Review: Decaffeination of fresh green tea leaf (Camellia sinensis) by hot water treatment (Liang et al., 2007)

There are many things about college I love, but almost none more than free access to an unlimited amount of academic research journals. Since the start of this blog, I have promised all of you that I one of the things I would do would be to read and review and summarize tea research articles for you. The post before last I gave you some very small tidbits, but this post I will go much more in depth and I hope you will join the discussion by commenting.

This article, Decaffeination of fresh green tea leaf (Camellia sinensis) by hot water treatment (see bottom for citation and reference link), is not the most interesting one I've ever read. What it ends up saying is that if you boil the fresh tea leaves for 3-5 minutes in the step before rolling and drying (basically replacing the pan frying or steaming part of making Chinese green tea), you can remove around 90% of the caffeine without removing the catechins (what we love about tea).

As many of you know, there is a very large and old witch tale flying around that says that if you boil tea for 30 seconds and then discard that, your tea will be decaffeinated. This study just goes to show again that this does not happen. At 1 minute at boiling of fresh tea leaves (not even what would qualify as green tea) only about 50% of the caffeine was removed while you need to reach at least 80% to be able to consider it decaffeinated. Greater than 80% caffeine removal only happened when the time reached at least 3 minutes at boiling water. (Don't forget that what happens to fresh leaves is different from what happens to any type of later processed tea leaves that we drink from too.) When they tried 3 minutes boiling on processed leaves (what could actually be counted as green tea), they found that the leaves were essentially decaffeinated. Unfortunately by then the cell walls were so broken that 90% of the catechins were also removed, therefore making your leaves useless and likely disgusting tasting. Did I really need to tell many of you that, though? Hahaha. I'm pretty sure that when you read "3 minutes boiling on (green tea)" many of you cringed and/or may have nearly cried at the thought like me. Hahahahaha.

So why does processing the fresh leaves into green tea change the amount of catechins removed? Catechin removal requires breaking the cell walls, which happens when leaves are broken by stirring during pan frying and definitely when rolled and dried. I wonder if this then means that there would be a significant difference between the amount of catechins released into the tea from something like Longjing vs. Gunpowder green teas though? Since one is very obviously rolled more than the other.

Another one of the points it makes in discussing how to decaffeinate the fresh leaves is that the water should be at boiling for decaffeination to happen. Somewhere below the boiling point (between 100C and 75C) the caffeine is no longer removed from the fresh leaf. In their table of results from this part (see Table 2), it's interesting to see that while that basically no caffeine is removed from the fresh leaves at 50C (the lowest of the tested temperatures), a significant amount of catechins are (~10% after 5 minutes). Because their study was about decaffeination as opposed to catechin extraction though, they did not test what happened in processed green tea leaves at these lower temperatures. It seems counter-intuitive to me that while normally catechin removal would require breaking the cell walls, brewing at lower temperatures before those steps also released them.

Even after processing the fresh leaves into actual tea, does do lower temperatures affect how much caffeine and catechins go into the water? Or are the cells so broken by then that there is no difference based on brewing temperature? Based on personal experience, I do believe that lower temperatures change the ratio of what goes into the tea since the taste is different. I just wish I had scientific evidence backing me up and explaining exactly what that taste difference was caused by. Maybe I'll find that in another article.

Reference:
Huiling Liang, Yuerong Liang, Junjie Dong, Jianliang Lu, Hairong Xu, Hui Wang, Decaffeination of fresh green tea leaf (Camellia sinensis) by hot water treatment, Food Chemistry, Volume 101, Issue 4, 2007, Pages 1451-1456, ISSN 0308-8146, 10.1016/j.foodchem.2006.03.054. (http://www.sciencedirect.com/science/article/pii/S0308814606002998) Keywords: Decaffeinated tea; Green tea; Black tea; Caffeine; Catechins; EGCg; HPLC; Polyphenol oxidase