In short, the 2021 NOSE symposium, From Aroma to Anosmia, was a huge success! Although we were hesitant at first to host (yet another..) online event, this turned out great, since in this way we were able to invite prestigious speakers from abroad (i.e. Dr Andrew Ketter, Prof Robert Datta), and could host a large and international audience, with around 150 attendees! By popular request, here is a summary of this exciting event. Whenever speakers agreed for their presentation to be shared, you can download these by following the links after each paragraph.
As starting point for the symposium, we took the nice and comprehensive review by Richard Stevenson (Chemical Senses, 2010 https://pubmed.ncbi.nlm.nih.gov/19942579/) that classifies human olfaction into three major functions: related to ingestive behaviour, avoiding environmental hazards, and social communication. For each of these functions, we invited a speaker on the topic, which was introduced by one of our NOSE members. The symposium kicked off with a soft manifesto on the importance of smell (throughout history), by Caro Verbeek.
One of the functions of smell is its approach function, introduced by Catrienus de Jong and Garmt Dijksterhuis of Wageningen Food and Biobased Research. This function results in our being attracted to food; i.e. if it’s not spoiled, as then its odour will make us avoid the food. They started with pondering the importance of eating, and hence food, for our survival. We are intelligent ‘tubes’ on the lookout for food, and smell is one of our guiding senses. Food material emanates volatile chemical substances, which our nose is able to pick up. Combined with its taste, and other in-mouth sensations this results in flavour. The retronasal olfactory part of flavour is the topic of many investigations, and still holds many mysteries. Just combining a set of odourants to create a specific flavour is only part of the picture. Typically such odourants can interact, resulting e.g. in fishy taste when mixing potato odours with geranium odours in a product. Most food products contain hundreds of different chemicals, making up the typical flavour of the food. Analysing what compounds make up what specific flavour needs delicate research, combining analytical flavour chemistry with sensory smelling and tasting studies, and knowledge of the way the compounds interact. Some compounds, when mixed, may smell totally different from how they smell individually, other compounds are able to suppress the smell of another compound in the mixture. This very often cannot be predicted from the molecular structure, and can only be assessed by smelling the mixtures.
Sometimes a food product has developed an unwanted off-flavour. The challenge is to find what compound is responsible for it. When this is known, one can try to omit this compound from the food product, to get rid of the off-flavour. Another route is adding another compound that is able to suppress the unwanted off-flavour. Odourants do not appear all at the same time, when eating a food product. Depending on mastication and salivation, and on the specific properties of the odourant molecules, some appear immediately after biting, while others get released much later during chewing. Together they paint a very dynamic picture of a food product.
This all can be assessed with a PTR-MS by measuring the breath coming out of the nose during mastication of a food product. To have more insight in which compounds are responsible for the flavour (odour) perception and how they affect each other in terms of masking, enhancing and interaction, two in vivo analytical techniques were discussed.
Composcent 2.0 enables omitting compounds from a complex odour mixture. This is achieved by separating the mixture by means of GC and deciding which compounds remain in the in the odour profile and which to omit. The mixture, excluding the omitted volatiles, is collected again and sensorially evaluated to measure the effect.
Olfactocan 2.0 enables scanning for enhancing or masking compounds. The separated compounds coming out of a GC are mixed with a (off-)flavour from an olfactometer and assessed by a sensory panellist. With this technique it is possible to find new masking or enhancing odourants or a change of flavour based on the pairing of specific compounds.
The presentation concluded with pointing out that the complexity and the dynamism of the flavour of food calls for careful analysis of the compounds, married to smelling and tasting by sensory testing. The introduction of new food product flavours or the removing of off flavours, should be done hand-in-hand with the development of new products, or reformulating existing products.
Yet another function of the human sense of smell is social communication. Typically, we think body odor is disgusting and we want to avoid it, but there’s actually a lot more to it that escapes our awareness. NOSE member Jasper de Groot introduced this topic by summarizing evidence from clinical, psychological, and neuroscience research that human odors can convey a diverse range of information, including your identity, diseases, old age, gender, and emotions. As experiments like these have almost exclusively involved Western Caucasian participants, the question is really whether there is some kind of universal language of smell. A few studies have examined the link between ethnicity and the composition of body odor. Using chemical analysis techniques, Prokop-Prigge et al. (2016) examined differences in armpit odorant molecules among 30 individuals of African-American, Caucasian, and East Asian descent. While the type of molecules found in armpit sweat appeared universal, the authors did find that characteristic armpit odors varied in numbers with respect to ethnic origin, with higher quantities of molecules found from East Asians to Caucasians to African Americans.
This formed a bridge to the main talk in this session by Dr. Andrew Kettler, an Ahmason-Getty Fellow for the University of California, Los Angeles. His talk, entitled “The smell of slavery: Olfactory racism and the Atlantic world” (like his book [link]), displays the impact that smell had in the making of racial metaphors that were used to justify enslavement. He described how vision lacks the ability to singularly control racial categorizations and how Western Europeans defined Africans as a “pungent object” to create levels of ownership through the development of embodied cultural knowledge (associating “Africa” with the smell of “waste/excrement”) and to prevent them from accessing public spaces. On the flipside, a greater consciousness about the sense of smell in communities of African descent created a powerful tool to stress their individualism and power through acts of resistance. In sum, Kettler’s talk provided a deeply researched historical understanding of the development of olfactory racism, a shocking and sensitive topic that still influences us today.
Avoidance and warning
NOSE members Monique Smeets and Ilja Croijmans hosted the next theme, on the avoidance and warning function of human olfaction. Monique Smeets explained that an odour can act as a signal of threat, so that once detected, the individual can make sure to avoid it. Broadly defined, there are 2 categories of odours that are reflective of hazards: nonmicrobial hazards (e.g., predators, fire, degraded air, and poisons), and microbial threat s (e.g ., feces, vomit, and organic decay). One important reason for this division is that each class of hazards seems to be associated with a different emotion, namely, fear and disgust. The link with emotion is an important one because emotions are strong motivators of behavior: humans are much more inclined towards action when something makes them fearful or disgusted then when they are indifferent to a situation even if there are rational reasons why they should act. To react quickly and effectively is very important when it comes to threat detection: time is of the essence as slow response could cost you your life.
The focus of this session was on the emotion of disgust and disgust sensitivity in relation to the smell of microbial threats. Disgust helps drive avoidance behavior in multiple ways. One is via what we call “sensory rejection”. When people experience disgust they lower their brow, compress the nose and raise their lips. For fear we see an entirely different expression: brow-raising and eye-opening, elongation of the nose and lowering of the lips. According to Darwin these emotional expression did not evolve randomly but served a evolutionary purpose, which for disgust is to reduce sensory intake. When people are in a disgust state, much less air is inhaled compared to a fearful state. This shows that when people are disgusted they engage in sensory rejection: seeking to avoid inhaling foul air to reduce the risk of contamination of whatever is in the air.
After the brief introduction by Smeets, Croijmans presented work that they did together with Joshua Tybur (Vrije Universiteit), that further explored the relationship between disgust sensitivity and odour sensitivity. There are a number of hypotheses on why people may vary in their sensitivity for disgusting objects: some people may never feel disgust at all, whereas other people have a very low threshold to feel disgust. Croijmans explained there are at least three non-mutually exclusive theories:
- people may learn when to feel disgust by learning from, and imitating their parents. This is called parental modelling
- there may be certain environmental pressures that make that people are more or less sensitive to disgust
- or disgust sensitivity is primarily determined by certain genes.
A recent study by Tybur, Wesseldijk and Jern (2020) showed that disgust sensitivity is not due to parental modelling, but a combination of genetic and environmental pressures. In the work that Croijmans presented next, they further explored the possibility that sensory sensitivity, more specifically odour sensitivity, drives disgust. If people are more sensitive to smells that signal pathogens in the environment, do they also show a greater proneness to express disgust? This would be expected if there is an underlying biological explanation that drives both disgust sensitivity (e.g., genes) and one’s sensitivity to odours. The results did not confirm this hypothesis. In two studies with relatively large sample sizes, only a relationship was found between smell pleasantness, or how pleasant each individual thought the smells in the experiment were, and their disgust sensitivity. There was no relationship between how strong people found the odors, which is a first indication of smell sensitivity. And there was no relationship between disgust and smell sensitivity as measured by a detection threshold test. Thus they found no evidence that disgust sensitivity is related to intensity, or sensitivity for odours. This suggests that variation in disgust sensitivity reflects how environmental variables are interpreted rather than detected.
Losing the sense of smell
And to top things off – what would a symposium on smell these days be, without a session on corona? – we also dedicated part of the symposium to smell loss in relation to covid-19. Here, Sanne Boesveldt first presented data from the Global Consortium for Chemosensory Research (GCCR, https://gcchemosensr.org/), and described how such a worldwide network started out. It truly is amazing how much science can accomplish, and how fast, if everyone – researchers, clinicians, patient organizations – joins forces and puts their best effort towards the same goal: trying to understand what’s going in in terms of smell and taste loss in covid-19 (see also https://www.nosenetwork.nl/an-update-on-smell-taste-loss-with-covid-19-from-gccr/). Next, it was Kirsten Jaarsma who put ‘Cinderella on stage’, claiming that the olfactory sense has been regarded as the stepsister of the human senses, but that all has changed in the past year, ‘thanks’ to covid. Nowadays, it seems that everyone know what smell loss is, because they’ve suffered from it themselves, or at least heard about it in the media. She also highlighted some of the events the patient organization reuksmaakstoornis.nl has organized last year, such as launching an online campaign, being present in almost every talkshow on tv, and hosting a (private) facebook-group for patients with smell or taste loss due to corona infection to share experiences. Finally, it was time for Robert Datta to explain what we already know (and don’t know, or speculate) about the mechanisms by which the corona-virus causes smell loss. Truly interesting, and we learned a lot of new things.
As a bonus-presentation, William Tullet provided a brief introduction to Odeuropa, an EU funded project on sensory mining and olfactory heritage, and specifically asked for input and everyone interested to join their network: https://odeuropa.eu/.
All in all, this was an afternoon well-spent behind the computer screen, with a great diversity of talks and people and lots of interactive discussions. Thanks to everyone who participated!