An Update on Tea and Caffeine

I’ve been hearing for a while that decaffeinating your tea at home by first brewing for 30 seconds or so, then re-brewing the same leaves in fresh water, does not actually work.  I have recommended this method in the past, and I see it suggested all the time by all sorts of tea authorities, so I’ve been concerned.  I finally got around to reviewing the research literature on caffeine in tea and learned some interesting facts, suggesting that this home method of decaffeination, for loose leaf tea anyway, is a myth.  This is a rather long, detailed, post that covers several aspects of caffeine in tea.  I really got into this research, so if I lose you along the way, skip to the end to my summary!

In a study published this year in Thailand (Ref: Caffeine in Chiang Rai tea infusions: Effects of tea variety, type, leaf form, and infusion conditions. Food Chemistry, Volume 114, Issue 4, 15 June 2009, Pages 1335-1338.), researchers looked at green and oolong teas where the leaves had been either ground, rolled, or left as loose leaf.  Teas were brewed at 80, 90, and 100° Celsius (100°C = 212°F, which is boiling) for various periods, from 30 seconds to an hour.

For rolled and loose leaf tea, they found that for a period of 30 seconds to 15 minutes, the caffeine content of the brew increased rapidly, then it slowed down from 15 minutes to 60 minutes. So clearly, if caffeine content is still increasing in the cup after 10 minutes, 30 seconds of brew time isn’t enough to make any difference in trying to reduce the caffeine content of your tea.  This is clear evidence that the home method of decaffeination does not work for loose leaf tea.

However, they noted that for ground leaves, the caffeine infused much more rapidly than it did for non-ground leaves, such that the caffeine concentration in the brew was independent of the brew time.  In other words, caffeine concentration in the brew was stable 30 seconds to 60 minutes, indicating that the vast majority of caffeine had come out of the leaves in the first 30 seconds.  Ground leaves have a larger surface area, allowing the caffeine to be released more quickly.  So perhaps for finely ground tea, such as is commonly found in tea bags, the home method should work.  But read on.

Also note that they confirmed that water temperature is the most important factor in determining how much caffeine is released from loose tea leaves.  The hotter the water, the more caffeine is released.  This may be part of the reason why green tea usually seems to have less caffeine in it, because it is usually brewed with cooler water.

An earlier study from 1996 (Ref: Tea preparation and its influence on methylxanthine concentration.  Food Research Journal, Vol. 29, Nos 3-4, pp. 325-330, 1996), researchers looked at caffeine content of bagged and loose leaf black and green teas, including the interesting variable of making multiple cups out of the same leaves, a common practice in Asia.  Each tea was brewed for 5 minutes in freshly boiled water, allowed to cool, then re-brewed the same way two more times to produce 3 different cups to test for each of the teas.  They found the bagged tea released its caffeine more quickly, such that a larger percentage of the total caffeine extracted was present in the first cup brewed.  For loose leaf tea, caffeine was released more slowly, but the total amount of caffeine released over the three cups was larger than it was with tea bags.  The basic idea is that  you’ll get somewhat more caffeine with loose leaf tea if you re-use your leaves than you will if you re-use your tea bags.  These researchers weren’t really looking for advice for consumers, though.  They were concerned about epidemiological studies in which tea intake is reported and used to estimate caffeine intake.  The point here is that if you don’t know whether your study population is re-using their tea leaves or whether they are using bags or not, you have no real clue as the their caffeine intake.  Still, it reinforces the idea that bagged teas give up their caffeine more quickly than loose leaf teas.

On more of a side note, I found a 2007 study where researchers examined a new way to produce decaffeinated tea that doesn’t require expensive equipment or undesirable chemicals (Ref: Decaffeination of fresh green tea leaf, Camellia sinensis, by hot water treatment.  Food Chemistry 101(2007) 1451-1456).  These researchers looked at fresh tea leaves, meaning right off the plant and unprocessed.  The “hot water treatment” mentioned in the title is similar to the home decaffeination method mentioned above.  They just brewed the fresh leaves in water to extract the caffeine.  As in the first study, they found that water temperature was the most important factor in getting the caffeine out of the leaves, and they were able to easily get most caffeine out using the hot water treatment (it took about 3 minutes for fresh leaves and they removed roughly two thirds of the caffeine by weight).  They also looked at the catechin levels (catechins are the polyphenols in tea that are good for you) and found that for fresh leaves, 95% of total catechins remained after caffeine extraction.  Sounds great so far.  But when green teas are processed they are usually steamed to prevent oxidation and then rolled and dried.  So they also looked at steamed leaf, rolled leaf, and dry leaf, and found that they were all effectively decaffeinated just like the fresh leaves, but that catechin levels were much decreased compared to fresh leaf (dry leaf catechins were reduced by a whopping 68% compared to fresh leaf).  They hypothesized that the rolling and drying processes used in tea production may destroy leaf cells, facilitating loss of catechins.  So they suggested using this hot water decaffeination process during green tea processing as a replacement for the initial steaming of the leaves (this will have the same effect as steaming, but will also decrease caffeine).  However, this won’t work for black tea because black tea must be first oxidized, and then it is too late to save the catechins.  So hot water treatment in black tea processing would cause too much loss of catechins along with the loss of caffeine.  Based on the results of this study, it also makes sense the using the home decaffeination method yourself, which is most definitely after the fresh-leaf stage, you would start to lose the health benefits of your tea along with any caffeine you might lose.

So here is my summary:

  1. The home decaffeination method — which involves brewing your tea for about 30 seconds, throwing that out, the pouring fresh hot water on the same leaves for the normal amount of time – does not work for loose leaf tea, but probably does work for ground tea (and therefore, probably most tea bags).  It is not clear exactly how much caffeine you could get out of ground tea in 30 seconds, but it certainly appears to be most of it.  But keep in mind that you may also be losing some of tea’s healthful catechins when you do this.  It looks like it’s better to just buy decaf!
  2. If you are in the habit of re-using your tea bags and loose tea leaves, you will get more total caffeine from the loose tea leaves over multiple cups than you would from tea bags.  Not a whole lot more, maybe 10-20 mg. or so over 3 cups.  I just found this counter-intuitive, and therefore, interesting.  But it doesn’t mean much, practically speaking.
  3. The most important factor in determining how much caffeine is infused into your cup from loose tea leaves is the water temperature:  the hotter the water, the more caffeine you’ll get.  If you use tea bags, the water temperature doesn’t matter as much.
  4. I do not see consistent differences in caffeine content of black tea vs. green tea, provided that they are brewed in the same way.  I would like to look at more data, though.   However, the popular notion that green tea has no caffeine is false.  Brewed green tea probably tends to have less caffeine because it is frequently brewed at a cooler temperature than black tea.
  5. I saw numbers for caffeine content for a 6 oz. cup of tea (all kinds) range from 45 mg to 76 mg.
  6. I’ll definitely be looking at more research, so I’ll let you know when I find out something else interesting!