The Science of Flavor, Part 1: The Physiology of Taste and Smell

When we are seeking delicious food, we are seeking more than flavor. Besides our hands, the brain dedicates more nerve space to the face, especially the mouth and nose, than any other part of the body. Beyond the obvious seat of four out of five of our senses, the head is also the first part of our bodies to interact with the world at large. Whether we are looking for savory or sweet foods to stimulate our desires, we are actually seeking complex stimulation of a complicated evolutionary developed sensory system. It shouldn’t be a surprise that these senses are so deeply wired into our being.  It is likely that our sensory abilities started with the very first organic cell consuming the first bit of food.

But we are seeking more than that now.  We have become refined in our desires, and have become savvy in editing our world to create the experiences that bring us the most sensual joy.  A chef friend of mine once said that the act of dining is the only act we make in which we engage all five senses: seeing the setting and then taking in the decorated plates in from of us, we are hearing our surroundings, the soft music, the buzz of voices; we are smelling the myriad smalls from our own table and others; feeling the soft tablecloth or cold metal ware in our hands, and of course tasting the food in front of us.

Seeing, hearing, feeling, smelling… these are familiar to any of us. But what is tasting?  Most people confuse the term “to taste,” which has come to mean examining the flavor of the food, the sum total of what makes something phenomenal or just so-so. But scientifically and sensually, taste is a very specific set of chemical sensors isolated to the mouth parts of our body.  Flavor on the other hand, which is what many people refer to when the mention taste, is much more involved than that.

Flavor has to do with taste, one of the five physical senses that is specifically isolated to the mouth in combination with aroma. The tactile sense of the tongue is significantly entirely different from smell/aroma, which is the multifaceted and incredibly complicated realm of the nasal passages and olfactory system.  It is here that thousands and thousands of aromatic molecules are parsed, read, and turned into electrical signals and sent to the brain for interpretation. Curiously, scents are wired more directly to the brain than any other sense, triggering emotions and memories often as vivid as deja vu.

“[This] is likely due to brain anatomy. Incoming smells are first processed by the olfactory bulb, which starts inside the nose and runs along the bottom of the brain. The olfactory bulb has direct connections to two brain areas that are strongly implicated in emotion and memory:  the amygdala and hippocampus. Interestingly, visual, auditory (sound), and tactile (touch) information do not pass through these brain areas. This may be why olfaction, more than any other sense, is so successful at triggering emotions and memories.” [https://www.psychologytoday.com/us/blog/brain-babble/201501/smells-ring-bells-how-smell-triggers-memories-and-emotions]

There is even some discussion now that the previous “lock-and-key” understanding of aromatic molecules and our sensory systems is still rather Newtonian--2D--in our equations, but could even be due to a far more complicated method of quantum field differences across the surface of otherwise identical molecules. Note this report in BBC Science (this is the field of Quantum Biology).

Since new foods--and hence new flavors--are being found, bred and grown in our ever-shrinking world, the combinations of these aromatics due to the nature of fragrance and flavor are constantly toyed with and explored by both scientists and chefs alike. It is hard to imagine a more tactile and yet deeply personal exploration of almost anything on earth beyond the infinite combinations of flavors and the experiences of gustatory pleasures.

As a result of the assumed “simplicity” of taste versus the incredibly convoluted sciences dedicated to identifying and replicating aroma, the complexities of the role of our tongue/mouth are still unfolding in both labs and consumer tasting panels everyday. As suggested above, Flavor is the combination of taste and aroma, and is thusly the subject of every kitchen and campfire from Antartica to Siberia, from prehistory into the distant future, and everywhere (and everywhen) in between.  The experience of flavor is subjective in every way: With the connections of memories to aromas and the combination of tastes, physical sensations of the mouth, the experience of flavor is extremely personal.  The experiences of taste, however, are molecular.  Let’s take a look at how taste works.

Long before we had institutions to tell us what to avoid, our tongues have long been on the job, letting us know that food we are tasting is rich in certain beneficial attributes.  The various tastes of the tongue are actually sophisticated chemical receptors, capable of detailed analysis and quick reporting through signaling to the brain.

The mouth is the first defense against infiltration of bad food, bad chemicals, and other poisons into our body. It is also the first sensory organ to sense incoming material for nutrition rich foods, as well as the first place that saliva secretion actually begins to enzymatically break down our foods.  Our digestive organs, starting with our mouths, are the most integrated of any of our systems with the earth outside. As a result, they are incredibly complex in their immunity, their symbiotic relationships with bacteria, viruses and fungi, as well as with the nervous system. And all of this starts with the mouth.

It is quite necessary that what we put into our mouths be thoroughly analyzed by the tongue before we swallow it and assimilate into our bodies.  Looking at humans strictly as organisms concerned with survival, the nuances of taste is meant to be the first defense against the opposite: untimely mortality.  In fact, both smell and taste are products of billions of years of evolution asking one question for each sense, since the first motile cell. For smell the question is “do I move towards this or move away from this?” and for taste: “should I assimilate this or spit it out?” Though simplistic, one could say that life on Earth has survived to become us by moving towards and eating the right things and spitting out and running away from the wrong things.

Let’s take a look at the 5 tastes we know about (so far):

1. Sweet is from sugars --a much more rare treat than say starchy carbohydrates our evolving ancestors would gorge upon, such as ripe fruit harvests--have the benefit of quickly available, thus storable energy in the form of fats.  Overwhelming the digestive system with too many simple sugars to process forces the body to store the sugars as fat. In our current age, this twist of evolution leads to obesisty, and is the seeming cause of almost 20% of all deaths worldwide. In ancient times, a vast feast of fruit over the course of a week might mean survival in the barren winter, as energy stored is used efficiently by the body.
   
2. Salty is from salt-- NaCl, or sodium chloride-- an extremely important nutrient--and the only rock we eat.  Salt has been a highly prized and essential nutrient since before recorded history, was responsible for the creation of the first mega-barons of industry in the Mediterranean thousands of years ago was worth its weight in gold until as recently as 150 years ago. Our need for salt is so thoroughly integrated into our lives that you probably did not realize the words salary, saute, sauce, and sausage are all rooted in the Latin word for salt, Sal. All animals die without ample sodium in their diet.  “In humans, sodium plays a key role in the regulation of blood volume, blood pressure, osmotic balance and maintain a constant pH. Sodium is also essential for the functioning of neurons and osmoregulation between cells and the extracellular fluid. In humans, this process is regulated by the Na+/K+ pump.”  https://www.mineravita.com/eng/sodium_role.php
   
3. Sour is the detection of the tongue of free H+ ions, or acids.  This sour taste was probably not favorable in the earlier days of evolutionary history, as it is an indicator of unripe fruits, or perhaps to tell the difference between desirable or non-desirable mushrooms or plants before ingesting.  Later in our culinary explorations as humans developed fermentation in its many forms, our tongue was then used to determine whether something was fermented and to what degree, which has led in these modern times to the advance degrees in beer, wine and cheesemaking, et al.
   
4. Bitter is an interesting flavor, if only because the evolution of taste has taken bitter as the warning against danger to something in which we artistically revel.  Bitter is the indication that the item contains a concentration of chemicals that are dangerous or poisonous to us, and needs to be avoided. If evolution taught us anything in the past, it is that bitter is bad.  Bittering agents are a form of chemical defense in plants and animals, and a warning that the mouth can detect and reject before the damage is done to the liver (As above, tasting a mushroom in the field to determine edibility may depend on its bitterness. Not all poisonous things are bitter, of course. And not all bitter things are poisonous.  The same entrepreneurial spirit that drove our sour sensors to develop some of the finest foods in the world, so did bitter begin to seep into our culinary and medicinal worlds.  The Chinese were the most detailed docu-mentors of medicines in our early historical periods, and bitter compounds were used for any number of curative properties.  Nowadays, bitter foods that we tend to revere are coffee, chocolate, grilled or charred food items, and certain spices. And of course bitters are used extensively in the study of Mixology and Bartending, lending sophistication to flavor combinations that might otherwise be lost in a bowl of soup. If innovators have taught us anything, it is that bitter is good!
   
   
5. Umami Finally, and perhaps most interesting to this current work is the taste of umami, the fifth taste. Umami is a Japanese word that essentially means “pleasant savory taste.”  The word was coined by a Japanese food scientist Kikinae Ikeda to describe the intense and mouth coating  flavors of Dashi, a broth that forms the basis of Japanese cooking made from varying combinations of seaweed, dried bonita fish flakes, and dried shiitake mushrooms.  Bonita fish flakes are dried, smoked type of tuna, that are strained out of the broth before use and only for flavor. It is now universally accepted that Umami is indeed a fifth taste, a different experience of the taste sensors in the tongue that allow us to identify food items rich in proteins and the necessary ingredients and building blocks for genetic material.  Find much more on this fifth taste on the next post, where I will greatly expand on what makes umami tick.
   
   

The tongue is a much more complicated sensory organ than we have given it credit for being.  It wasn’t long ago that we used to believe (and were subsequently taught in schools) that certain portions of the tongue had higher concentrations of four taste receptors: Sweet on the tip of the tongue, Salty in the middle, Sour on the edges of the tongue toward the cheeks, and then Bitter, far in the back of the tongue toward the throat.  It is now understood that we have taste receptors for these four tastes on all parts of the tongue, even the soft palate, the epiglottis, the cheeks and upper esophagus, and that these are not the only four tastes at all, but perhaps the most important taste of Umami.

Taste, of course, is hard to isolate when combining foods and flavors.  The flavor of food starts with the aromas hitting our nasal passage before the food ever touches the tongue, and our experience of the food is much more than its simple make-up of the five tastes outlined above.  Beyond aroma, the mouthfeel of foods can affect the way the tongue interacts with the molecules of flavor. Mouthfeel is affected by starches, mucilaginous ingredients in plants and mushrooms, the temperature of the food eaten, from very cold to very hot, the crunchiness of an item, the crumb, the chewy-ness or suppleness, etc. etc.  In addition to the physical feeling from the texture of the food, we also find chemical reagents, such as capsicum, which activates the same heat-sensitive cells as literally being burned, or the very odd molecule in szechuan peppercorns, used liberally in Szechuan cooking.  This strange molecule activates the touch sensors of the tongue, and stimulates them to fire to the brain the same signal as being vibrated softly at 5o Hz.

The science of taste (for now) is pretty basic.  The science of aroma is much more convoluted. The science of flavor, therefore, is even more complicated again, combining the various smells and taste sensations.  But truly our experiences of eating is much more than a simple combination of our senses, but a deeply personal experience of memories, new experiences, aromas, tastes, sensations, and textures.  Science can tell us how to combine various aromas and textures to create the desired effect, but it cannot tell us what we subjectively like.  That, dear reader, is up to you

Zachary Hunter

Zachary Hunter is a lifelong devotee to flavor, a professionally trained chef who has been obsessed with mushrooms and uncovering the unknown with regards to edible mushroom chemistry and physiology. He is a member of the NAMA's (North American Mycological Association) Culinary Committee. He lives in Oaxaca, Mexico with his wife Kim, where the run MycoAdventures in the mountains of Oaxaca and beyond.