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.

About Steve Slatcher

Wine enthusiast
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