It is commonly said that 99 percent of tea is water. Frankly, I don't know if anyone has sat down and done the math on this, but the point is the same - water matters when it comes to your tea.
In fact, I have a crazy pipedream. One day I wish I'll have the means and opportunity to travel in search of the best water for different teas. I want to try melting the snow off of Winter-flowering plum blossoms and brew with that. I want to visit different mountain springs throughout Japan and China and brew up the locally-grown teas there. In the back of my mind I think this would also really ingratiate me with the local farmers, who'll nod to each other and say "this guy gets it."
I've experienced the difference water makes. When I moved back to Minnesota from NYC, my green teas suddenly tasted lackluster. After some trial and error, I determined it had to do with the hardness of the local water. Switching to spring water helped a great deal. This is the same reason the water in our tea house gets filtered - but not too much, because water that's too soft is a problem too!
So, what makes good water for matcha? Let's get into it.
About Water Hardness
Water Hardness refers to the concentration of minerals within water, with more mineral-rich water referred to as "hard" water, and more mineral-poor water referred to as "soft." Water tends to pick up minerals as it moves along riverbanks and pebbles and rocks slowly dissolve.
When it comes to tea, you want some minerality to your water, as it gives the tea texture and a sense of vibrancy. However, if the water is too hard, delicate teas often lose much of their flavor. There seems to be a limit to how much "stuff" water can chemically hold, and if there are already a lot of minerals, there's not much room left for the aromatic molecules of tea.
It is often said that tea is best brewed with the water that is local to its point of origin. For matcha, this means generally softer water is best. This tends to be true for most green teas and lighter white teas - the delicate flavors often get easily lost if the water is too hard. Whether the tap water you live is hard or soft is something I can't speak to, but often you can get pure filtered water and add concentrated mineral drops to it, or else jugs of spring water are typically the right hardness.
What About Temperature?
Water that's too cold won't whisk the matcha well. It also won't release the aromatic molecules in the powder as easily. Water that's too hot will extract bitter molecules within it that are best left locked in the particles. In general, the bitter catechins of tea are extracted more easily with hotter water. It is true that these catechins are good for you, but in the case of matcha, you are getting them into your body whether they are extracted from the powder or not - so, better to leave the matcha tasting smoother, and that way you'll drink more of it!
I have found the ideal temperature for whisking matcha to be around 160 degrees Fahrenheit. If you don't have a smart kettle, this is roughly when the tiniest bubbles just barely start to form in water (near sea level at least). At this temperature, the matcha still feels like a pleasantly hot drink, but you don't need to wait for it to cool down before drinking it - thus, it doesn't brew unnecessarily long as it sits there. It also seems to balance aroma, whisk-ability, and lack of bitterness very well.
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That's pretty much what I've got as far as the highlights of water for tea. I encourage you to do some experiments with different waters side-by-side and see what you like best!
Thanks for reading and happy sipping,
Simon