Why do some things smell and taste different?

Sometimes, we find that the smell of food is very different from its actual taste: for example, the smell of soil after rain is fresh, but if you really put a mouthful of soil in your mouth, it will become an unpalatable earthy smell. There is also snail rice noodle, which smells stinky, but becomes delicious when you eat it.

So why do some things smell and taste completely different? You might be smart enough to say, isn't this just the difference between smell and taste? That's right, but you're only half right. This "smell and taste different" phenomenon is indeed related to the perceptual differences between taste and smell, but there is actually another situation that is almost entirely dependent on smell.

In our interaction with food, the sense of smell is actually divided into two perceptual pathways, namely "prenasal olfaction" and "retronasal olfaction". [1] Prenasal olfaction refers to the smell we smell directly through our nostrils. Usually, our prenasal olfaction begins working before the food enters our mouth. Prenasal olfaction provides us with our first impression of food and is the key to our judgment of the aroma and attractiveness of the food, and more importantly, whether the food in front of us is fresh and whether it is still edible. Imagine you smell the aroma of a steaming dish of stir-fried vegetables before eating. This is the perception brought to you by prenasal olfaction. This perceptual process can affect our expectations of food and even make us feel hungry and salivate.

Figure caption: Anterior nasal olfactory pathway (left) and posterior nasal olfactory pathway (right), Image source: Reference [2]

However, the retronasal sense of smell is activated after the food enters the mouth. When we chew our food, the food releases more odor molecules, which enter the nasal cavity through the back of the mouth and are then captured by the olfactory receptors. This is the retronasal sense of smell, which provides us with the smell perception of food as we chew and swallow. The signals of retronasal smell and taste merge with each other in the brain to form a complete taste experience. Therefore, the taste of the food we eat is actually composed of the integration of retronasal smell and taste.

This division of labor makes the experience of anterior nasal olfaction and retronasal olfaction completely different. For example, the smell of food we smell (anterior nasal olfaction) may make us expect the food to be sweet, but when eating, the combined experience of retronasal olfaction and taste may be different from expectations. For example, some foods that smell very fragrant may not have the same sweetness when chewing, or even have a slightly bitter or sour taste. In addition, some foods will react chemically with enzymes in saliva during chewing to produce new odor molecules. Take onions as an example. When we cut onions, the enzymes in the cells will break down some sulfides and release gases with spicy smells. However, during the chewing process, these decomposition products may change further, making the smell perceived in the retronasal olfaction not completely consistent with the initial smell we smelled.

In addition to the differences in perception pathways, psychological expectations also affect our judgment of the taste of food. The anterior nasal sense of smell will largely produce psychological cues, guiding us to have expectations about the flavor of food. Studies have found that people will have taste associations when they smell specific odors. For example, when they smell flowers, they usually expect sweetness, while when they smell the aroma of lime, they may expect sourness or a refreshing taste. If the retronasal olfactory experience does not match expectations, we will feel that the taste and smell of the food are inconsistent. For example, some foods that smell like chocolate tend to be surprising or even disappointing if they taste bitter or salty.

It can be seen that behind the difference between smelling and eating is the complex process of integrating the two senses of smell, frontal olfaction and retronasal olfaction, with the sense of taste, which together form our overall experience of food. Next time if you want to try a delicacy that "smells bad but tastes good" (such as snail rice noodle), you might as well pinch your nose and take a bite first, so as not to miss the delicious taste because of the "smoked" frontal olfaction.

References:

[1] Hannum M, Stegman MA, Fryer JA, et al. Different olfactory percepts evoked by orthonasal and retronasal odorant delivery[J]. Chemical senses, 2018, 43(7): 515-521.

[2] Chapter15:Taste Flashcards, Quizlet, 2021. https://quizlet.com/460557239/chapter-15-taste-flash-cards/

Author: Shi Jianyi Popular Science Creator

Reviewer: Pan Chunchen, deputy chief physician of the Department of Otolaryngology, First Affiliated Hospital of University of Science and Technology of China

The article is produced by Science Popularization China-Creation Cultivation Program. Please indicate the source when reprinting.