Smell is probably the only sense that has two pathways. When we sniff an object, our nose senses that object from the outside, from the aromas it is giving off. When we put that item in our mouth, our sense of smell can also detect what is in our mouth, due to aromas making their way from the palate and into the nasal passages located behind the roof of the mouth. Although everyone will have experienced some loss of ‘taste’ when having a cold, most people are not aware of the fact that the inability to taste food during sickness is related to the blockage of pathways for smelling. When we consume food and drink, what we often perceive as ‘tastes’ are in fact aromas from the palate that reach our olfactory receptors through the nasal passages. This perception, called retronasal olfaction, can be pictured as an aroma compound travels from our palate and passes through the back of our mouth to the back of the nasal cavity, ending up on the nasal receptors that detect the aroma. This is why most food ‘tastes’ more than just sweet, umami, sour, salty or bitter on the palate: our nose also receives signals while eating and drinking. If a person loses their sense of smell from diseases such as COVID-19, most food becomes ‘tasteless’.
The graphic demonstrates the two pathways of smelling: orthonasal olfaction and retronasal olfaction. Orthonasal olfaction is more intuitive as we actively or passively sniff things even from birth. Retronasal olfaction was not well understood by researchers until recent decades, but easy experiments can be done to trace the smell from within the mouth. When eating chocolate, use a nose clip to close your nose – you will find out that the chocolate seems to lose all of its ‘flavours’ in the mouth. This is because most of the ‘flavours’ of chocolate are volatile aromas that are detected by our retronasal olfaction. Since wine has so many volatile aroma compounds, losing the ability to smell will make any wine taste nothing but sour, astringent or alcoholic.
Studies have demonstrated that our brain responds to orthonasal and retronasal olfaction differently. Moreover, different foods cause different brain activities. For example, chocolate gives more intense brain activity retronasally, while lavender triggers more of the orthonasal responses. It is hypothesized that because chocolate is considered a consumable food, our retronasal olfaction is more sensitive to produce the desire to eat chocolate. Lavender is less of a food but more of an aromatic object in nature so the orthonasal pathway is more active. In most cases, retronasal olfaction activates many more parts of our brain, despite some aroma compounds being more easily detected via the orthonasal pathway. Therefore, to have a comprehensive experience with aromas in food and drinks, we should combine smelling and tasting.
The bouquet one might discover when smelling wine directly may differ from that experienced when tasting and ‘inhaling’ its scent retronasally
In wine, the difference between orthonasal and retronasal olfaction can be quite obvious. For example, the bouquet one might discover when smelling wine directly may differ from that experienced when tasting and ‘inhaling’ its scent retronasally. One major cause of such difference has to do with the volatility of the aroma compounds. For instance, not all terpenoids in grapes and in wines are volatile. In fact, most of them are attached to other compounds like sugar, keeping them non-volatile. This is because plants have worked out a mechanism to store the aromatic terpenoids and release them gradually into the air. The fruit doesn’t want to give out all of its aroma at once and therefore immediately lose attraction to animals during the season of dissemination.
Since wine has so many volatile aroma compounds, losing the ability to smell will make any wine taste nothing but sour, astringent or alcoholic
The most common combination of a non-volatile compound and a terpenoid is a glycoside, which consists of a sugar molecule attached to a terpenoid compound. The bond within the glycoside can be broken quickly by enzymes so that the terpenoids are released as volatile aromas. During fermentation, the enzymatic reaction within the yeast cells cleaves the bond, releasing quite a lot of terpenoids. This is one of the many factors that make the aromas of wine more complex than the aromas of grapes. During a wine’s ageing process, terpenoids can decrease or increase in concentration, or transform into something else. If the majority of terpenoids in wine are released to become volatile during winemaking, those aromas tend to be lost quickly. This can explain why certain wines smell very aromatic in their youth but become bland and dull in a short period of time. However, if there are more terpenoids locked in the non-volatile, sugar-attached form, the aromas of those wines may not be very expressive at the beginning, but will tend to smell more complex after ageing. These wines may also be more aromatic retronasally than orthonasally. This is because our saliva contains enzymes that can break the bonds between sugars and terpenoids. This is why when we chew on the skins of grapes like Muscat, they become more aromatic in the mouth as the saliva breaks apart the sugar molecule and the terpenoid. If the concentration of enzymes in our saliva is high, we are likely to perceive far more aromatics on the palate than we will by sniffing a wine in a glass.
