Science to Live By: Our Senses (Part Four): Smell and Taste

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The aroma of Chicken Barbeque
The aroma of Chicken Barbeque

© J. Dirk Nies, Ph.D.

Reflecting upon the nearly intractable, innate fixedness of his son, my dad once told me that he had been able to teach me only two things: how to build a fire and how to barbeque chicken!  To this day, the smell of aromatic smoke wafting in the air can set my mouth to water.  A warm, sumptuous, smoky fragrance magically conjures up memories of lively gatherings of family and neighbors in the backyard enjoying a meal al fresco on a summer’s evening. The photograph taken by my dad in 1965 captures just such a memory—with my best friend helping out, I’m bending over our impromptu barbeque pit attending to the chicken.

Our senses of smell and taste are intimately linked.  They are complementary forms of chemoreception.  Unlike our other senses, smell and taste respond to the presence of individual molecules. Through olfaction (the sense of smell) and gustation (the sense of taste), we recognize and interpret the world at the molecular level!

And what an immense world this is.  The number of known natural and synthetic molecules is huge and growing rapidly.  The Chemical Abstract Service of the American Chemical Society maintains the most authoritative collection of chemical substance information in the world.  Its registry of substances contains more than 82 million distinct organic and inorganic compounds, with an astonishing 15,000 new substances added to the registry database each day!  Although most are not edible or volatile enough to smell, these substances represent an immense pool of potential chemoreceptive possibilities.

Molecules possess different sizes and shapes.  They vibrate and flex at different frequencies.  Some are acidic, some are basic, and some are both (amphoteric).  Some are positively charged while others are negatively charged.  Some possess reactive groups that break apart or create links with other molecules.  All these things can contribute to the smells and taste we perceive.

Despite the huge variety of compounds present in food and drink, the number of tastes we can distinguish with our taste buds is very small – sweet, sour, salty, bitter, savory (umami), pungent and metallic.  These are the primary notes, the musical scale of taste.

Taste is perceived by taste buds found on the tongue, the back of the throat and the roof of the mouth.  These protruding structures contain specialized receptor cells on their surface.  Scientists have identified specific receptors for the primary tastes of sweet, bitter and savory.  Sourness (acidity) and saltiness are sensed in our taste buds using ion channels, pores in cell membranes that permit the flow of charged ions across the membrane (acids and salts yield charged ions when dissolved in water).

Bitter is our most sensitive taste category, which is very helpful for our survival because so many toxic compounds are bitter.  Many pharmaceutical medicines also fall in this category, which is why they make such bitter pills to swallow.

Given that our sense of taste is so limited in scope, what then imparts so many flavors to our food and drink?  How do the varieties of flavors we savor and crave arise?  What makes the delicious difference between sugar and maple syrup, vinegar and lemon juice, dark roast coffee and Earl Grey tea, between grape juice and Beaujolais, cottage cheese and blue cheese, between salsa and barbeque sauce?  The answer is our prolific, precocious sense of smell.

The vast array of gastronomical delights arises from the thousands of different smells we can distinguish.  Smell adds nuance and character to these fundamental taste categories.  Working together, our olfactory and gustatory perceptions define and create the array of flavors we enjoy at breakfast, lunch and dinner, and during reviving snacks between meals.

Odors are small, volatile molecules.  When we breathe in through our noses, airborne molecules come in contact with a layer of cells called the olfactory epithelium.  Covering roughly two square inches in our nasal cavity, the olfactory epithelium holds thousands of olfactory nerves.  Mucus, which acts as a solvent for odor molecules, covers these cells.  To keep from getting saturated with odor molecules, the body renews this mucus every 10 minutes or so.  Odor molecules dissolved in the mucus are available to bind (like a key in a lock) to specific receptor molecules found at the end of each olfactory neuron.  The neuron recognizes this binding and fires a message to the olfactory bulb situated at the top of the nasal cavity and at the base of the brain.

The olfactory system differs from our other senses in remarkable ways.  It is the only human sense that is connected directly to the forebrain, that part of the brain responsible for receiving and processing sensory information, perceiving, thinking and understanding.  All other sense data is transmitted first through the thalamus—the structure perched on top of the brainstem that acts like a switchboard for information being sent to the cerebral cortex.  Unlike most neurons in our bodies, olfactory receptor neurons can regenerate and do so frequently.  Interestingly, it is primarily during exhalation that the olfaction contribution to flavor occurs, in contrast to non-food smells which occur during inhalation.

Adding to the variety and perception of flavor afforded by our senses of smell and taste is chemesthesis.  Chemesthetic sensations arise in the eye, nose, mouth and throat when chemical compounds stimulate nerves involved in sensing touch, inflammation, pain or temperature.  Examples are the burning, sometimes numbing sensation in our lips caused by capsaicin present in chili pepper, the cool feeling in our throats by menthol in cough drops, the tingling of the nose and mouth by carbon dioxide present in effervescent drinks, and the reactive, tear-inducing amino acid sulfoxides found in onions.

Flavor is highly influenced by temperature (fond of lukewarm root beer?) and texture (soggy potato chips anyone?).  Rounding out our experience of flavor are the color, appearance and presentation of food, our appetite, our palate, the time of day, the season of the year, the occasion, and our personal associations of a particular food with pleasant or unpleasant experiences.  Years ago I once became nauseated after eating a certain candy, and the mere thought of that confection still makes me feel queasy.

Smell and taste convey awareness of the world at the exquisitely fine resolution of molecules.  When compared to a symphony, taste can be thought of as providing the unadorned notes of the melody, with smell providing the orchestration.  Working in harmony, they bring flavor into our lives.