Our sense of smell is often overlooked, but it plays a crucial role in our daily lives. In this post, we will explore the science behind olfaction, taste, and flavour perception. 

Olfaction, or the sense of smell, works by detecting molecules in the air. When you inhale, aromatic molecules are picked up by the smell receptors of the olfactory epithelium, a special tissue responsible for detecting odours. 

The olfactory epithelium is located in the upper part of the nasal cavity, and it is connected to the olfactory nerve, which is the pathway that transmits signals from the receptors (epithelium) to the olfactory bulb. 

Once the signals reach the olfactory bulb, they are processed and sent to other areas of the brain for further identification, such as the olfactory cortex, orbitofrontal cortex, and the limbic system, which are responsible for interpreting the signals and creating the final perception of a scent. These areas of the brain are also involved in processing our emotions and memory, connecting them to the different odours we sense.

The brain is able to distinguish between different aromas because each type of molecule activates a unique combination of receptors. For example, the molecules that give roses their characteristic smell activate a different combination of receptors than the molecules that give lemons their smell. 

The sense of olfaction is not just about detecting and identifying different scents, it is also closely linked to our sense of taste. 

The link between olfaction and taste

Our sense of smell actually plays a dominant part in the perception of the flavours of the food and drinks we consume. Indeed, when eating or drinking, pretty much 80% of the perception of flavours comes from our nose (our sense of smell) and only 20% from our palate (our sense of taste). [Let’s use this 80:20 ratio as a guideline only - in reality, smell accounts for most of the overall ‘flavour perception’, easily up to 95% of it]

This is because with our tongue we can only detect five different things: sweet, sour, salty, bitter and umami (these are called the basic tastes). With our nose, on the other hand, we can detect more than 10,000 different odours. Even when we sip, or chew, the different flavours we perceive are actually picked up by our nose and not by our palate. This is because our sense of olfaction is divided into two parts: orthonasal olfaction, and retronasal olfaction. 

Orthonasal olfaction refers to the process of detecting odours through the nose by inhaling air through the nostrils. This is the most common way that we perceive smells, and it allows us to identify and detect the various scents we encounter in our daily lives. 

Retronasal olfaction, on the other hand, refers to the perception of odours that occur when air is exhaled back through the nostrils after being inhaled through the mouth. 

Retronasal olfaction plays a key role in our sense of taste, as it allows us to perceive the flavours of the food we consume. When we eat or drink something, the aromatic molecules of the substance enter the mouth and are mixed with saliva. As we chew or sip, these molecules are released into the air and are inhaled back through the nostrils, activating the olfactory receptors in the nasal cavity. 

When we perceive an aroma, whether orthonasal or retronasal, the brain receives a signal from the smell receptors. This signal, along with signals from other senses (taste, sight, touch, etc.), is used by the brain to create the final perception of the flavour of the food we consume. But before explaining this process, let’s talk about the sense of taste. 

The sense of taste, or gustation, works by detecting the chemical composition of substances that we put in our mouth. There are only five primary tastes that the human tongue can perceive: sweetness, sourness, bitterness, and umami. Some scientists also argue that metallic and fat are basic (or primary) tastes too. 

Each of these tastes is caused by the activation of specific types of taste receptors by different types of molecules. For example, the sweetness of sugar is detected by sweet taste receptors, while the bitterness of coffee is detected by bitter taste receptors. When we eat or drink something, the molecules of the substance dissolve in our saliva and come into contact with the taste buds on the surface of the tongue and palate.

Each taste bud is made up of a group of taste cells, which have tiny hair-like structures called microvilli on their surface. These microvilli are covered with special proteins called taste receptors, which bind to specific molecules in the food or drink, triggering a chemical reaction. This reaction sends a signal to the gustatory cortex in the brain, which interprets the signal as a particular taste. 

How the final perception of the food and drink we consume is created in the brain? 

When the brain receives signals from the sense of smell and taste (and other senses too), it integrates them to form a comprehensive perception. 

The gustatory cortex, located within the insular and opercular region of the frontal lobe, specifically processes and interprets signals from the taste receptors in the tongue. 

The olfactory system, on the other hand, processes signals from the taste receptors in the nose and sends them to the olfactory bulb in the brain, which then relays them to the olfactory cortex. 

Additionally, the orbitofrontal cortex and the limbic system play a role too in the perception of flavour, by integrating the information from the gustatory cortex and olfactory cortex, with other sensory information from memory and emotion. This explains why certain flavours can evoke specific emotions and memories, and how our mood at a given time can impact the perception and quality of the flavours perceived.  

It is a bit more complex than just putting something in your mouth and deciding whether you like it or not.

To summarise in short what happens when we eat or drink something: 

• Aroma receptors in the olfactory epithelium in the nose pick up different molecules, via orthonasal olfaction (through the nostrils) and retronasal olfaction (from the back of the palate). They send a signal through the olfactory nerve to the olfactory bulb. From here, the signal is sent to other areas of the brain for identification and further processing (the olfactory cortex). 

• Taste receptors on the tongue detect the different basic tastes and send a signal to the gustatory cortex. 

• Other signals about the food and drink we’re consuming are sent to the brain by different senses: think of the temperature, texture, or how appealing (or not) it looks. Those are all elements that will contribute to the final flavour perception. 

• The primary gustatory cortex and olfactory cortex process these signals and generate the perception of taste and aroma respectively.

• The orbitofrontal cortex and the limbic system, both receive the signal from the gustatory and olfactory cortex and integrate information related to memory and emotion, creating the final, multi-sensory perception of the flavours processed.

This is obviously a very simplified version of the process and other areas of the brain are also involved in the perception of flavour. Overall, it is a fairly complex topic (although a fascinating one) and I’ll break it down slowly in different posts, to uncover all things flavours, spirits, cocktails and scents. 

In the next post, we will describe and analyse everything that you need to know about aromatic molecules.