Retronasal vs Orthonasal: The Complexity of Smell

In sensory analysis, we use our senses to evaluate products. The senses from which we can gather the most interesting data are taste and smell, and I am particularly drawn to the study of the latter, as the number of stimuli it can perceive is immense.

Taste

Taste is detected on the tongue through five specific receptors, each corresponding to a different family of molecules. These are translated into five sensations we call tastes: sweet, sour, salty, bitter, and umami.

Smell

Smell, on the other hand, is an epithelium composed of non-specific receptors, meaning it can perceive hundreds of thousands of different stimuli. The richness of these stimuli is further enhanced because the brain interprets varying concentrations and relative mixtures of molecules in distinct ways. It is the sense that can provide the most varied information, but it is also one of the earliest to appear evolutionarily. This creates certain connections to parts of the brain often linked to memory. As a result, it is not difficult to relive a situation when perceiving a familiar aroma.

Precisely because of my fascination with the sense of smell, I felt somewhat frustrated when I began studying sensory analysis. Smells (perceived via the orthonasal route) and aromas (perceived via the retronasal route) were treated as different attributes. The only distinction between them lay in the path the air takes to reach the olfactory epithelium. In the case of smells, volatile molecules enter through the nose, while with aromas, the air travels through the mouth and rises to the olfactory epithelium via the throat.

What difference could the path possibly make?

When I delved deeper into sensory study, I came to understand that the difference is significant, but the key is not in the path the air takes, but in the conditions of the food.

The issue is that not all molecules have the same volatility; this varies depending on numerous factors. Typically, when we “smell” a product, it has relatively little surface area exposed to the air and is cold. As a result, only certain molecules are present in the air and can be perceived by our olfactory epithelium.

When we place the product in our mouth and chew it, we agitate it, increasing its surface area exposed to the air and its temperature. This helps less volatile molecules reach our olfactory epithelium. Therefore, the olfactory profile and the relative intensity of aromas can differ greatly between orthonasal and retronasal olfaction.

Sensory analysis always involves an additional layer of complexity. Its methodologies transform the senses of tasters into true sensors, far more sophisticated than any instrument we might find in a laboratory.

Words by Andrés Martínez Sánchez , CPO SENSESBIT