Odor Mixtures Research Articles

Toward a Better Understanding of Perceptive Interactions between Volatile and Nonvolatile Compounds: The Case of Proanthocyanidic Tannins and Red Wine Fruity Esters-Methodological, Sensory, and Physicochemical Approaches.

The impact of commercial proanthocyanidic tannins on fruity pool of esters, representing the fruitiness of Bordeaux red wines, was assessed in model solutions. It was shown that the presence of tannins in the matrix significantly attenuated perception of fruity notes (p = 0.013). Physicochemical analysis demonstrated that the presence of proanthocyanidic tannins in dilute alcohol solution resulted in a decrease in ester partition coefficients and thus in a decrease in ester contents in the headspace (p < 0.05). This fact highlighted the changes that may occur in wines at a pre-sensory level, prior to sensory evaluation. Finally, a new sensory tool was developed, consisting in an ISO glass containing two compartments separated by a glass wall, providing a way to compare perceived odors according to whether or not the components of the odor mixtures were actually mixed in solution (p < 0.001). This new tool was used to demonstrate the impact and the only implication of pre-sensory level in the consequences of physical mixture between proanthocyanidic tannins and esters on their odor perception.

Perceptual interactions among food odors: Major influences on odor intensity evidenced with a set of 222 binary mixtures of key odorants

Perceptual interactions among food odors: Major influences on odor intensity evidenced with a set of 222 binary mixtures of key odorants

A New Classification of Perceptual Interactions between Odorants to Interpret Complex Aroma Systems. Application to Model Wine Aroma.

Although perceptual interactions are usually mentioned and blamed for the difficulties in understanding the relationship between odorant composition and aromatic sensory properties, they are poorly defined and categorised. Furthermore, old classifications refer mainly to effects on the odour intensity of the mixture of dissimilar non-blending odours and do not consider odour blending, which is one of the most relevant and influential perceptual interactions. Beginning with the results from classical studies about odour interaction, a new and simple systematic is proposed in which odour interactions are classified into four categories: competitive, cooperative, destructive and creative. The first categories are most frequent and display a mild level of interaction, being characterised mostly by analytical processing. The last two are less frequent and activate (or deactivate) configurational processes of object recognition with deep effects on the quality and intensity of the perception. These interactions can be systematically applied to interpret the formation of sensory descriptors from the odorant composition, suggesting that qualitatively the system works. However, there is a lack of quantitative data to work with odour intensities reliably, and a pressing need to systematise the effects of creative interactions.

Toxicological assessment method for evaluating the occupational risk of dynamic olfactometry assessors

The paper aims to propose a new method to evaluate the occupational exposure risk for examiners involved in dynamic olfactometry. Indeed, examiners are possibly exposed to hazardous pollutants potentially present in odorous samples. A standardized method to evaluate the examiners' occupational safety is not yet available and the existing models present some critical aspect if applied to real odorous samples (no uniform reference concentrations applied and presence of compounds for which no toxicity threshold is available). A deepening of assessment procedure to evaluate the occupation exposure risk for olfactometric assessors is necessary. This paper proposes a standardized approach for risk assessment in dynamic olfactometry. The proposed approach allows the quantification synthetic and conservative risk indices. In this model, the use of the hazard index for the odorous mixture was proposed to assess the non-carcinogenic risk; the calculation of the inhalation risk was applied to estimate the carcinogenic risk. Different databases can be used to retrieve proper occupational exposure limits, according to the proposed hierarchical basis. These implementations allow obtaining the complete characterization of real samples which can be used to calculate the minimum dilution factor for protecting the panellists’ health.

Odor of achlorophyllous plants' seeds drives seed-dispersing ants.

Seed dispersal by ants is an important means of migration for plants. Many myrmecochorous plants have specialized appendages in their seeds called elaiosome, which provides nutritional rewards for ants, and enable effective seed dispersal. However, some nonmyrmecochorous seeds without elaiosomes are also dispersed by ant species, suggesting the additional mechanisms other than elaiosomes for seed dispersal by ants. The seeds of the achlorophyllous and myco‐heterotrophic herbaceous plant Monotropastrum humile are very small without elaiosomes; we investigated whether odor of the seeds could mediate seed dispersal by ants. We performed a bioassay using seeds of M. humile and the ant Nylanderia flavipes to demonstrate ant‐mediated seed dispersal. We also analyzed the volatile odors emitted from M. humile seeds and conducted bioassays using dummy seeds coated with seed volatiles. Although elaiosomes were absent from the M. humile seeds, the ants carried the seeds to their nests. They also carried the dummy seeds coated with the seed volatile mixture to the nest and left some dummy seeds inside the nest and discarded the rest of the dummy seeds outside the nest with a bias toward specific locations, which might be conducive to germination. We concluded that, in M. humile seeds, volatile odor mixtures were sufficient to induce seed‐carrying behavior by the ants even without elaiosomes.

嗅神経細胞における匂いの混合物の受容メカニズム

嗅神経細胞における匂いの混合物の受容メカニズム

Perceptual interactions among food odors: Major influences on odor intensity evidenced with a set of 222 binary mixtures of key odorants

This study explored the impact of perceptual interactions on the odor intensity of 222 binary mixtures designed from 72 odorants found in food products. Odor intensity was rated by 30 trained subjects. The results showed that in most cases, the components’ odor was perceived within the mixture and their intensity remained the same as in the unmixed situation in 54.3% of cases. Masking was the second major effect (44.8%) and occurred more frequently when components’ pleasantness was significantly different. Synergy occurred in a small number of cases (0.9%) and only for four compounds. The overall odor intensity of the mixture was determined to be equal to the strongest component in most cases (73.9%), while partial addition was observed as the second most frequent effect (21.7%), especially when the components had equal intensity. Overall, this work provides general rules about the outcome to expect when mixing key components of food aromas.

Flexible categorization in the mouse olfactory bulb

The ability to group sensory stimuli into categories is crucial for efficient interaction with a rich and ever-changing environment. In olfaction, basic features of categorical representation of odors were observed as early as in the olfactory bulb (OB). Categorical representation was described in mitral cells (MCs) as sudden transitions in responses to odors that were morphed along a continuum. However, it remains unclear to what extent such response dynamics actually reflect perceptual categories and decisions therein. Here, we tested the role of learning on category formation in the mouse OB, using invivo two-photon calcium imaging and behavior. We imaged MC responses in naive mice and in awake behaving mice as they learned two tasks with different classification logic. In one task, a one-decision-boundary task, animals learned to classify odor mixtures based on the dominant compound in the mixtures. As expected, categorical representation of odors, which was evident already in naive animals, further increased following learning. In a second task, a multi-decision-boundary task, animals learned to classify odors independent of their chemical similarity. Here, odor discrimination was based on the meaning ascribed to them (either rewarding or not). Following the multi-decision-boundary task, odor representations by MCs reorganized according to the odor value in the new category. This functional reorganization was also reflected as a shift from predominantly excitatory odor responses to predominantly inhibitory odor responses. Our data show that odor representations by MCs are flexible, are shaped by task demands, and carry category-related information.

Zebrafish (Danio rerio) behavioural response to an odorant mixture containing attracting and repelling odorants

Abstract Odorants guide essential activities such as foraging and fleeing predators. Studies usually examine behavioural and physiological effects of individual odorants, while animals in the environment are exposed to multiple stimuli simultaneously. In this study, we exposed zebrafish to a mixture of attraction-evoking and aversion-evoking amino acids, and behavioural responses were observed. Attraction to l-alanine and avoidance to l-cysteine were observed, and so these amino acids were used to make the mixture (zebrafish also avoided l-serine, but this was weaker than with l-cysteine exposures). When exposed to the mixture, fish responded with avoidance, which suggests that aversion-evoking stimuli outweigh attraction-evoking stimuli. Attraction towards the mixture was seen only when the concentration of l-cysteine was decreased from 0.1 to 0.001 mM. Olfactory ablation surgery confirmed that the behaviours were olfactory-mediated. Overall, this study demonstrated that odorant stimuli that repel outweigh stimuli that attract until their concentration decreases by as much as 100-fold.

Composition and compound proportions affect the response to complex chemical signals in a spiny lizard

Most animal signals across sensory modalities are multicomponent traits that can be broken down into discrete elements. If different elements are perceived as unique, independent units (elemental perception), instead of as integrated percepts (configural perception), single changes in the presence/absence or the abundance of specific elements of a multicomponent signal may be enough to impact communication. Here, we found that male Yarrow’s spiny lizards (Sceloporus jarrovii) can discriminate single compounds of a multicomponent chemical signal (femoral gland secretions), different concentrations of a signaling compound, and a single compound from a mixture of compounds. In addition, one chemical compound elicited a response similar to that evoked by the complete natural scent. We conclude that perception of chemical signals in S. jarrovii lizards is elemental but also configural. The elemental perception of signaling compounds seems to occur with high sensitivity and narrow resolution, so that minor changes in single key elements may affect chemical communication. Given the multicomponent nature of most animal signals, hypotheses regarding signal function and evolution would be enhanced if researchers could determine whether these results apply to signals in other sensory modalities and identify the key elements of complex signals, from a receiver’s perspective. Most signals in animal communication are quite complex. For example, odors are mixtures of multiple volatile chemical compounds, and the way in which receivers perceive and process these mixtures to extract relevant information influences the structure and evolution of chemical signals. In a series of behavioral trials, we investigated how male Sceloporus jarrovii lizards may perceive conspecific odors by testing their response to individual and combined mixtures of two compounds present in femoral gland secretions at two different concentrations. We demonstrate that lizards can discriminate structurally similar compounds and that the response to a compound changes when said compound is part of a larger mixture. Compound concentration affected the perception of individual compounds but not complex mixtures. Deciphering what elements and/or configurations are perceived in an odor mixture is the only way to understand the role of mixture composition and its impact on communication.

Configural memory of a blending aromatic mixture reflected in activation of the left orbital part of the inferior frontal gyrus

Blending aromatic mixtures components naturally fuse to form a unique odor - a configuration- qualitatively different from each component's odor. Repeated exposure to the components either in the mixture or separately, favors respectively, configural and elemental processings. The neural bases of such processes are still unknown. We examined the brain correlates of the experienced-induced configural processing of a well-known model of binary blending odor mixture, the aromatic pineapple blending (AB, ethyl maltol + ethyl isobutyrate). Before fMRI recording, half of the participants were repeatedly exposed to the mixture (AB, group Gmix), with the other half exposed to its separate components (A and B; Gcomp). During the fMRI recording, all participants were stimulated with the mixture (AB) and the components (A and B). Finally, participants rated the number of odors perceived for each stimulus. Gmix perceived the AB mixture as less complex than did Gcomp. While Gcomp perceived the mixture as more complex than its components, Gmix did not. These results show the presence of experience-induced configural or elemental processing of the AB mixture in each group. Contrasting the brain activity of Gcomp and Gmix, when stimulated with AB, revealed higher activation in the left orbital part of the inferior frontal gyrus. This result sheds light on this area's function, commonly found activated in olfactory studies, and closely connected with the lateral orbitofrontal cortex. We discuss the role of this area as a mediator of configural percepts between temporal and orbitofrontal areas involved in configural memory processes.

Characterization of sensory perceptions elicited by white wine spiked with different aroma, taste and mouth-feel active molecules

The present work was aimed at understanding the sensory responses induced by dry white wine modified with increasing concentrations of different sensory active molecules. The tasting panel was composed by 34 trained subjects characterized according to gender, smoking habits, 6-n-propylthiouracil (PROP) taster status, and sensitivity to tartaric acid, tannic acid and sucrose. Additional taste/mouthfeel responsiveness was evaluated in a basal white wine added of tartaric acid, tannic acid and sucrose. The addition of a fruity odorant mixture to the base white wine enabled the assessment of orthonasal aroma and taste intensities (sweetness, sourness, saltiness). The diversity of taste/mouthfeel responsiveness in water or wine enabled to group individuals as high or low sensitives. Likewise, the tasting panel showed two groups responding differently to aroma and flavor. Both high and low aroma sensitivity individuals showed equal (p&gt;0.05) and congruent response to in-mouth sweet flavor perception. Moreover, the high smell sensitive group was less sensitive to sourness and saltiness than the low smell sensitives for the spiked wines across all flavor concentrations. Differences in PROP taster status and sensitivity to other tastants in water solutions were not correlated with the taste/mouthfeel perceptions in wine. The individuals most sensitive to sucrose in wine taste showed higher response to the fruity aroma. In conclusion, taste, mouthfeel and flavor perceptions showed a high variability among individuals evidencing the advantage of grouping tasters with different chemosensory sensitivities to understand cross-modal sensory interactions.

Dogs can detect the individual odors in a mixture of explosives

There is a consensus that humans, rodents, and primates process odor mixtures configurally: that is, the mixture is perceived as a unique whole and not as a collection of its different components. However, it is commonly believed by dog trainers that dogs can analyze mixtures and break them down into their individual components: that is, that they process odor mixtures analytically. There is, however, little experimental evidence to support this belief. This experiment was designed to determine whether dogs trained on mixtures of three or five different odors would detect the individual odors when each component was tested. Our results show that the dogs were able to respond to each of the components on their first trial with each separate element of the mixture (p < 0.001). This finding presents the first experimental proof that dogs trained to detect a mixture will later detect its individual components. These results may be attributed to previous extensive general training of the tested dogs to detect odors, and to their being reared and maintained in an enriched olfactory environment.

Discrimination of Complex Odor Mixtures: A Study Using Wine Aroma Models.

There are key unanswered questions when it comes to multicomponent odor discrimination. This study was designed to assess discrimination of odorant mixtures that elicit a singular percept. We collected data to address the following two questions: (1) What odor features do humans notice when attempting to discriminate between subtly different odor mixtures? (2) Are odor mixtures easier to discriminate when an odorant is added, compared with when a component is removed? Using modern aroma chemistry techniques, an odor mixture resembling a generic white wine was constructed. This wine odor mixture was modified using a series of three esters which are commonly found in white wines that vary in chain length and branching. Participants performed a sequence of discrimination tasks for the addition/subtraction of modifiers to the base wine at different concentrations. Only one of the esters (ethyl propanoate) led to a discriminable odor mixture. As concentration of the modifying odorant was increased, discrimination of odor mixtures was first reported because of changes in odor mixture familiarity and then intensity. We found similar sensitivity to changes in odor mixtures regardless whether the modifying compound was added or subtracted, suggesting that perceptual stability of odor mixtures is equally dependent on both imputing missing information (pattern completion) and disregarding extraneous information.

Mixture Coding and Segmentation in the Anterior Piriform Cortex.

Coding of odorous stimuli has been mostly studied using single isolated stimuli. However, a single sniff of air in a natural environment is likely to introduce airborne chemicals emitted by multiple objects into the nose. The olfactory system is therefore faced with the task of segmenting odor mixtures to identify objects in the presence of rich and often unpredictable backgrounds. The piriform cortex is thought to be the site of object recognition and scene segmentation, yet the nature of its responses to odorant mixtures is largely unknown. In this study, we asked two related questions. (1) How are mixtures represented in the piriform cortex? And (2) Can the identity of individual mixture components be read out from mixture representations in the piriform cortex? To answer these questions, we recorded single unit activity in the piriform cortex of naïve mice while sequentially presenting single odorants and their mixtures. We find that a normalization model explains mixture responses well, both at the single neuron, and at the population level. Additionally, we show that mixture components can be identified from piriform cortical activity by pooling responses of a small population of neurons—in many cases a single neuron is sufficient. These results indicate that piriform cortical representations are well suited to perform figure-background segmentation without the need for learning.

Fruit flies can learn non-elemental olfactory discriminations.

Associative learning allows animals to establish links between stimuli based on their concomitance. In the case of Pavlovian conditioning, a single stimulus A (the conditional stimulus, CS) is reinforced unambiguously with an unconditional stimulus (US) eliciting an innate response. This conditioning constitutes an ‘elemental’ association to elicit a learnt response from A+ without US presentation after learning. However, associative learning may involve a ‘complex’ CS composed of several components. In that case, the compound may predict a different outcome than the components taken separately, leading to ambiguity and requiring the animal to perform so-called non-elemental discrimination. Here, we focus on such a non-elemental task, the negative patterning (NP) problem, and provide the first evidence of NP solving in Drosophila. We show that Drosophila learn to discriminate a simple component (A or B) associated with electric shocks (+) from an odour mixture composed either partly (called ‘feature-negative discrimination’ A+ versus AB−) or entirely (called ‘NP’ A+B+ versus AB−) of the shock-associated components. Furthermore, we show that conditioning repetition results in a transition from an elemental to a configural representation of the mixture required to solve the NP task, highlighting the cognitive flexibility of Drosophila.

Endogenous zinc nanoparticles in the rat olfactory epithelium are functionally significant

The role of zinc in neurobiology is rapidly expanding. Zinc is especially essential in olfactory neurobiology. Naturally occurring zinc nanoparticles were detected in olfactory and nasal respiratory epithelia and cilia in animals. The addition of these nanoparticles to a mixture of odorants, including ethyl butyrate, eugenol, and carvone, considerably increased the electrical responses of the olfactory sensory receptors. Studies of these nanoparticles by ransmission electron microscopy (TEM) and selected area electron diffraction revealed metal elemental crystalline zinc nanoparticles 2–4 nm in diameter. These particles did not contain oxidized zinc. The enhancement of the odorant responses induced by the endogenous zinc nanoparticles appears to be similar to the amplification produced by engineered zinc nanoparticles. Zinc nanoparticles produce no odor response but increase odor response if mixed with an odorant. These effects are dose-dependent and reversible. Some other metal nanoparticles, such as copper, silver, gold, and platinum, do not have the effects observed in the case of zinc nanoparticles. The olfactory enhancement was observed in young and mature mouse olfactory epithelium cultures, in the dissected olfactory epithelium of rodents, and in live conscious dogs. The physiological significance of the detected endogenous metal nanoparticles in an animal tissue has been demonstrated for the first time. Overall, our results may advance the understanding of the initial events in olfaction.

Brief olfactory learning drives perceptive sensitivity in newborn rabbits: New insights in peripheral processing of odor mixtures and induction

Perception of the wide, complex and moving odor world requires that the olfactory system engages processing mechanisms ensuring detection, discrimination and environment adaptation, as early as the peripheral stages. Odor items are mainly elicited by odorant mixtures which give rise to either elemental or configural perceptions. Here, we first explored the contribution of the peripheral olfactory system to configural and elemental perception through odorant interactions at the olfactory receptor (OR) level. This was done in newborn rabbits, which offer the opportunity to pair peripheral electrophysiology and well characterized behavioral responses to two binary mixtures, AB and A'B’, which differ in their component ratio (A: ethyl isobutyrate, B: ethyl maltol), and that rabbit pups respectively perceived configurally and elementally. Second, we studied the influence on peripheral reactivity of the brief but powerful learning of one mixture component (odorant B), conditioned by association with the mammary pheromone (MP), which allowed us to assess the possible implication of the phenomenon called induction in neonatal odor learning. Induction is a plasticity mechanism expected to alter both the peripheral electrophysiological responses to, and perceptual detection threshold of, the conditioned stimulus. The results reveal that perceptual modes are partly rooted in differential peripheral processes, the AB configurally perceived mixture mirroring odorant antagonist interactions at OR level to a lesser extent than the A'B’ elementally perceived mixture. Further, the results highlight that a single and brief MP-induced odor learning episode is sufficient to alter peripheral responses to the conditioned stimulus and mixtures including it, and shifts the conditioned stimulus detection threshold towards lower concentrations. Thus, MP-induced odor learning relies on induction phenomenon in newborn rabbits.

A biohybrid nose for evaluation of odor masking in the peripheral olfactory system

Olfaction is a synthetic sense in which odor mixtures elicit emergent perceptions at the expense of perceiving the individual components. The most common result of mixing two odors is masking one component by another. However, there is lack of analytical techniques for measuring the sense of smell, which is mediated by cross-odorant interactions. Here, we propose a biohybrid nose for objective and quantitative evaluation of malodor masking efficiency of perfumed products. This biohybrid nose is constructed by integrating mammalian olfactory epithelium with microelectrode array chip to read out the olfactory information as electrical signal from multiple tissue sites. The intrinsic odor response of olfactory epithelium is found to be represented by widespread spatiotemporal oscillatory activity. The masking efficiency of fragrance is quantified by calculating the relative difference between the malodor and the binary mixture (malodor + fragrance) response patterns. Results indicate that masking efficiency of fragrance is concentration-dependent, whereas completely masking may occurs when fragrance is employed at a concentration 2–3 orders of magnitude higher than malodor. This study demonstrates for the first time that capitalizing on the biological sense of smell to create biohybrid system provides an effective technique to resolve more complex biosensing-related issues such as odor interactions in mixtures.

Decoding Odor Mixtures in the Dog Brain: An Awake fMRI Study.

In working and practical contexts, dogs rely upon their ability to discriminate a target odor from distracting odors and other sensory stimuli. Using awake functional magnetic resonance imaging (fMRI) in 18 dogs, we examined the neural mechanisms underlying odor discrimination between 2 odors and a mixture of the odors. Neural activation was measured during the presentation of a target odor (A) associated with a food reward, a distractor odor (B) associated with nothing, and a mixture of the two odors (A+B). Changes in neural activation during the presentations of the odor stimuli in individual dogs were measured over time within three regions known to be involved with odor processing: the caudate nucleus, the amygdala, and the olfactory bulbs. Average activation within the amygdala showed that dogs maximally differentiated between odor stimuli based on the stimulus-reward associations by the first run, while activation to the mixture (A+B) was most similar to the no-reward (B) stimulus. To clarify the neural representation of odor mixtures in the dog brain, we used a random forest classifier to compare multilabel (elemental) versus multiclass (configural) models. The multiclass model performed much better than the multilabel (weighted-F1 0.44 vs. 0.14), suggesting the odor mixture was processed configurally. Analysis of the subset of high-performing dogs' brain classification metrics revealed a network of olfactory information-carrying brain regions that included the amygdala, piriform cortex, and posterior cingulate. These results add further evidence for the configural processing of odor mixtures in dogs and suggest a novel way to identify high-performers based on brain classification metrics.