Taste is a chemical sense, and a part of flavor. Taste is the chemical analysis of molecules in a solution. Smell is the chemical analysis of airborne molecules. Flavor is a combination of taste, smell and other factors, including temperature and texture. Together they make eating a fully immersive multimedia experience.
Researching taste and smell is not easy. First, it is hard to control the amount of molecules reaching smell receptors. It is hard to quantify the presence or absence of a smell. Second, things change. Taste buds, for example, die off every 4 – 11 days. You can replicate previous finds on new cells. There is no way to see if taste buds care reliable after two weeks. It’s a whole new tongue by then, in research terms. Third, both taste and smell adapt quickly. If you have tried hunting for a smell , you know you often have leave and come back into the same environment several times. Smells seem to both linger and magically disappear.
Gustation (Taste)
Taste detects chemicals dissolved in a solution. Fluids are ready for analysis. Anything more solid is broken up by chewing, and dissolved by saliva. Saliva also combines tastes together which creates a more complex pattern.
Aristotle proposed there are two basic tastes: sweet and bitter. Current models suggest five basic tastes: sweet, sour, bitter, salty and savory.
Sweet
We have a survival need for energy in general, and carbohydrates in particular. The brain runs on glucose. Neurons are little internal combustion engines. Engines need fuel, heat and oxygen to work. Sugar is the fuel, heat comes from an enzyme (ATP), and oxygen comes from the blood. To identify food high in energy, we need a carbohydrate detector.
In the internal world of fire building, carbs are kindling, fats are branches, and proteins are logs. Carbs give a quick release of energy. Fats are deconstructed into sugars, making them useful for releasing energy over a longer period of time. Proteins produce a steady stream on energy over a much longer time frame.
Sweetness is a good indicator of high energy food. Except for some artificial sweeteners, which fool the system, sweet equals brain food. Carbohydrate provide immediate, direct energy. They can also be stored in glycine, which when mixed with water produces glucose.
Milk contain the carbohydrates lactose. It is not very sweet. Table sugar (sucrose) is 5x sweeter. That’s why, if you need fast energy, you reach for sugar, not milk.
Glucose is about 75% as sweet as sugar, twice as sweet as glycine, and half as sweet as fructose. Aspartame and saccharine are non-nutritive artificial sweeteners which are several hundred times sweeter than sugar.
Bitter
People are very sensitive to the taste of bitter. It is important for survival because most poisons are bitter. We are familiar with bitter is medications, such as quinine (which a treatment for malaria). By diluting, cooking or brewing bitter substances, we have some to expect and appreciate a touch of bitterness is coffee, cocoa, olives, and citrus peel. It is also a component in cheese, alcohol and tonic water.
Sour
Sour is an acid detector. It helps us to maintain a proper pH and electrolytic balance. We are rather selective in which acids, and how much, we like. We like a little bit of the citric acid of lemons, but tend to enjoy the acetic acid of vinegar less. We may enjoy vinegar in pork adobo and put it on our chips. But lemon cake is still more popular than vinegar cake.
We seek out the malic acid of apricots, blueberries, cherries and grapes. We look for the tartaric acid in wine. We tolerate the fumaric acid used to extend the self life of meat and baked goods. But we avoid drinking hydrochloric acid. We prefer sour as a balance to sweet or as part of a larger flavor profile.
Salty
Sodium detection is important to our survival, so it is not surprising that we detect it. Depolarization of neurons based on an influx of sodium. When voltage regulated gates open, sodium rushes in, which causes the voltage in the adjacent region of the axon to open its gates. This cascade of sodium flows is called a neural impulse. Having the correct amount of sodium is crucial for your brain to work.
Savory
The newest item added to the list of basic tastes is savory, also called umani (Japanese for good flavor). It is a glutamate detector. Glutamate is our body’s primary neurotransmitter, which is produced in the brain. Umami is the taste underlying taste of soy sauce, fish sauce and other fermented food.
Other Factors
Temperature. I love hot pizza, warm pizza and cold pizza. The same can’t be said for onion rings or spring rolls. Americans are particularly keen to drink soft drink with ice. I find it harder to appreciate the taste, and prefer my drinks without ice; chilled maybe. Some wines are chilled, others consumed at room temperature, and some served warm. Temperature changes the taste of foods.
Texture. Crisp, crunchy, tender and chewy all describe texture. There is no agreed upon list of textures. Obviously, watery soup and creamy pudding are similar but not the same. They are different from firm or crispy and crunchy. But there is also hard, rough, soft and slimy. The list tends to be adjectives that describe our experience, rather than actual perceived sensations.
Pain. We don’t sense pain directly. It is usually a combination of temperature and spicy. If something is too hot and we burn the tongue, that spot won’t respond to any sensation; no taste is reported. Pain overrides everything else.
Spicy. In an indirect fashion, spicy is a response to capsaicin oil. Capsaicin stimulates pain and temperature sensations. These are not taste receptors but provide allied information. Water doesn’t easily wash away the oil, so milk is a much better countermeasure; it coats and neutralizes the oil. In Chinese food, spicy is desirable sensation called là. Some people are particularly fond of peppers.
Cool. This sensation is not related to testosterone. It is a subjective experience, often tied with other flavors, such as cool mint. Not a direct sensation. experience.
Numb. Allergic reactions can cause the tip of the tongue to go numb. In Chinese food, má is the desired effect of tingling and numbness of the lips.
Astringent. Probably not a basic taste but a common experience. Astringent foods make the mouth pucker or feeling dry. The former in found in persimmons. Green tea causes less puckering but more dryness.
Metallic. Detecting metallic in food is almost always unpleasant. It is a sign of contamination. Acid reflux can also leave a metallic taste.
Fat. There is some evidence that fat detection should be added to the list of basic tastes. Recent research has identified receptors for it, and the survival value is certainly high enough to warrant one. Fat provides a consistent release of energy.
Starch. In practice, people act as if there are starch taste receptors. We seem to crave cabs, seek them out, and use them to fuel our activities. More complex than sugar and more easily converted than fats, carbohydrates are a good source of energy. It is certainly a good candidate for being a basic taste.
Calcium. In addition to sodium, our bodies need calcium. Some receptors on the tongue are sensitive to calcium but it has not been established as a basic taste.
Hardy. Experientially, people know what a hardy meal is like. It is substantial, and feels long lasting. If is probably not a basic taste but an additional marker.
Tongue
The tongue only extends outward an inch or two but it fills the mouth and continues down the throat. It is anchored at the hyoid bone midway down your neck, and at the base of the mouth. It is a strong, flexible muscular structure.
Used in speech, manipulation of food, and the detection of chemicals in solution, the tongue looks pink, and moist. It is covered with a layer of mucus.
Small rounded mounds spot the tongue, protruding from the surface. These papillae are unevenly distributed across the tongue in four regions.
The filiform are cone-shaped papillae spread out over the entire surface of the tongue. They are thread-like and contain no taste buds. They give your tongue texture, which helps in speech, and in the manipulation of food.
In the back of the tongue, there is a flat mound-shaped region. It is covered with circumvallate papillae, which are the largest type. Overall these region contains 10,000 taste buds., about 50% of your taste buds.
The foliate is a series of folds on the back and sides of the tongue. These 20 or so foliate papillae contain a total of several hundred taste buds. It holds about 25% of your taste buds.
The last 25% of your taste buds are are located on the sides and tip of the tongue. These fungiform papillae are mushroom-shaped, hence the name.
Each taste bud has several receptor cells. All tastes can be sensed across the tongue but different regions have different levels of sensitivity.
The first receptors to be identified detect bitterness. There are two dozen or so different types but, 9n general, bitterness is detected by type 2 taste receptors, such as TAS2Rs. These are coupled with a g-protein. When stimulated, an ion channel opens, a g-protein changes its shape and sticks its foot in the gap, keeping the channel open, and allowing a cascade of reactions to occur. Bitterness is most tasted in the back of the tongue.
Sweetness receptors are also g-protein regulated. These are type 1 receptors. They are sensitive to many types of natural sugars, and somewhat sensitive to some artificial sweaters. The difference in sensitivity may explain why artificial sweeteners don’t quite taste the same as the real thing.
One of my favorite flavors is CocaCola with cane sugar presented in a glass bottle. I don’t like the corn syrup or aluminum can versions. Just doesn’t taste right.
Umami is a glutamate detector. Like sweetness, it is a tip of the tongue (and sides) phenomenon. The TAS1R1+3 receptors are in the fungiform papillae.
Sensory coding is a whole tongue activity. Same receptors respond with different amounts of activity, there is some regionality. But taste is the combination of all the sensors. It is the pattern of neural activity across many receptors.
Yet we tend to use our tongues to increase whatever regionality there is. When we are unsure of what we are about to eat, we tend to use the tip of the tongue to test it. In practical terms it means we sense sweet and umami more than salty (sides of tongue) and bitter (back of tongue).
Another factor is the variability between people. Some people, particularly those with underlying health problems, have difficulty tasting food. For them, everything tastes the same, or doesn’t really taste like anything.
Others are highly sensitive to taste. The mechanisms are not well understood. These super-tasters may have an increased number of fungiform papilla, making it easier to detect tastes. They require less fat and sugar to get the same effects. But they require more salt.
Taste is more than a tongue experience. There are receptors in the mouth and throat too. This is clear when you experience an aftertaste that differs from the original taste. Many medicines and many artificial sweeteners produce a second taste after they are swallowed.
Taste Disorders
Age and genetics both influence taste. People lose sensitivity as we age. There is less saliva, and less papillae. These factors can also appear in genetic variability. People vary greatly.
But most taste disorders are caused by environmental factors.
Ageusia is the complete loss of taste. It is not generally genetic but is the result of head trauma, infection or chemicals, including medications. Similarly, hypogeusia, the reduced sense of taste, is typically caused by infection, liver damage, and traumatic brain injury.
Some viruses cause partial or total loss of taste. About 50% of COVID-19 patients experienced some loss of taste. Dysgeusia is the distortion of taste sensation. In this condition, usually caused by meditation or vitamin deficiency, everything is tinged with a bitter or metallic taste. Lesions and adrenal diseases can produce a craving for salt or sour tastes. This heightened sense of taste is called hypergeusia.
Allergies and sensitivities also impact taste. Allergic reactions are usually too slow to affect taste directly. But being allergic to one substance can cause the taste of other things to be off. Similarly, sensitivity, which doesn’t activate the immune system, doesn’t produce an allergic reaction but does make some things less desirable. MSG sensitivity, sometimes Chinese Food Syndrome, results in headaches and skin flushing but they are delayed symptoms.
Similarly, habitual behavior can impact behavior. Drugs, alcohol and general health issues can lower sensations. Smoking, in particular, dulls taste perception. Tars make other tastes harder to identify. And nicotine damages the blood supply to the mouth, killing taste buds. Chefs may smoke for social reasons or to reduce stress but they lower their ability to taste the food they are making.
Brain
Getting to the brain isn’t exactly a straight path. There is no single nerve, like the optic nerve, that carries all of the information. There are three nerves associated with taste.
Sensations from the back of the tongue are carried by the glossopharyngeal nerve (cranial nerve IX). Sensations from the epiglottis region of the throat are carried by the vagus nerve (cranial nerve X). They follow similar but slightly different paths from the tongue to the nucleus solitarius, a group of nuclei located in the medulla oblongata.
The sensations from the rest of the tongue are carried by the facial nerve (cranial nerve VII). It has its own route to the brainstem but ultimately reaches the same region.
The medulla oblongata’s nucleus solitarius is a relay center for taste information. It is also the region which regulates the gag and chocking reflexes.
From the nucleus solitarius taste information is sent to the frontal lobes, stopping briefly at the hypothalamus along the way. When the neural patterns of taste sensations reach the frontal lobe, specific tastes are identified, and mixed with smells to produce flavors. Flavor information is -aired with long term memories of places, events and people. The ambiance and location data is added, and the whole experience is evaluated and enjoyed.
The brain also provides long term influences. Learning and culture do impact we find acceptable. All cultures find some foods to be disgusting but they varying greatly on that what item is. There is no agreement on pleasant or disgusting. Parents sometimes rely 0n a theory of acquired taste, hoping their offspring will change their views. Taste preferences do change over a lifetime but are not readily predictable.
Smell
Want to jump ahead?
- What Is Perception? What
- Perceptual Efficiency
- Vision
- Taste
- Smell
- Touch
- Receptor
- Pressure
- Haptic Perception
- Temperature
- Pain
- Itch
- Hearing
- Ear
- Cochlea
- Pathway
- Temporal Lobe
- Vestibular
- Visceral
- Proprioception
- Time
Photo by Nick Fewings on Unsplash
