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.