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Smell plays an incredibly important role in the way our brains perceive flavor. The five tastes (sweet, salty, sour, bitter, and umami) are only one part of the flavor equation, which also includes sight, sound, and touch. Our sense of smell is what truly allows us to appreciate and savor food.
Aromas are actually a complex blend of many molecules. When they enter our nose, those molecules bind to receptors, which send signals directly to our brain. The brain quickly identifies the aroma based on those signals. Despite the belief that the human sense of smell is lacking, fMRI has shown that the brain can distinguish between three aroma molecules that differ by just one carbon atom. That's why, with a single sniff, we can readily distinguish between fresh and spoiled milk, or a firm, green banana versus an overly ripe, mushy one.
Our sense of smell is what truly allows us to appreciate and savor food.
Obviously, we smell food by inhaling through the nostrils (called orthonasal olfaction). These scents are perceived as coming mainly from outside the body. But we also smell food while it is in our mouths. This process, called retronasal olfaction, is key to our flavor experience. When we eat, aroma molecules in our food drift through the mouth, towards the back of the throat, and up an airway (the retronasal passage) that connects the back of the mouth to the nose, where they undergo the same treatment as molecules that entered through the nostrils. Chewing releases more of those compounds, so does swallowing. As additional aroma molecules are released, they travel to the nose and stimulate the brain.
So much happens in the mouth when we eat: we simultaneously experience the tactile sensations of food, the five tastes on the tongue, and the physical acts of salivating, chewing, and swallowing. Food aromas captured through retronasal olfaction during that time feel as though they come from within the mouth, too. This gives the illusion that flavor occurs there. But it is the brain that registers the separate signals from the mouth (touch and taste) and nose (smell), and combines them with other signals into the unified concept of flavor.