Unique by David Linden

Author:David Linden [J. LINDEN, DAVID]
Language: eng
Format: epub
Publisher: Basic Books
Published: 2020-09-29T00:00:00+00:00

MOST OF OUR EXPERIENCE of flavor comes from the sense of smell. You may not think either Coke or 7UP has much of an aroma, but without smell, they are indistinguishable sweet fizzy water. A slice of steak is merely salty chewy stuff with a bit of umami mixed in. Lemonade is just sweet and sour water. Not surprisingly, people who have lost their sense of smell (a condition called anosmia) take little pleasure in food and often struggle to eat enough to maintain a healthy weight. In addition, they often report difficulty sleeping, cognitive disruption, and loss of motivation and feelings of social connectedness. Most importantly, anosmia sufferers have a significantly elevated risk of depression and suicidal thoughts. Anosmia is a serious, life-threatening condition that goes well beyond the evaluation and enjoyment of food, as we also rely on smell for social and sexual cues, detecting danger, learning, and even navigation.

In relation to food, the sense of smell supports three main decisions. The first is: Where can I find food? The second: Should I put this food item in my mouth? The third, also informed by the sense of taste: Should I swallow this item that’s in my mouth or spit it out? Considering these questions is key if we are to develop an understanding of the human sense of smell and how it varies among individuals. The first and second decisions involve evaluating odor molecules originally located outside of the body. These odorants are inhaled through the nostrils to reach a patch of about twenty million specialized olfactory receptor neurons, located in the upper wall of the nasal passages. This sniffing route is called orthonasal olfaction (figure 13).

However, when food is in the mouth, odor molecules released from it are carried by exhaled breath through a passage at the rear of the roof of the mouth called the nasopharynx and so reach the patch of olfactory receptor neurons through the back door. This smelling through exhalation is called retronasal olfaction and is only found in a subset of mammals, including primates and dogs. Importantly, sniffed and exhaled odorants do not reach the array of olfactory receptor neurons in exactly the same pattern and concentration. This means that if you sniff something, the odor you experience is somewhat different than if you hold it in your mouth and exhale. This is probably why some foods, like certain ripe cheeses, can have an offensive odor when sniffed but have a fine flavor (to which odor is a major contributor) once they are in your mouth.28

Let’s consider the smell of a ripe tomato. The tomato is composed of thousands of different molecules. Of those, only about 450 will be sufficiently small and volatile to be released into the air, where we can measure and identify them using a machine called a gas chromatograph. Of those that travel on the air, only sixteen will bind to the specialized odor receptors on the olfactory receptor neurons with sufficient affinity to reach the threshold of perception by humans and so contribute to the blended odor we call tomato smell.

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