I have noticed it frequently and decided to check with some of my peers and friends about this and they too have felt such a thing. The taste comparison occurs after the boiled water is cooled down to room temperature. My question is: does water even have a taste? And whether heating it may change the taste.
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3$\begingroup$ Well, taste of everything is temperature dependent for 2 reasons:: volatility and sensory response dependency on temperature. $\endgroup$– PoutnikCommented Sep 2, 2019 at 12:53
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$\begingroup$ Does that warm-water taste you're after remain once the heated water is cool again? $\endgroup$– cbeleitesCommented Sep 2, 2019 at 18:55
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$\begingroup$ You taste senses feel your standby mouth taste, teeth last brushed five hours ago, some remains of a recent lunch, a coffee twenty minutes ago, and that one teeth that should see a dentist. ;) If you now sip some fresh water, the mouth chemistry suddenly changes, and you notice a difference. $\endgroup$– KarlCommented Sep 2, 2019 at 21:54
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$\begingroup$ Actually feeling of hot and cold in mouth are parts of the taste - that's how capsaicin and menthol work. $\endgroup$– MithoronCommented Sep 2, 2019 at 23:01
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$\begingroup$ @cbeleites yes. The taste seems to remain after being cooled too. It's easy to identify water that has been boiled and cooled by taste $\endgroup$– evamPUNditCommented Sep 3, 2019 at 0:22
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1 Answer
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The changed taste that stays after cooling the heated water
Hot (boiling) water has a much lower capacity for dissolving gases. When boiling water, $\ce{CO2}$ is removed and through the equilibria behind ($\ce{CO2 + H2O <=> H2CO3}$ and $\ce{H2CO3 <=> H+ + HCO3-}$) so is $\ce{HCO3-}$.
Basically, this removes $\ce{CO2}$, overall carbonic acid species and also acid equivalents: the resulting water will be more basic (less acidic).
$\ce{CO2}$, $\ce{H2CO3}$ and $\ce{HCO3-}$ are not only present in carbonated (sparkling, fizzy) water but also in tap water. For example, this analysis sheet of the Munich municipal water supply cites 300 mg/l $\ce{HCO3-}$, 15 mg/l $\ce{CO2}$ and 0.5 mg/l $\ce{CO3^2-}$ and a slightly basic pH.
It is known that mammals have $\ce{CO2}$ receptors in their acid taste cells, and this is thought to contribute to the water taste. (I don't have the full text, but Zocchi et al: The cellular mechanism for water detection in the mammalian taste system, Nature Neuroscience volume 20, pages 927���933 (2017) looks promising.)
BTW, two further candidates for that bland/flat/stale taste are deionized water and reverse osmosis drinking water.
Warm vs. cold water taste
There will be further changes with temperature, including acid base equilibria and pH, partial pressure of volatile molecules and so on between cold and warm water. And this also applies to our sensing in the taste cells. Put this together with taste/smell sensory input being subject to further processing in our brain to produce the taste/smell impression, and I think it is very hard to compare taste/smell of cool and hot things.
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$\begingroup$ in the question I meant the taste of boiled water, that is left to cool down. The taste is compared between the unboiled water with the boiled, then cooled one $\endgroup$ Commented Sep 4, 2019 at 5:34