In 1901 German researcher D.P. Hanig published his Ph.D. thesis, a work that ultimately would have a dramatic impact on the direction of Western investigation of taste sensation. His work included a graph of taste sensitivity on the base, tip, and right and left edges of the tongue. The results appeared as orderly curves that rose and fell to indicate differences in response to sweet, bitter, and sour stimuli. In this illustration, the tip of the tongue was sensitive to sweet substances while the base was responsive to bitter flavors. The left and right edges were stimulated by sour foods. Subsequent generations of scientists reworked and summarized this data in different ways, even adding the fourth taste sensation for salt.
The tongue taste map, an expanded illustration of Hanig's findings, visually depicts distinct zones of sensitivity for each taste. Much of our common knowledge about taste can be traced to Hanig's thesis and to this familiar illustration, which is still widely used in textbooks today, but unfortunately the general understanding is wrong. When creating the original graph, Hanig neglected to include a scale to indicate differences in measurable sensitivity, so the results were simply qualitative in value. The tongue taste map depicts areas of absolute difference in taste sensitivity where only relative variations were found.
This misinterpretation ultimately was refuted in 1974 when researchers used electrophysical tests to demonstrate that the taste buds on the tongue and soft palate are sensitive to each of the tastes. Although they noticed some variation in sensitivity within each area, the scientists concluded that these differences were of no practical significance; taste sensation is not localized in highly specific areas of the tongue. Contemporary research has shown that the individual receptor cells responsible for transmitting signals to the brain may have a higher level of sensitivity for a particular stimulus, but they are non-specific and capable of reacting to the presence of all of the basic tastes.
Interestingly, the sensation of smell is a key component of taste. Volatile chemicals pass through the nasal passages and react with receptors to create the sensation of smell. Scientists estimate that nearly 75% of taste is actually the detection of the aroma associated with foods. The quantification of the number of smells varies by source, but most researchers recognize the human ability to distinguish between thousands of scents. While it is difficult to separate taste and smell when they occur together, the absence of smell has a profound effect on flavor. It is this powerful combination that creates tea's unique sensory appeal. For anyone who has ever pondered the allure of a favorite brew, findings from current studies may hold the answer. Researchers across the globe have long recognized the existence of four basic taste sensations - sweet, sour, bitter, and salty. Yet, a new taste, long known to the Eastern palate but only recently accepted in the West, is partly responsible for tea's popularity.
The scientific interest in this taste dates back to 1909 when Kikunae Ikeda, a researcher at Tokyo Imperial University, began new experiments with the goal of understanding why certain foods seemed to have a unique, deeply satisfying taste. From kombu seaweed, he isolated glutamate (glutamic acid) as the chemical compound responsible for this previously unexplainable flavor, which he called umami, from umai, the Japanese word for tasty. While there is no direct translation to English, umami, often referred to as the "fifth" taste, has been given a variety of descriptors including hearty, savory, brothy, and mouthfullness. It should be noted that umami also retains a broader, more subjective cultural meaning and can be used in reference to anything with a pleasing taste.
Glutamic acid is the most frequently occurring amino acid in nature, and it is a constituent of nearly all animal and vegetable proteins, particularly those in tea leaves. When heated, fermented, or allowed to mature, these proteins break down into their component parts - amino acids. When consumed in this "free" form, glutamate imparts a pleasant, flavorful taste.
While generally recognized for its flavor enhancing quality, glutamic acid historically had been rejected as a basic taste because scientists were unable to identify a corresponding taste reception mechanism. Recently however, two major breakthroughs in the identification of an umami taste receptor finalized its status as the fifth basic taste. In 2000 researchers from the University of Miami announced their identification of an umami taste receptor, which they named taste-mGluR4. Last year, a separate group of U.S. scientists identified another amino acid receptor, T1R1-3, which is responsive to glutamate and most of the other naturally occurring amino acids.
This latest finding is also of interest to tea drinkers because of another amino acid called L-theanine, which is unique to the tea plant Camellia sinensis; L-theanine accounts for 60-70% of the total amount of amino acids in tea leaves. Tea's satisfying taste is directly related to the umami flavor and the presence of L-theanine.
Through understanding the science of taste, one gains a greater appreciation for the remarkable flavors that can be found in the world of tea. Centuries of scientific investigation have revealed the basis for what tea lovers have known for millennia.
Collings, V.B. "Human Taste Response as a Function of Location of Stimulation on the Tongue and Soft Palate." Percep. Psychophys. Vol. 16. p. 169-174.
Lindeman, Bernd. "A Taste for Umami." Nature Neuroscience. Vol. 3. Issue 2. Feb. 2000. p. 99-100.
Nelson, Greg, et al. "An Amino-Acid Taste Receptor." Nature. Vol. 416. Mar. 14, 2002. p. 199-202.
Ninomiya, Kumiko. "Umami: A Universal Taste." Food Reviews International. Vol. 18. Issue 1. May 3, 2002. p. 23-38.
Wilson, K.C. and M.N. Clifford. Tea: Cultivation to Consumption. New York: Chapman & Hall, 1992.
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