Caffeine evolved as a natural chemical defense mechanism in some plants. By affecting the neurological functioning of insects, caffeine interferes with some behaviors, thereby lessening the threat posed by certain pests. Caffeine provides further protection to plants by acting as a potent antibiotic and antifungal. The accumulated residual caffeine presence in soil surrounding plants may also retard weed growth (Weinberg, p. 236-237).
Caffeine is largely responsible for the widespread adoption of tea. Even as early as 200 B.C., Chinese scholars hailed the drink for its mood-enhancing properties; the physician Hua Tuo observed simply that drinking tea "makes one think better" (Weinberg, p. 28). The physiological and psychological effects of caffeine consumption are well researched. The most common findings are that caffeine increases alertness and inhibits sleep. Scientists have also demonstrated that moderate doses of caffeine benefit athletic tasks requiring physical endurance. It enhances memory, reasoning abilities, and vigilance for those performing repetitive tasks. Caffeine also improves mood through its effect on dopamine production in the brain (Lieberman, p. 96-99).
Everyone knows that tea contains caffeine, but just how much is another topic altogether. Available information spans a broad range of claims including common fallacies like "green tea contains no caffeine" and "oolong tea has 2/3 the caffeine of black tea." Regretfully, many of these assertions are simple restatements of other unsubstantiated claims. Despite what many declare, there is no simple rule for predicting the quantity of caffeine in a particular tea; it is the product of the complex interaction of factors.
Regional and local agricultural conditions affect the amount of caffeine present in the tea plant. A clear illustration of this point comes from the production of Japanese gyokuro. The shading of the tea bushes preserves high levels of caffeine and amino acids in the young tea shoots (Wilson, p. 427). Soil composition and altitude are other important contributing factors.
Plant variety also plays a role. All true teas come from Camellia sinensis, but each of the two common varieties has a set of distinct characteristics. Camellia sinensis sinensis (the China variety) is a slow-growing shrub that produces small, narrow leaves; Camellia sinensis assamica (the Assam variety) is a quick-growing tree that yields large, broad leaves. Chinese and Japanese green teas are typically made from var. sinensis while black teas are generally made from var. assamica (Wilson, p. 413). Some sources suggest that leaves from the China varietal contain slightly less caffeine than those plucked from an Assam bush.
The leaf style of a particular tea also hints at caffeine concentration. In fact, this is perhaps one of the best indicators of caffeine content in dry leaves. In young buds and the first leaf, caffeine accounts for nearly 5% of the dry weight. At 3.5%, the second leaf contains just slightly less. In the upper and lower portions of the stem, the dry weight of caffeine accounts for only 2.5% and 1.4% respectively. Thus, a fine plucking of "two leaves and a bud" will likely contain more caffeine than large, souchong leaves.
While all of these factors play a role, brewing time further affects the amount of caffeine that infuses. Because caffeine is highly water soluble, nearly 80% will be extracted within the first 30 seconds of steeping. Beyond that time, the remaining caffeine infuses more slowly. The U.S. Department of Nutritional Services found that extending the brewing time for a black tea from 3 minutes to 5 minutes resulted in only an additional 4 mg of caffeine in the infused liquor.
So how much caffeine does a cup of tea contain? In one study, researchers used extended multiple infusions to fully extract caffeine from tea samples. From black, green, and Formosa oolong samples, they determined the caffeine content per gram to be 32.8 mg, 36.6 mg, and 23.8 mg respectively (Hicks, p. 325). A Department of Nutritional Services report lists the caffeine content of a cup of tea as 23-110 mg, 12-55 mg and 8-36 mg for black, oolong, and green teas, respectively. As caffeine-in-tea statistics are nearly always listed by style, many infer that there is a correlation between fermentation level and caffeine. This is false. Fermentation does not alter the amount of caffeine in tea leaves.
A generally accepted statistic is that a cup of tea contains 40-50 mg of caffeine, roughly half the content of a cup of coffee. Numbers higher or lower than this range can be accounted for by the list of factors previously mentioned.
No discussion of caffeine would be complete without mentioning decaffeinated teas. Current decaffeination processes use one of three organic solvents: methylene chloride, ethyl acetate, or carbon dioxide. Of these, teas decaffeinated by carbon dioxide are considered the most natural. Additionally, it has been found that green teas decaffeinated with this method contain approximately 95% of their original polyphenol content.
When pressurized, carbon dioxide (CO2) takes on the properties of a liquid and acts as a highly selective caffeine solvent. Moistened leaves are placed in a sealed chamber and exposed to the "super critical CO2" for several hours. The caffeine-bearing liquid is then poured off and filtered with water or activated charcoal. The process is repeated until the required amount of caffeine has been removed. Caffeine collected by the filtration process is a byproduct, marketed for use in drugs, caffeinated bottled beverages, etcetera.
While some imbibers choose to limit their caffeine intake, most enjoy the lift that a cup of tea provides. It is this link between tea and the effect of caffeine that has secured the beverage's place as a fixture in daily life throughout the world.
Hicks, Monique B., Y-H. Peggy Hsich, and Leonard N. Bell. "Tea Preparation and Its Influence on Methylxanthine Concentration." Food Research International. Vol. 29. Issue 3-4. April-May 1996. p. 325-330.
Lieberman, Harris R. "The Effects of Ginseng, Ephedrine, and Caffeine on Cognitive Performance, Mood and Energy." Nutrition Reviews. Vol. 59. No. 4. April 2001.
Weinberg, Bennett Alan and Bonnie K. Bealer. The World of Caffeine: The Science and Culture of the World's Most Popular Drug. New York: Routledge, 2001.
Wilson, K.C. and M.N. Clifford. Tea: Cultivation to Consumption. New York: Chapman & Hall, 1992.
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