Article from TeaGeek.net. Used with permission.
Caffeine is perhaps the most well-known chemical component of tea. Whether it's reviled or revered, caffeine inspires more strong opinions and beliefs than any other tea component. But how much caffeine is there, really, in your cup? That's no easy question to answer.
Looking at the dry leaf, the variation in a black tea can be immense. By dry weight, caffeine can make up as much as 6% of the leaf, to as little as below 1%.
Many factors influence caffeine levels in any given cup of tea. First, the varietal of the tea plant makes a difference--''Camellia sinensis'' var. ''assamica'' (the "India varietal") produces as much as 33% more caffeine than ''Camellia sinensis'' var. ''sinensis'' (the "China varietal"). This difference may explain the perception and claims that green tea (usually made from China varietals) has lower caffeine than black tea (usually made from India varietals).
Other differences include which leaves are picked--generally, younger leaves contain more caffeine than older leaves; the fourth has roughly a third less caffeine than the youngest first leaf. Oxidation levels may influence the level of caffeine, but oxidation isn't something that is easily measured scientifically, making it difficult to determine variations caused by that factor alone. Also, plants that were propagated by seed produce more caffeine that plants propagated by cutting or other cloning methods. Plants fed with nitrogen fertilizer produce more caffeine than those that aren't, or that are grown in low-nitrogen soil.
Another variation comes based on when the tea was picked. For example, in Kenya, summer-picked teas have lower levels than winter-picked. Sometimes the seasonal difference is more than 50%, depending on speed of plant growth. And after the leaves are picked, the temperature and length of time they are withered before final processing slightly affects caffeine levels, as does other aspects of processing like drying. And whole-leaf teas seem to release different amounts than broken leaf like you might find in a tea bag.
In addition, what technique was used to brew the tea and what temperature of water is used affect how much caffeine makes it from the leaf into the cup. You sometimes hear that tea can be decaffeinated by giving the leaves a quick brew which you discard, then brew again. However, research has shown that only about two-thirds of the caffeine comes out in the first five minutes of brewing. This means that a second brew will generally be lower in caffeine than the first, but it would be incorrect to call it "decaffeinated," especially if the first brew is shorter than five minutes.
The upshot of all of this is actually rather simple--it's almost impossible to know accurately how much caffeine is in your cup. And if you see statistics about caffeine levels that don't tell you the varietal, propagation method, which leaves were picked, how long they were withered, harvesting season, and leaf grade, then the results need to be taken with the proverbial grain of salt.
The good news, however, is that the highest levels measured in most teas tend to be about half that of coffee--less than 100mg per cup of tea. Luckily, the negative effects of caffeine tend to start showing up at the 300-500mg per day level, and then generally among sensitive groups such as pregnant women with risk factors for miscarriage. For most people, then, two or three cups of tea a day is seen as a safe level for all but the most sensitive, assuming no other sources of caffeine like cola or coffee. (Of course, everyone is different, so consult health care professionals for recommendations specific to your situation and health conditions.)
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