Questions regarding the caffeine content of tea are bound to come up frequently and are worth addressing, as there are many factors to consider. Many people are aware that by dry weight tea contains more caffeine than coffee, but because you use much less to brew a cup and whole leave varieties aren't ground in the way coffee is, that cup contains a substantial amount less than a cup of coffee. Water temperature and the time the tea is brewed for also affect caffeine content of the final product. Because black teas tend to be brewed at temperatures around boiling (as coffee is) and for 3-5 minutes or more, the majority of the caffeine in the leaves is extracted (close to 70% at 5 minutes according to the 1996 paper 'Tea preparation and its influence on methylxanthine concentration,' from Food Research International Vol 29, Nos 3-4, pp. 325-330.) With that figure in mind, it's worth pointing out how incorrect the myth is, that you can easily and mostly decaffeinate any tea by steeping it for 30 seconds and pouring off that liquid before re-steeping to drink.
Moving on, green teas and the lighter oxidized oolongs will quickly become bitter it infused for too long at too high a temperature, so brewing them properly will necessarily impose certain limits on how much caffeine is extracted per cup. Without a controlled study measuring caffeine levels in a cup of tea brewed at 165 degrees F, 170, 185, 195, etc. for the same length of time, it's very hard to say exactly how much less caffeine is being extracted. Yet the fact that a properly brewed cup of green tea isn't overly bitter implies that brewing at a lower temperature extracts a much smaller portion of all of the bitter, water-soluble compounds in tea (caffeine included.)
While even things like water and altitude may play a part in influencing caffeine extraction, an important point to consider is that you can only extract what is there in the first place. As different teas have different levels of caffeine to begin with, this may be the most crucial aspect to focus on and one of the easiest to experiment with. The common stereotype is that the darker the tea, the more caffeine, but as we've seen above, there are usually other variables that aren't considered leading to this idea. The reality seems to be that the level of oxidation in tea (how green or black it is) has very little effect on the caffeine content. Rather some common things that increase the caffeine level of tea are the amount of buds and young leaves (as in a white tea*), if the tea is made from the Assamica variety (Indian, African and Ceylon black teas) rather than the sinensis variety (most greens, whites, oolongs and Chinese black teas) and if the tea is in a tropical, fast-growing climate. So for example, a white or black tea made solely from the buds of an Assamica variety tea and grown in a lush Sri Lankan rainforest might achieve the highest level of caffeine (as a % of weight) of any tea. At TeaLula we carry a white tea that exactly fits this description called "Ceylon Silver Needles" and because it is made from just the young buds, there is very little bitterness and it can be brewed with boiling water for up to 10 minutes, producing a lovely golden brew with a delicate floral aroma and sweet aftertaste that just so happens to be one of the most caffeinated cups of tea you can drink.
*While it may seem hard to believe that white tea, of all teas, is the most caffeinated, this has been well studied and the results show an average of 4.85 grams of caffeine per 100 grams of white tea, versus 2.90 g/100g in green tea and 3.50 g/100g in black tea.
**Credit must be given to Nigel Melican of Teacraft Ltd., one of the world's leading tea authorities, for so thoroughly researching this topic and finding the excellent studies I referenced in this post.
Tags: Tea and Health