I've heard some mixed opinions regarding how water that's been boiled too long/re-boiled affects tea. Apparently whenever water is boiled, oxygen evaporates, leaving the tea brewed from said water tasting flat. Frankly, I don't see the logic behind this. If the oxygen is boiled out, doesn't that just leave hydrogen on its own, which isn't even water? If this is a misconception, does re-boiling water affect the tea in any way? If it isn't, can someone explain the chemistry behind it?

LowInFat wrote:I've heard some mixed opinions regarding how water that's been boiled too long/re-boiled affects tea. Apparently whenever water is boiled, oxygen evaporates, leaving the tea brewed from said water tasting flat. Frankly, I don't see the logic behind this. If the oxygen is boiled out, doesn't that just leave hydrogen on its own, which isn't even water? If this is a misconception, does re-boiling water affect the tea in any way? If it isn't, can someone explain the chemistry behind it?

I tried searching for existing threads but couldn't find any.

I was going to let someone with more chemistry knowledge than me speak up. But when we talk about the oxygen leaving the water, we are not talking about the oxygen in the water molecules themselves, but rather there are other gases in gas form so to speak dissolved into the water. So there is some oxygen, nitrogen, carbon dioxide even inside the water itself. Again not the water molecules but you can imagine molecules of Oxygen, nitrogen and the like floating around among all the water molecules.

A way to think about this is if you've seen those gels that have bubbles or other little beads floating in them, its very similar to that except on a much smaller level. Where the gas molecules that the place of the bubbles or the beads, and the water is the gel.

Now yes the dissolved gases work their way out when boiled, but if the kettle sits for awhile some will naturally start to work their way back in slowly. And I have a hard time completely believing that the water gets that dead, as the turbulence of the pouring of the water in the brewing should introduce a good bit of air back into the water right before it is brewed. Granted I do not have the equipment to be able to run any sort of test to see how much it would be off.

As a newbie to tea in general I did not notice any discernable taste difference between freshly boiled water and re-boiled water in tea. My masala chai tasted just as good second boil as it did the first. But then again I don't have a refined palate.

When I grew up, my family didn't allow re-boiling anyway, according to some Chinese kitchen wisdom. I don't know if it's backed up by scientific mechanism, but I just follow it and don't like re-boiled water. That's also why I don't like to use most of the electric kettles nowadays and use small kettles and/or thermos instead.

I think a possible reason against re-boiled water is, no water is pure water, but most water is nearly pure water with certain other contents. When water is reboiled, some water will evaporate and what's left is more concentrated. This may affect taste (I didn't taste it, some ancient people said so...) and may have some heath outcome - the "health outcome", even if existing, I guess, would be too subtle and probably can be ignored, as long as one doesn't drink re-boiled water all the time. Why would one do it all the time anyway

I've looked a tiny bit into this back when I was trying to figure out how to make perfectly clear ice (I have many eccentric hobbies..).

The reason why ice isn't clear when frozen is the tiny bubbles of gasses that are somewhat invisible in water as a liquid cause the ice become porous and cloudy as only water can become part of the crystalline structure of ice. While there's many ways to make clear ice, and thereby pure ice, most people tend to agree that while boiling water will make some oxygen (and presummably other gases) disappear from the water, but after a bit as it sits, it simply regains the gasses naturally.

Though I'm no chemist/physicist. This is just how I understand it. If anyone's interested in how gasses interact with water, looking up clear ice making can actually be fairly interesting.

gingkoseto wrote:some water will evaporate and what's left is more concentrated

This seems true to me. Forget who told me this, but supposedly the minerals (and potential chemicals such as fluoride) concentrate together the longer you boil. This could explain the metallic taste you will find if you boil water too long, or the concentrated mineral deposits if you have ever boiled anything dry and looked at the bottom deposits.

Is it really getting concentrated enough to be a health concern after long term reboiling? I guess that all depends on your water and your bodies sensitivity. I just don't like it because it makes my water taste off, much more noticeable in electric, but still there in the clay and glass kettle too.

Gases tend to be less soluble in water at higher temperatures (although apparently that does not hold for all other solvents). This link shows the solubility curve for various gases in water (oxygen is near the bottom), and then this link has a good description of the equilibrium process behind the solubility changes.

What teaisme mentions may be of more concern -- as you boil your water, water vapor will escape, but minerals and such will not, thus the mineral concentration increases. You would have to seriously overboil your water to worry about this problem, though.