein
IL
infralimbic cortex
IMAC
in-mouth air cavity
Imax
maximum intensity of the release profiles
IMP
inosine-5'-monophosphate
IPL
internal plexiform layer
ISO
International Standards Organization
K
gas matrix partition coefficient
K
d
dissociation constant
k
i
molar concentration
K
i
molar fraction
k
m
mass fraction
L
liter
LCN1
lipocalin-1
LEC
lateral entorhinal cortex
LH
lateral hypothalamus
LISS
ligand-induced selective signaling
LN
local interneurons
LOT
lateral olfactory tract
M
a
molar mass of air (28.8 g mol
−1
)
MC
mitral cell
MCL
mitral cell layer
MDR
multidrug resistance
mGluR
metabotropic glutamate receptor
M
liq
molar mass of liquid (g mol
−1
)
MMP-3
matrix metalloproteinase 3
MOE
main olfactory epithelium
Mp
multipolar cell
MRI
magnetic resonance imaging
MRP
multidrug resistance-associated protein
MSG
monosodium glutamate
MUC
mucin
MUC1
mucin 1
MUC5B
mucin 5B
MUC7
mucin 7
NaCl
sodium chloride
NHDC
neohesperidin dihydrochalcone
NIF
nasal impact frequency
NLOT
nucleus of the lateral olfactory tract
NMP
negative mucosal potential
NMR
nuclear magnetic resonance
NPY
neuropeptide Y
NTS
nucleus tractus solitarius
OASIS
original aroma simultaneously input to the sniffing port method
OAV
odour activity value
OB
olfactory bulb
OBP
odorant-binding protein
OC
olfactory cortex
OCT
octanol/al
OE
olfactory epithelium
OEC
olfactory ensheathing cell
OFC
orbito-frontal cortex
OM
olfactory mucosa
OME
odorant metabolizing enzyme
ONL
olfactory nerve layer
OR
olfactory receptor
ORN
olfactory receptor neuron
OSN
olfactory sensory neuron
OT
odour threshold
OT
olfactory tubercle
P
pressure (Pa)
saturated vapour pressure of compound i (Pa)
PCx
piriform cortex
PEEK
poly-ether-ether-ketone
PET
positron emission tomography
pgC
periglomerular cells
Pgp
P-glycoprotein
PI3
phosphoinositide-3
PI3K
phosphoinositide-3-kinase
PIP3
phosphatidylinositol-(3, 4, 5)-trisphosphate
PLC
phospholipase C
PLco
posterolateralcortical nucleus of amygdala
PN
projection neuron
pPCx
posterior piriform cortex
PPI
polyproline I
PPII
polyproline II
PRH2 protein
proline-rich protein HaeIII subfamily 2
PROP
6-n-propylthiouracil
PRP
proline-rich protein
PRV
phase ratio variation
PTC
phenylthiocarbamide
PTFE
polytetrafluoroethylene
PTR-MS
proton transfer reaction mass spectrometry
QSAR
quantitative structure activity relationships
QSPR
quantitative structure property relationships
R
gas constant (8.314 J mol
−1
K
−1
)
RAS
retronasal aroma simulator
RI
retention index
RMS
rostral migratory stream
RNA
ribonucleic acid
RNAi
RNA interference
SCM
single chain monellin
SCOPe
structural classification of proteins
sIgA
secretory immunoglobulin A
SL
semilunar cell
SNIF
surface of nasal impact frequency
SNP
single nucleotide polymorphism
SO
septal organ
SON
supraoptic nucleus
SP
superficial pyramidal cell
SPLUNC2
short palate, lung and nasal epithelium clone 2
SPME
solid phase micro-extraction
ST
sulfotransferase
SVP
saturated vapour pressure
T
temperature (K)
T1R1
taste receptor type 1, member 1
T1R2
taste receptor type 1, member 2
T1R3
taste receptor type 1, member 3
TAS1R
taste 1 receptor
TAS2R
taste 2 receptor
TDS
temporal dominance of sensations
TGF-α
transforming growth factor-alpha
TI
time-intensity
TM
trans-membrane
Tmax
time to reach maximum intensity
ToF
time-of-flight
TRP
transient receptor potential
Trp
tryptophan
TT
tenia tecta
UGT
UDP-glucuronosyltransferase
V1
first branch of the trigeminal nerve
V2
second branch of the trigeminal nerve
V3
third branch of the trigeminal nerve
VEG
von Ebner's glands
VEGP
von Ebner's gland protein
VNO
vomeronasal organ
VNR
vomeronasal receptor
VSN
vomeronasal sensory neuron
VTD
venus flytrap domain
WL
wiskey lactone
XME
xenobiotic metabolizing enzyme
Chapter 1Olfactory system in mammals: structural and functional anatomy
Anne-Marie Le Bon, Frédérique Datiche, Jean Gascuel & Xavier Grosmaitre
Center for Taste and Feeding Behaviour, CNRS, INRA, University of Bourgogne Franche-Comté, Dijon, France
1.1 Introduction
The survival and reproductive success of living organisms, including human beings, depends on the detection of sensory stimuli. Living organisms do not eat or reproduce with whatever is available; instead, they show considerable selectivity by taking advantage of their chemical and physical senses. In this regard, the sense of smell and its capacity to detect myriad of odorant molecules is of critical importance for humans and most animal species. This sense significantly contributes to the identification of food and assessment of its palatability, as well as to the detection of chemical compounds carrying specific information concerning dangers, social interactions and reproductive behaviours. In mammals, these diverse roles are accomplished by a complex olfactory system. The primary tissue responsible for sensing volatile odorants is the olfactory epithelium (OE) which is localized in the nasal cavity. Sensory neurons residing in the OE convey olfactory information to the olfactory bulb (OB) which, in turn, transfers this information towards multiple higher cortical regions collectively referred to as the olfactory cortex. Other olfactory subsystems such as the vomeronasal organ coexist with the main OE in many species. These subsystems are separate entities that are dedicated to distinct functional roles.
The principal aim of this review is to gather the results of very recent as well as major studies on the processing of olfactory information by the olfactory system and to highlight its plasticity. We first describe the physiology of the main OE and the molecular mechanisms of odorant detection. We then show how endogenous and exogenous factors may induce different forms of plasticity of the OE. We also outline the main features of other olfactory subsystems. Next, we examine how the olfactory signal generated at the peripheral level is transformed at the first processing center in the brain, the OB. Finally, we provide an overview of the higher olfactory pathways involved in the processing of olfactory information and we consider the pathways that shape odour perception.
1.2 Organization and function of the peripheral olfactory system
1.2.1 Physiology of the peripheral olfactory system
Stimulation of the olfactory system begins when odorant molecules are detected by the olfactory neuroepithelium located in the upper part of the nasal cavity. The odorant molecules can reach the epithelium by two pathways: via the nose (orthonasal olfaction) and via the mouth (retronasal olfaction). Odorants perceived by the orthonasal pathway originate from the external world whereas odorants perceived retronasally emanate from food or drink (aroma compounds) (see Chapter 13 for more details on these pathways).
The nose and the nasal cavity are separated into two halves along the midline by a cartilaginous structure called the nasal septum. The lateral wall of each nasal cavity is typically shaped by three bony protuberances termed the inferior, middle and superior turbinates. Animals can have more turbinates, for example, the rat has four. The turbinates and the septum are covered with an epithelium. Depending on its location, this epithelium is either nonsensory (respiratory) or sensory (olfactory). The nonsensory respiratory portion of the nasal cavity warms, cleans and humidifies the inspired air.
There is widespread acknowledgement that the human OE is located in the superior region of the nasal cavity, predominantly on the dorsal side of the nasal vault, the septum, and the superior turbinate. However, recent studies have reported a more extending distribution of OE on the middle turbinate (Escada et al. 2009). Actually, the location of the OE is variable among people. Besides, its organization is thought to change over time: ageing induces conversion to or ingrowth of respiratory epithelium and loss of olfactory neurons (Nakashima et al. 1991). Environmental compounds or pathophysiological processes such as infection or inflammation can also modify the distribution of OE. The OE in the adult has therefore a non-contiguous and patchy distribution. Globally, the human olfactory region covers between 1 and 2 cm2 in each cavity (Moran et al. 1982). This area is modest relative to those of other vertebrates such as rodents and dogs (Gross et al. 1982, Harkema 1991).
In the superior part of the nasal cavity, a horizontal bone, called the cribriform plate of the ethmoid, separates the OE from the brain. The cribriform plate is a highly perforated bone: the perforations provide access for the olfactory nerve bundles to the OB. This is the only site in the body where the central nervous system is in direct contact with the outer surface. The nerves serving the olfactory region are called the first cranial nerves or the olfactory nerves. They concentrate multiple axons of olfactory neurons located in the lamina propria. These axons convey the nerve impulse generated by the odorant detection into the OB.
1.2.2 Structure of the olfactory epithelium
The human OE has a structure similar to that of other vertebrates (Morrison and Costanzo 1992). It is a pseudo-stratified columnar epithelium that lies on a dense connective tissue, the lamina propria. Together, the OE and the lamina propria form the olfactory mucosa (OM). The human OE is about in height and has a slight yellow-brownish colour. It is composed of several distinct cell types, notably olfactory sensory neurons (OSNs), sustentacular cells (a type of nonsensory supporting cells), microvillar cells, two types of stem cells (horizontal basal cell and globose basal cell) as well as Bowman's glands and duct cells (Figure 1.1B).
Figure 1.1 Schematic drawing of the rodent olfactory system (sagittal cross section through the nasal region of the head, lower jaw is not shown). Inset A shows the different cell layers observed in the olfactory bulb and the neuronal connections. Inset B represents the various cell types and structures located in the olfactory mucosa. Inset C schematizes the connectivity of glutamatergic neurons in the PCx (Source: Adapted from Ekberg and St John 2014, Haberly 2001). Abbreviations: AOB, accessory olfactory bulb; aPCx, anterior piriform cortex; DP, deep pyramidal cells; EPL, external plexiform layer; FB, feedback interneurons; FF, feed forward interneurons; G, glomeruli; GG, Grueneberg ganglion; GL, glomerular layer; Gr, granule cells; GrL, granular cell layer; IPL, internal plexiform layer; LOT, lateral olfactory tract; M, mitral cells; MCL, mitral cell layer; Mp, multipolar cells; OB, olfactory bulb; OE, olfactory epithelium; ONL, olfactory nerve layer; pgC, periglomerular cells; pPCx, posterior piriform cortex; OSN, olfactory sensory neuron; SL, semilunar cells; SO, septal organ; SP, superficial pyramidal cells; T, tufted cells. A colored version of this figure can be found in the online version of this chapter.
Vertebrate OSNs are slender and bipolar neurons spread in the epithelium with a density of 106-107 per cm2. Their cell bodies are generally located within the lower two thirds of the neuroepithelium. At the apical surface of the epithelium, about 10-25 cilia protrude from the OSN dendrites (Morrison and Costanzo 1992). These olfactory cilia float in the mucus which covers the epithelial surface and their plasma membrane contains the olfactory receptors (ORs). On the opposite side, the axons of OSNs penetrate through the basement membrane into the lamina propria where they are ensheathed by the olfactory ensheathing cells (OECs) (Figure 1.1B). OSNs and OECs together with fibroblasts form the olfactory nerve bundles. Serous glands called olfactory glands or Bowman's glands, bundles of the accessory olfactory nerve (surrounded by accessory OECs), as well as trigeminal nerve bundles (surrounded by Schwann cells) are also located within the lamina propria. The olfactory nerve bundles project through the cribriform plate towards the OB where the OSNs' axons synapse with mitral/tufted cells and interneurons (Figure 1.1A).
Stem cells divide to give rise to sustentacular cells and immature OSNs which mature and migrate apically. In rodent OE, there are two kinds of stem cells: horizontal basal cells and globose basal cells (GBCs). These two types are morphologically and functionally distinct. In humans, however, only one basal cell type has been reported. These human basal cells morphologically resemble the GBCs in the rat (Hahn et al. 2005).
Additional cell types, called microvillar cells, have also been described in the olfactory neuroepithelium of vertebrates. These cells, which are located near the epithelial surface, are flask shaped and have an apical tuft of microvilli extending into the nasal cavity. They provide trophic factors such as neuropeptide Y (NPY) to the OE under the control of odorant or trigeminal nerve stimulation, or both (Montani et al. 2006). Microvillar cells might therefore play a role in the regulation of cellular homeostasis in the OE.
Like other epithelia, the peripheral OE constantly regenerates itself throughout life. OSNs only live for 1-3 months after which they undergo apoptosis and are replaced by new neurons originating from basal cells (Mackay-Sim and Kittel 1991). The continuous turnover of OSNs protects the OE against damage induced by environmental factors that can result in cell death. This replenishment after damage is critical to maintain the functional integrity of the OE.
1.2.3 Molecular mechanisms of odorant detection
The airborne odorants diffuse into the aqueous nasal mucus before reaching olfactory cilia where ORs are localized. In the mucus, proteins called odorant-binding proteins (OBPs) are thought to carry odorants, which are commonly hydrophobic molecules, through the mucus towards the ORs (Heydel et al. 2013). In addition to the solubilisation of odorants, OBPs may have other functional roles. Recent studies have revealed that OBPs directly interact with ORs thus modulating their function (Vidic et al. 2008) or contribute to the clearance of odorants from the microenvironment of the receptor (Strotmann and Breer 2011).
Binding of odorants to specific ORs is a key event that induces olfactory signaling. ORs were first identified from rats in 1991 by Linda Buck and Richard Axel (Buck and Axel 1991) who received the Nobel Prize in 2004 for this discovery. These authors revealed that OR genes belong to a large multigene family that encode G protein coupled receptors (GPCRs). Further studies confirmed that OR genes constitute the largest multigene family in mammals. Comparison of diverse genome sequences showed that the numbers of OR genes vary greatly among species (Niimura 2012). Rats and mice have 1,400–1,700 OR genes in their genomes, cows and horses have higher numbers (2,200-2,600) and recently, it has been reported that the genome of African elephants contains more than 4,200 OR genes (Table 1.1). Compared with other mammals, primates tend to have smaller numbers of OR genes (600-800). A fraction of mammalian OR genes has been shown to be pseudogenes (i.e., genes that are not functional). The fraction of OR pseudogenes varies widely among species (Niimura et al. 2014). In human genome, more than half (52%) of the entire set of OR genes are pseudogenes, leading to 396 intact (potentially functional) OR genes (Matsui et al. 2010).
Table 1.1 Numbers of OR genes in the genome sequence from 13 placental mammalian species (Source: Adapted from Niimura et al. 2014). An intact gene was defined as a sequence starting from an initiation codon and ending with a stop codon that did not contain any disrupting mutation. A pseudogene was defined as a sequence with a nonsense mutation, frameshift, deletion within conserved regions, or some combination thereof. A truncated gene represents a partial sequence of an intact gene
Species
Total number
Intact genes
Truncated genes
Pseudogenes
number
%
number
%
number
%
Human
821
396
48.2
0
0
425
51.8
Chimpanzee
813
380
46.7
19
2.34
414
50.9
Orangutan
821
296
36.1
37
4.51
488
59.4
Macaque
606
309
51.0
17
2.81
280
44.7
Marmoset
624
366
58.7
27
4.33
231
36.9
Mouse
1,366
1,130
82.7
0
0
236
17.3
Rat
1,767
1,207
68.3
52
2.94
508
28.7
Guinea pig
2,162
796
36.8
26
1.20
1,340
62
Rabbit
1,046
768
73.4
22
2.10
256
24.5
Horse
2,658
1,066
40.1
23
0.87
1,569
59
Dog
1,100
811
73.7
11
1.00
278
25.3
Cow
2,284
1,186
51.9
41
1.80
1,057
46.3
Elephant
4,267
1,948
45.7
89
2.09
2,230
52.3
An important feature of OSNs is the fact that each cell expresses only one allele of a single OR gene: this has been proven through extensive studies in the mouse OSNs (Chess et al. 1994, Malnic et al. 1999, Serizawa et al. 2004). OSNs expressing the same OR would be distributed randomly within one of four circumscribed zones in the OE (Ressler et al. 1993, Vassar et al. 1993). However, some studies suggest that OR gene expression zones broadly overlap rather than bear sharp zonal boundaries (Iwema et al. 2004, Miyamichi et al. 2005). All OSNs expressing the same OR in turn converge upon spatially invariant glomeruli in the OB, the site of the first synaptic relay in olfactory sensory processing (Mombaerts et al. 1996, Ressler et al. 1994, Vassar et al. 1994). Thus, activation of specific ORs by an odorant elicits a characteristic pattern of activity in the OB.
The OR functionality was demonstrated through a number of in vitro and in vivo studies. Odorants may be recognized by multiple ORs, and one OR may recognize multiple odorants (Kajiya et al. 2001, Malnic et al. 1999). This implies that different odorants are recognized by different combinations of ORs. This scheme of combinatorial coding is now widely admitted to explain how odorants are encoded at the peripheral level. However, some ORs (such as the human receptor OR7D4) have been shown to bind to a limited number of structurally related odorants (Keller et al. 2007). ORs can therefore be classified into two groups: ORs that are broadly tuned and ORs that are narrowly tuned. However, the way a receptor can recognize an odorant still remains poorly understood and further studies are necessary to investigate the physicochemical laws that govern OR-ligand interactions.
ORs belong to the class-A of the GPCR family that includes a number of diverse membrane receptors. Bovine rhodopsin or β2-adrenergic receptors, class-A GPCRs whose structural features have been widely investigated, were used as templates to perform homology modeling. These experiments indicated that ORs fold into quite similar tertiary structures, consisting of seven trans-membrane (7-TM) helices connected by extra-cellular and intra-cellular loops (Baud et al. 2011, Singer 2000). The 7-TM helices form a bundle in which a pocket is dedicated to odorant binding. Studies combining molecular modeling and site-directed mutagenesis helped specifying the nature of the binding sites of some ORs (Baud et al. 2011, Gelis et al. 2012, Katada et al. 2005, Launay et al. 2012). The binding pockets were predicted to be located between TM3, TM5 and TM6 and the main amino acids in contact with the ligands could be identified. For a given OR, the binding mode differs from one odorant to another but some amino acids, all hydrophobic, are involved in binding whatever the ligand (Charlier et al. 2012).
In the cilia of OSNs, ORs are coupled to a specific G-protein called Golf. When a cognate ligand binds to an OR, this interaction activates the Gαolf subunit which elevates intracellular cAMP through type III adenylate cyclase enzymatic reaction. Binding of cAMP to the cyclic nucleotide-gated channel allows influx of cations, mainly calcium, into OSNs. Elevation of intracellular calcium induces the opening of the calcium-gated chloride channel that produces an efflux of chloride ions to amplify cellular depolarization (Kleene 2008). The cAMP pathway is thought to be the main signalization pathway involved in peripheral olfactory transduction. However, some studies suggest the involvement of cAMP-independent signaling pathways, including guanylate cyclase and phospholipase C (PLC) signaling, in olfactory transduction (Lin et al. 2004, Meyer et al. 2000). Recently, it has been demonstrated that a subset of mouse OSNs located in the most ventral zone of OE can mediate both the phospholipase C signaling pathway and the cAMP pathway upon binding to structurally similar ligands (Yu et al. 2014). In consequence, some ORs could possess conformational plasticity leading to preferential interactions with different downstream elements, depending on ligand that binds to the OR.
1.2.4 Plasticity of the olfactory epithelium
Several endogenous and exogenous factors induce different forms of plasticity at the OE level.
1.2.4.1 Development and ageing
Evidence has been accumulated that the peripheral olfactory system is functional before birth. Behaviour studies have shown that prenatal olfactory experience provoked by odorants present in the amniotic fluid contributes to postnatal preferences and behaviours such as suckling and feeding (Logan et al. 2012, Schaal et al. 2000). In mouse embryonic development (lasting 19 days from conception), the OE is fully formed at embryonic day 10 (E10) and at around E14, multiple short cilia can be observed on neuron dendrites (Cuschieri and Bannister 1975). Several works reported that ORs and components of the main olfactory signaling pathway (such as protein Golf
