Here is a very interesting data of the experiment conducted by The National Agriculture and Food Research Organization (NARO). This data shows how much the elution % of major components (catechin, theanine and caffeine) contained in green tea changes with the temperature change.
In CASE-A, green tea was brewed in cold water at 10 ° C (50F) . In Case-B, the temperature was at 0.5 ° C (32.9F) in iced water.
The Y axis shows elution % from tea leaves (relative ratio when the elution rate obtained in hot water at 80 ° C (176F) for 2 minutes is set to 100% as the standard value), and the X axis shows time of brewing. The elution% was measured every 10 minutes for 60 minutes.
In CASE-A, the amount of caffeine (orange line) eluted after 60 min is about 50% compared to be brewed at 80 ° C (176F). In CASE-B, the caffeine elution was only about 20% even after 60 min. This result shows that lower the brewing temperature, the amount of caffeine eluted in water is decreased more. So if you want to lower caffeine intake, cold brewing is very good option.
While, theanine (yellow line), the source of umami is much faster to elute even in low temperature. In CASE-A, 100% of theanine is eluted in 30 minutes. In CASE-B, the theanine elution % is slower than CASE-A, but 80% of theanine is eluted in 60 min, while the caffeine elution is only 20% at the same time. This result shows that you can still enjoy umami from theanine with lower caffeine intake by brewing in cold water. In addition, the EGCG (EpiGalloCatechin Gallate) - blue line, the main source of astringency is much less eluted in low temperature vs. brewing in 80 ° C (176F).
Summary:
- Brewing green tea in cold water can reduce caffeine intake
- Lower the brewing temperature, the caffeine intake can be reduced further.
- Refreshing taste of cold brewing comes from less astringency (=catechin) and bitterness (=caffeine) elution!
You can now find from a scientific point of view, why cold brewing contains lower caffeine, and why it generates refreshing taste with good umami and less bitterness and astringency. Hope this helps for your healthy life !
Reference:
https://www.naro.affrc.go.jp/laboratory/nifts/t_contents/mizudashi_ryokucha/detail.html