Retronasal smell, retronasal olfaction, is the ability to perceive flavor dimensions of foods and drinks. Retronasal smell is a sensory modality that produces flavor. It is best described as a combination of traditional smell (orthonasal smell) and taste modalities. Retronasal smell creates flavor from smell molecules in foods or drinks shunting up through the nasal passages as one is chewing. When people use the term "smell", they are usually referring to "orthonasal smell", or the perception of smell molecules that enter directly through the nose and up the nasal passages. Retronasal smell is critical for experiencing the flavor of foods and drinks. Flavor should be contrasted with taste, which refers to five specific dimensions: (1) sweet, (2) salty, (3) bitter, (4) sour, and (5) umami. Perceiving anything beyond these five dimensions, such as distinguishing the flavor of an apple from a pear for example, requires the sense of retronasal smell.
History Evolutionarily, smell has long been presumed to be a less-important sense for humans, especially compared to vision. Vision appears to dominate human stimuli perception, but researchers now argue that smell cues are highly informative to humans despite being less obviously so. Before his death in 1826, French gastronome Brillat-Savarin published his book, The Physiology of Taste; Or, Meditations on Transcendental Gastronomy: Theoretical, Historical, and Practical Work, in which he makes the first mention of the importance of smell in the “combined sense” of taste. He defines taste in terms of the five taste dimensions in addition to flavor created with the nasal apparatus. Avery Gilbert, in his book The Nose Knows, reviews the work of Henry T. Finck, an American philosopher from the late 1800s who published a groundbreaking essay titled “The Gastronomic Value of Odours.” Flink called flavor a “second way of smelling,” and much subsequent scientific investigation in the early 1900s focused on attempting to break down smell dimensions into basic categories, a feat that has proven too complicated due to the vast number and complexity of odors. Food connoisseurs and chefs are increasingly capitalizing on the newly ascertained understanding of the role smell plays in flavor. Food scientists Nicholas Kurti and Hervé This expanded upon the physiology of flavor and its importance in the culinary arts. In 2006, This published his book, Molecular Gastronomy: Exploring the Science of Flavor, in which he explores the physical mechanisms that bring about flavor perception. Kurti and This influenced others, such as Harold McGee, whose 1984 book, On Food and Cooking: The Science and Lore of the Kitchen, has been extensively revised in 2004 and remains a key reference on the scientific understanding of food preparation. His book has been described by television personality Alton Brown as “the Rosetta stone of the culinary world.” Such a breakthrough in the understanding of the mechanisms behind experiencing the flavor of different foods is likely to continue inspiring those in the culinary arts to create novel combinations and recipes. Today, one of the most active food psychologists, Paul Rozin has been the first to successfully map the role of retronasal smell in flavor. In 1982, he explained that smell is a “dual-sense” and made the explicit differentiation between retronasal smell and orthonasal smell. Rozin describes orthonasal smell as “breathing in” and retronasal smell as “breathing out.” In 1982, he devised an experiment in which he trained participants to accurately recognize smells orthonasally before introducing them to the back of the mouth, at which point the success rate fell drastically, demonstrating that smell operates through two distinct mechanisms. His favored example of this duality is Limburger cheese, which is known for its repulsiveness to the nose yet pleasantness to the mouth. Originally published in 2012, Neurogastronomy by Gordon M. Shepherd provides an overview of the way smell is perceived in humans. The book comprises a detailed review of how retronasal smell, in combination with taste, creates flavor. Shepherd describes the neural basis for identification, recognition, and preference for certain flavors, and explores potential political and social implications of a deeper understanding of flavor perception, such as causes of obesity and concerns of loss of smell sensitivity in old age.
Overview of the smell pathway To better understand this mechanism, a simple breakdown of smell pathway is provided below. When humans chew, volatile flavor compounds are pushed through the nasopharynx and smell receptors.
Olfactory epithelium The first stop in the olfactory system is the olfactory epithelium, or tissue resting on the roof of the nasal cavity which houses smell receptors. Smell receptors are bipolar neurons that bind odorants from the air and congregate at the olfactory nerve before passing axons to the dendrites of mitral cells in the olfactory bulb. Sensory receptors in the mouth and nose are polarized at resting state, and they depolarize in response to some change in environment, such as coming in contact with odor molecules. Odor molecules, consisting of hydrocarbon chains with functional groups, bind to sensory receptors in the nose and mouth. Properties of functional groups include: (1) length of carbon chain, (2) terminal group, which concord with differences associated with different smells, (3) side group, (4) chirality, (5) shape, and (6) size. When odor molecules bind to sensory receptors, they do so in according to these properties. Each olfactory cell has a single type of receptor, but that receptor can be “broadly tuned” and odor molecules further interact at the receptor level, meaning that, in certain cases, an odor molecule alone may not bind to a receptor, but in the presence of another odor molecule, the original would bind and thus create a sensation of smell only in the presence of the second molecule.
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