Unpleasant smells, the coronavirus, and corked wine

By now, I think it is quite well known that loss of sense of smell, or anosmia, is a key symptom of COVID-19, but this BBC News article explains how a short-term anosmia due to COVID-19 can turn into a longer-term parosmia, which is a distorted sense of smell. The parosmia seems to make many everyday substances, food for example, smell disgusting. I find this interesting, and not a little scary. I am not sure what the sufferers think, but given a choice between no smell and disgusting smells I think I would choose the former.

But to return to what I found interesting, when reading the BBC article I immediately thought of the disgusting smell of corked wine, which is primarily caused by the chemical 2,4,6-trichloroanisole, commonly abbreviated to TCA. Based on experience of corked wine you might think that TCA had two effects: one being to stimulate some smell receptor cells, to create a nasty smell; and the other to inhibit other smell receptor cells, to give the “fruit scalping”. However, experiments with newts showed only the inhibiting effect, and no receptor cell stimulation. We have to be a bit careful here, because of course the newt olfactory system might not behave like the human one but, on the face of it, the absence of any stimulation to create the corky smell seems rather puzzling. If the human nose behaved like the newt’s then could that mean that corkiness exists as a component of all wines, only to be revealed when other smells are suppressed? It seems unlikely, especially considering that other TCA-contaminated food and drink has the same musty smell.

Perhaps you can now see where this discussion is heading, and I must warn you that from this point it is all speculation on my part. It is possible there is a more solid basis in science, but I am not aware of one.

Could the COVID-19 parosmia be caused by some smell receptor types being inhibited, while others have been restored to a working state? And is that also the way that TCA gives rise to a musty smell, with some receptors types working and some inhibited? Note that in both cases it is not actually a case of aromas being removed from a blend, as there is not a one-to-one relationship between receptor types and aromas. Rather than aromas being removed, it is the taking out of action of receptor types, and that changes the shape of the “smell images” on the olfactory bulb, turning them into ones that are more similar to the images of bad smells (see my earlier post for a discussion of smell images). In the case of TCA, the smell that is distorted into mustiness could perhaps have nothing to do with the wine, but be the unnoticed background smell we have all the time from our own body (i.e. mouth, throat and stomach).

Speculation aside, I think you will agree that our sense of smell is an amazing thing, and of great value. Let’s hope that all COVID-19 sufferers manage to fully regain theirs. And may their wine never be corked.

More on aroma perception by sniff and sip

I am again writing here about comparing orthonasal and retronasal olfaction – smelling things through the nostrils, compared with through back of the mouth. If having two modes of smelling is a new concept for you, you might like to refer to an earlier post on the subject where I explain it in more detail. I started to get interested in the subject when I realised that a number of studies on wine aroma perception only looked at orthonasal olfaction, i.e. sniffing the wine, and wondered how much difference it would make if retronasal olfaction was studied instead. One of those studies was the relatively well-known one where a red dye was added to white wine, leading tasters to describe it in terms of red wine aromas (Morrot, Brochet and Dubourdieu, 2001, The color of odors, Brain Lang, 79 , 309-320). Another was a series of studies with the conclusion that we can reliably identify a maximum of four aromas in a multi-aroma mixture. Reading the recently published books by Gordon Shepherd and Jamie Goode has given me a few new insights, which I shall share here.

The most general point is that the smell of a wine is never perceived in isolation. It is part of what we might call the wine’s overall flavour, and it is impossible to totally separate smell from what we perceive through other senses. With retronasal olfaction, we are simultaneously tasting the wine on our tongue, and also feeling it in our mouth as possible astringency and alcoholic heat. Orthonasally, the interference from other senses is maybe less obvious, but we before we sniff the wine we still normally see it, and that affects expectations. This was in fact the point of the Morrot et al experiment mentioned above. Additionally we can also experience chemicals in our nostrils through our sense of touch, for example as alcoholic heat, or the pungency of hydrogen sulphide. These non-smell sense modalities are not only confusable with smells, but they can also affect our sensitivity to true smells

We should also note that we never smell the wine itself, but the volatile molecules that escape from it. In the glass, however much we may swirl and sniff, we have relatively little control over how the volatile molecules escape and reach our nasal cavity. But once in the mouth, the processes acting on the wine can be complex, and vary a lot from time to time, and person to person, even if we are largely unconscious of what is going on – the wine is mixed with saliva, warmed towards body temperature, moved around the mouth to a greater or lesser extent, and allowed to coat the mouth and throat, and its molecules enter the nose when we breathe out.

Then, there are the effects of background smells. You can become so used to a background smell that you no longer notice it. Normally this is an advantage when it comes to wine tasting. It will, for example, filter out the smell on your hands of cigarettes or mildly scented soap. But what if that background smell also happens to be a component of the wine? IN that case your desensitisation to the background smell will affect how you perceive the wine. There is also the possibility of cross-adaption, where one odour affects your sensitivity to other one. The most obvious example is perhaps how the TCA of a corked wine mutes its fruity aromas. If background smells may be important from the environment, retronasal olfaction must be affected by non-wine smells inside your mouth. Apart from the possible remnants of lunch, coffee and cigarettes, there are also the normal odours of your mouth and throat to consider. Unpalatable to think about perhaps, but perfectly natural, and a potential source of desensitisation and cross-adaption when tasting wine.

There have, by the way been a number of studies that have concluded that odour detection thresholds are higher for retronasal that orthonasal olfaction – in other words that we are more sensitive to smells when sniffing through our nostrils. But we need to be careful about drawing false conclusions from these results. In the experiments, the samples are presented to subjects as gases containing an odour, through a tube that goes either to the nostrils or to the back of the mouth, so it is difficult to see what direct relevance these results have to wine tasting.

Finally I would draw your attention to a 2006 study, not discussed in the above-mentioned books, that pursues the question of how many flavour components we can identify in a mixture, but unlike earlier studies that have looked at orthonasal olfaction alone, this one concerns how we perform with a liquid in the mouth, and involves both odourants and tastants – things we detect using both the nose and taste buds. Thus, it is more applicable to the question of how many component we can detect when tasting wine. I won’t give a detailed description of the results here, but just mention that they were very much in line which the findings of the orthonasal olfaction studies. To quote from the authors: “In conclusion, the present study has shown that humans have great difficulty identifying the components of odor–taste mixtures when more than two components are present”.

In summary, I would say that there are many potential reasons why orthonasal and retronasal olfaction might lead to different perceptions in wine tasting, but it is still far from clear how important the differences are in practical terms.

How smell is like vision, and what that means for wine

Considering the very different impressions that vision and smell make on us, there are surprising similarities in how the two senses are processed before they reach the brain. And it is quite possible that these similarities may throw some light on how we describe the aromas we find in wine.

The olfactory bulb (we actually have two of them) is an elongated protuberance lying close to the underside of the brain, but attached only at the back end. The surface of the human olfactory bulb has on it some 6,000 spherical bundles of cells called glomeruli, each one being connected by neurons to several thousand olfactory receptors in the nasal cavity. When odorous compounds enter the nasal cavity, each glomerulus is activated to a greater or lesser extent, creating a pattern of activity on the surface of the olfactory bulb that is a representation of the odours detected. That pattern can be regarded as analogous to the pattern of activity on the retina of the eye when an image falls on it. In fact the similarity does not stop there, because just as the image on the retina is further processed to facilitate detecting edges and motion, the activity pattern in the glomeruli is also enhanced by subsequent layers of cells in the olfactory bulb. Examples of smell images, reproduced from Gordon M Shepherd’s book Neurogastronomy (reviewed here), can be seen below – click to enlarge and make the text legible.smell images

In the same book, Shepherd proceeds to speculate that the smell images created by glomeruli activity are similar to visual images of faces. He suggests that this explains why smells, like faces, are difficult to describe in words but relatively easy to recognise. As a result, if asked to describe a smell we need to resort to comparisons with the smells of well-known objects. Also, neither smells nor faces are processed as the sum of distinctive component parts – we tend to recognise both of them holistically, not so much by the detail as by a general impression, and the relation between the parts of the image. Only occasionally can we recognise a face if we only see a small part of it, and usually only for faces we are very familiar with.

This speculation of Shepherd’s can be plausibly taken even further, and related to how we recognise and describe wines. Regardless of whether we are nosing a complex wine or sniffing a single chemical compound, at one level in our perceptual system the result is a glomeruli smell image. I would propose that, in the case of wine, certain aspects of that smell image may remind us of the smell images of other objects – blackcurrant maybe, or lemon – which then become the descriptors we use for the wine. In some cases, the aspects of the smell image that cause us to identify other objects in wine may arise from chemical compounds in common, but this need not necessarily be the case and similarities might be coincidental. The aspects in common may be as simple as discrete fragments of the smell image, or possibly with their root in common relationships between different parts of each image. To continue with the face analogy, the identifying of blackcurrant in a wine could be like saying that a baby’s face has his grandfather’s eyes – the eyes need not be identical, but there is however something that seems somehow similar, perhaps the relationship between the eyes and the nose. Something else that has a counterpart in wine is the idea that if we are very familiar with a face is it easier to recognise it from a partial image. In a smell image of wine, presumably the types of fruit etc. we may recognise in it are only partially represented, and that could explain why it we are more likely to recognise the aromas we are more familiar with, either from the experience of the actual fruits or from other wines.

I totally accept that most of this is speculation on the part of Shepherd and me, but nevertheless I think how we experience and describe wines is consistent with the idea of smell image recognition, and an interesting way of conceptualising it. Only time and more research will be able to refute or support these ideas.

How does a sniff compare to a sip?

I have received no comments directly addressing the issue posed in my last blog post: whether aromas detected on the palate mirror those on the nose or not. However, I did get 34 wine-enthusiast responses to a poll on the UK Wine Forum.


The results are not entirely clear cut, and my question could perhaps have been better, but it does seem that experiencing very different sets of aromas on the nose and palate tends to be the exception rather than the rule. I would thus very tentatively suggest that the results support the applicability to wine of the experiments regarding the number of aromas we can detect.

However we need solid experimental evidence to be sure. It is quite possible that those who detect different aromas when the wine is on the palate are imagining the different aromas – in the sense that there are no chemicals present that could account for them, that is. But, neither can we rule out the possibility that those who detect the same aromas on the palate do so due to expectations created by the wine’s nose aromas.

Aroma recognition by sniff and sip

In everyday life, and in wine tasting notes, we often distinguish between what we smell through our nose, and what we taste when something is in our mouth. However, in practice the distinction is not so simple, and smell is important in both cases. In the image below, you can see that in fact we have two very different openings through which odours can gain access to the olfactory bulb where smells are detected: through the nostrils (orthonasal olfaction), and through an opening between the mouth and the back of the nose (retronasal olfaction).

When we sniff a wine, we perceive its volatile molecules though our nostrils, and that is all. But when we sip it, we sense the wine through a number of distinct mechanisms. With the tongue we experience relatively simple flavours: sweet, sour, salt, bitter and umami. On all the inside surfaces of the mouth, we experience physical sensations such as the temperature of the wine, its weight and viscosity, and we may also feel a slight alcoholic burn. At the same time, and most importantly, the olfactory bulb senses the volatile components retronasally. It is this that gives rise to how we perceive the most interesting aspects of wine, which are its volatile components, e.g. blackcurrant, lemon, vanilla, coffee, leather – in fact everything we call flavours apart from sweet, sour, salt, bitter and umami. Because we experience the volatile components as the wine is in the mouth, we are given a strong impression of sensing them on the tongue, but this is an illusion.

So, and here I get to the nub of this blog post, if we smell for example apples and lemon when we sniff a wine, wouldn’t you then expect to taste apples and lemons when it is in your mouth. And if it smells of oak, shouldn’t it taste of oak? Probably very few of us have actually chewed on a piece of oak, and the same goes for other non-food items, but we seem intuitively to understand how things should taste if we can smell them. As the volatile molecules are the same, as a starting point I would indeed expect the taste to be consistent with the smell.

However there are complicating factors. One is that the relative concentrations that arrive at the olfactory bulb might differ in each case, as molecules are carried there in different ways, and the temperature of the liquid will be different. As a result, the dominant aroma might be different in each case.  The concentration differences might also be such that certain aromas are above their detection thresholds in one case but not the other. These effects can be mimicked to an extent by sniffing wine at different temperatures, in different glass sizes and shapes, and after the wine has been agitated to different degrees in the glass, all of which can cause a wine to smell differently.

Synaesthesia is another complicating factor. Above I analysed how the flavour of wine in the mouth can be broken down into the taste on the tongue, aromas in the nose, and physical sensations. However that is a simplification, because the different senses interact with each other. For example, Westerners are more sensitive to almond aromas if there is a drop of a sugar solution on the tongue, amazingly even if the sugar concentration is below the detection threshold. That is one interaction we know about, but it is a safe assumption that there are many others. Thus, synaesthesia might be another reason why aromas could be perceived differently orthonasally and retronasally.

But what happens in practice? Well, people seem to differ alot. Speaking personally, the flavours I get in the mouth are nearly always very similar to those I get on the nose. Sometimes one flavour might be more dominant on the palate than on the nose, or some might be wiped-out by excessive acidity or astringency, but that is about the extent of the differences for me. On the other hand, some tasting notes have totally distinct sets of aromas for orthonasal and retronasal olfaction. For example, take the one I referred to a few weeks ago in my first post about the number of aromas we can identify in a wine: fresh fruit aromatics of mandarin orange, black raspberry and grilled watermelon spring from the glass. On the palate, pretty nuances of rose petal, gardenia and oolong tea mingle with herbal notes of sandalwood, star anise, fresh thyme and fennel seed. So that is 3 aromas orthonasally, and 7 totally different ones retronasally.

In fact, it was my number-of-aromas posts that got me thinking about this, as the experiments, which found we can recognise no more than 4 aromas in a blend, were all based solely on orthonasal olfaction. So does retronasal olfaction make these experiments of limited applicability to wine, or are markedly different aromas detected retronasally merely a tasting note conceit?

I suspect the latter, but I really don’t know and I would love to hear how much wine orthonasal and retronasal aromas differ for you. I am going to try to run a few straw polls in various places, and will report back. Feel free to leave comments here too.

Update: My reporting-back can be seen here.