Tea tasting is the process in which a trained taster determines the quality of a particular tea. Most of us rarely think about the science of tasting tea moreover, the chemical process happening within our body as our brain decides whether or not we like or dislike a food or beverage. Relatively unknown to our conscious mind, gustation along with olfaction creates the human sense of taste, allowing us to perceive different flavors.
Our understanding of the flavor of tea requires interactions between several of our sensory systems. Taste and smell are the principal systems for distinguishing flavors. However, feel and temperature input from the oral mucosa also contributes to our perception of a tea’s quality. The gustatory system, including the mouth, tongue, and taste buds allows us to convert chemical molecules into specific taste sensations.
The Five Tastes
Until recently, Western scientists believed that humans had just four main tastes: bitter, salty, sweet, and sour. Umami, which is the Japanese word for “savory,” was added to this list of basic tastes. It is thought that umami receptors act similarly to bitter and sweet receptors, but very little is known about their actual function. Scientists do know that umami detects glutamates that are common in meats, cheese, and other protein-heavy foods. [i]
All the varieties of flavor we experience are a combination of some or all of these tastes. There is one type of taste receptor for each flavor, and each type of taste stimulus is converted by a different mechanism. Contrary to popular belief, every part of the tongue is sensitive to all five tastes. The once common ‘tongue map’ depicting specific regions for each taste was based on a misunderstanding in the early 1900s and is wrong.[ii]
The tiny pink bumps on the tongue are the papillae that conceal many of our taste buds. These are the gateway to detecting the sweetness of a cake, the saltiness of a potato crisp, the meatiness of a steak, the bitterness of beer, and the sourness of a lemon. The four types of papillae that speckle the tongue give it a rough surface that helps move the food around as we chew. Each taste bud is packed with taste cells, which are capped with sensors for the five basic tastes.[iii]
Of the five basic tastes, bitterness is the most sensitive and many perceive it as unpleasant, sharp, or disagreeable, but it is sometimes desirable and intentionally added via various bittering agents. There are several classes of bitter compounds that vary in chemical makeup. The human body has evolved a particularly sophisticated sense for bitter substances and can distinguish between the many radically different compounds that produce a bitter response. Evolutionary psychologists believe this to be a result of the role of bitterness in human survival: some bitter-tasting compounds can be hazardous to our health, so we learned to recognize and avoid bitter substances in general.
Like bitter tastes, sweet taste a similar specific mechanism called GPCR. Natural sweeteners such as saccharides activate the GPCRs to release gustducin. Synthetic sweeteners such as saccharin activate a separate set of GPCRs, initiating a similar but different process of protein transitions.
Umami receptors act similarly to bitter and sweet receptors. Umami is an appetitive taste and is described as a savory or meaty. It has a mild but lasting aftertaste that is difficult to describe. It induces salivation and a sensation of furriness on the tongue, stimulating the throat, the roof and the back of the mouth. By itself, umami is not palatable, but it makes a great variety of foods pleasant especially in the presence of a matching aroma. Like other basic tastes, with the exception of sucrose, umami is pleasant only within a relatively narrow concentration range.
Sour tastes signal the presence of acidic compounds in substances. There are three different receptor proteins at work in a sour taste. The first is a simple ion channel, which allows hydrogen ions to flow directly into the cell. The second is a K+ channel which has H+ ions in order to block K+ ions from escaping the cell. The third allows sodium ions to flow down the concentration gradient into the cell. This involvement with sodium ions implies a relationship between salty and sour tastes receptors.
The salt receptor, NaCl, is arguable the simplest of all the receptors found in the mouth. An ion channel in the taste cell wall allows Na+ ions to enter the cell. This depolarizes the cell and floods it with ions, leading to a neurotransmitter release.
The receptors in our gustatory system only paint part of the flavor picture. The volatile molecules that create aromas constitute about 80% of the flavor experience, compared to 20% from the taste. The olfactory system located in the back of the nasal cavity allows us to sense complexities of tea as we inhale. This system is extremely discriminative and sensitive. Humans can distinguish between 1,000 to a predicted high of 4,000 aromas. All of these aromas can be classified into six major groups: floral, fruit, spicy, resin, burnt, and putrid.
During inhalation, aroma molecules from food pile up at the back of the mouth and throat as we breathe. As we swallow, air whips down the nasal cavity and into the lungs, creating a kind of air curtain separating the throat and the mouth. When we exhale, air sweeps into the back of the mouth and throat sending aroma compounds into the nose.[iv]
All of these aroma molecules are generated from carotenoids, lipids, glycosides and Maillards. Carotenoids include beta-carotene have been identified as precursors for flavors like woody, floral, flowery, cooked apple, sweet floral, and fruity. Lipids are fragrant and leafy volatiles initially identified from jasmine flowers, with high concentrations fund in oolong and some green teas. Glycosides are flavorless compounds in fresh tea leaves. During the manufacturing process, injured tealeaf tissues release enzymes into cell walls or cavities creating sweet-herbaceous and cherry flower-like aromas in green tea. Maillards are responsible for putrid, nutty, and roasted flavors in black teas and oolongs.
Surprisingly, aromas are sensed by the limbic system in the brain, which controls emotion, behavior, motivation and long term memory. The olfactory bulb has direct connections to the amygdala and hippocampus. This may be why olfaction, more than any other sense, is so successful at triggering emotions and memories, which can affect how we perceive tastes.[v]
Parasympathetic nervous system
Saliva also plays an important role in the perception of taste. It acts as a solvent, transports solutes to the taste receptors, and provides a buffer for acidic foods. ii Astringency is a sensory attribute that is described as a drying-out of saliva, creating a roughening and puckery sensation felt in the mouth. Foods that are often astringent include red wine, green and black teas, soy-based foods, and certain fruits. In these foods, astringency is caused by the polyphenolic compounds they contain. Polyphenolic compounds may reduce the risk of a variety of illnesses, including cancer and coronary heart disease.[vi]
The Joy and Science of Tasting Tea
Developing one’s tea palate requires relentless tasting and documenting of the many varieties of tea. Understanding the complexities within our own bodies combined with the numerous ways teas are produced and processed makes tea tasting a rewarding challenge which can take a lifetime to master.
[i] Boundless. “Gustation: Taste Buds and Taste.” Boundless Psychology. Boundless, 20 Aug. 2015. Retrieved 17 Dec. 2015 from https://www.boundless.com/psychology/textbooks/boundless-psychology-textbook/sensation-and-perception-5/sensory-processes-38/gustation-taste-buds-and-taste-163-12698/
[ii] Nature. “Gustatory system: The Finer Points of Taste.” Nature: International Weekly Journal of Science. Nature, 20 June 2012. Retrieved 20 Dec. 2015 from http://www.nature.com/nature/journal/v486/n7403_supp/full/486S2a.html
[iii] Neuroscience. “Chapter 9: Chemical Senses: Olfaction and Gustation.” Department of Neurobiology and Anatomy, University of Texas Medical School at Houston, 1997. Retrieved 20 Dec. 2015 from http://neuroscience.uth.tmc.edu/s2/chapter09.html
[iv] The Salt. “Mechanics Of Eating: Why You’ll Miss Flavor If You Scarf Your Food” NPR, 10 Nov. 2015. Retrieved 20 Dec. 2015 from http://www.npr.org/sections/thesalt/2015/11/10/455475805/mechanics-of-eating-why-youll-miss-flavor-if-you-scarf-your-food
[v] Psychology Today. “Smells Ring Bells: How Smell Triggers Memories and Emotions” 12 Jan. 2015. Retrieved 20 Dec. 2015 from https://www.psychologytoday.com/blog/brain-babble/201501/smells-ring-bells-how-smell-triggers-memories-and-emotions
[vi] National Center for Biotechnology Information. “Tea Polyphenols: Prevention of Cancer and Optimizing Health.” NCBI, 7 June 2000. “Retrieved 20 Dec. 2015 from http://www.ncbi.nlm.nih.gov/pubmed/10837321