The perception of flavor – retronasal olfaction
We are honoured to present you a special blog edition written by our guest author Dr. Johannes Frasnelli.
Dr. Frasnelli specialises in odor perception. He conducts research in the field of neurophysiology of smell and taste as well as therapy in loss of the chemical senses.
Let’s say we go for a walk through the city with a friend. At some point we are getting hungry and we happen to walk by a rotisserie. As we walk through the fumes, we say to our companion: “This smells so nice; I would like to eat some grilled chicken.” So we decide to enter the rotisserie; as soon as we open the door the lovely smell of grilled chicken is getting even stronger. We order and finally the waiter serves a plate with a beautiful grilled chicken. We start eating, the chicken is impeccable, and with the first bite we take, we say to our companion: “This tastes so good, the chicken is just lovely.” Our companion agrees, since he is as happy with his chicken as we are. So, we both enjoy our food; the question is, are we both right? Are we right when we say that food “tastes” good?
(Click here to know where this rotisserie is! The chicken is amaizing)
Let’s first have a look at the sense of taste. With our sense of taste, we are able to perceive the five basic tastes. They are: salty, sour, bitter, sweet and umami. We all know salty (for example, salt), sour (for example, lemons), bitter (for example, coffee), and sweet (for example, cherries); umami is less known. Umami, which is a Japanese word and means “pleasant savory taste”; it indeed describes a savory taste as we find it in meat, tomatoes, mushrooms, parmesan cheese etc.
However, these five basic tastes are not enough to perceive the flavor of food. Rather, we perceive the flavor of food via the sense of smell! But how in the world would odor molecules reach the nose when we have food in our mouth – even if we close the mouth while chewing, as we were taught when we were children? The solution to this is that the odor molecules use the back entrance to get into the nose. From the oral cavity, they travel backwards, until they reach the throat; the throat is connected to the nasal cavity (the inside of the nose) in the top, to the oral cavity (the mouth in the middle), and in the bottom part it is connected to the larynx and eventually to the trachea and the lungs as well as to the esophagus.Odor molecules can easily travel from the mouth to the nose via this connection in the throat. So, they can reach the olfactory receptors, and they can evoke a smell perception. The interesting thing is that we do not realise that this happens in the nose, we have the impression that our perception stems from the mouth; and therefore we say “the chicken tastes good.” We call this perception of flavors retronasal olfaction, which means nothing else than “smelling the back of the nose”.
It may be difficult to believe but we can actually test whether this is true. For example, if we have a cold, our nose is stuffed. We cannot breathe through our nose, and we can’t smell anything. If we eat something during having a cold we also realise that every food tastes bland. We are still able to perceive its sweetness, its saltiness, its bitterness, its saltiness, and its savoriness, even if we have a blocked nose, but we would have a hard time to distinguish an apple from a pineapple. Both are sweet and a bit sour, and we would still perceive this, but in order to perceive the typical apple flavor and the typical pineapple flavor we have to rely on the sense of smell. The access to the receptors of the sense of smell is however blocked, since the nose is blocked when we have a cold; so we are not be able to perceive the flavors and consequently we cannot distinguish apples from pineapples.
We can test retronasal olfaction even if we have no cold in a fun experiment. You need two persons, one who is tested (the subject) and the other one who is testing (the experimenter). The experimenter prepares different food with a similar texture; alternatively you can also use candies with different flavors for this experiment. Similar texture is important, because we do not need to perceive the flavor to distinguish a banana from an almond, we can do that by feeling both fruits with the tongue. The experimenter picks two fruits (let’s say strawberries and ripe mangos; or apples and pineapples). The subject closes his eyes and pinches its nose. Then the experimenter hands the subjects a spoon with a small piece of the fruit. The subject inserts the fruit into the mouth while keeping the nose pinched. He should then try to identify the fruit. You will see that this is very difficult to do. At some point the subject can unpinch the nose, and all of a sudden he will have no problem in identifying the food.
Why is this? When the nose is pinched, much less air can travel to the nose, even from the back and the odor molecules cannot reach the receptors. When however the nose is unpinched, air travels freely through the nose, the odor molecules reach the receptors, and we perceive the flavor.
As we see, flavor perception is mostly driven by the sense of smell. To come back to the question we had in the beginning: we are not right if we say the food tastes good, we should rather say it smells good, even if we are already eating it. The thing is: we probably do not care, as long as the food is good.
Want to know more? Learn how does work an electronic nose compared to the human nose
About the author: Dr. Johannes Frasnelli Ph.D.
Dr. J. Frasnelli is a graduate of the Medical Schools of the University of Vienna (Austria; 2001; Dr. med. univ.) and the Technical University of Dresden (Germany; 2009; Priv.-Doz.). Since 2006 he work in Montreal, first as an Academic Trainee at the Montreal Neurological Institute, since 2008 as a Postdoctoral Fellow at the Department of Psychology at the Université de Montréal. He currently hold a fellowship of the CIHR. Dr. Frasnelli research interest is the neurophysiology of smell and taste as well as therapy in loss of the chemical senses.
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