Reboiling, oxygen and fresh water
Three pieces of tea folklore travel together: never reboil water, use water that is highly oxygenated and always start from a freshly drawn tap rather than water that has been standing. Published tea writing splits on all three, with some authors calling them baseless and others repeating them as settled fact. Neither side is quite right. The oxygen story does not hold up, but the caution behind it is not empty either, it is just aimed at the wrong gas.
What it is
The third claim, that water must be freshly drawn, is the mildest of the three and the only one with nothing much wrong with it. Water standing in a jug picks up nothing that matters over a few hours, so drawing it fresh is not doing what the advice implies. But it costs nothing, and water that has stood open does lose some chlorine, which is a real if accidental benefit. Harmless habit, wrong reason.
What it does to your tea
What reboiling reliably does is drive off dissolved carbon dioxide, which measurably shifts the water's chemistry: one controlled study found it raised pH by roughly 0.5 to 1.0 units, a shift the same study ties to reduced catechin content, because bicarbonate buffering, not hardness or pH alone, is what governs how tea holds up in the cup. A separate study found the water's dissolved bicarbonate concentration inversely correlated with how fresh and pure the tea's aroma scored. Reboiling also concentrates whatever minerals are already dissolved, since water evaporates and minerals do not, though the effect is modest for one reboil.
How to find out what yours is
What is missing is the direct test. No peer-reviewed study has compared reboiled against freshly boiled water on an actual tea tasting panel, so the sensory question the folklore claims to answer is still open. One small non-academic test, three trials by America's Test Kitchen, found no detectable difference and reasoned that most gas escapes on the first boil, which is suggestive rather than conclusive. Until someone runs that panel, the honest answer is that reboiling changes your water's chemistry by a measurable amount and nobody has shown what that does to the taste.
What people get wrong
The oxygen explanation does not hold up. A 1970 study designed specifically to test whether low dissolved oxygen makes water taste flat found no taste effect at all, and drinking water guidelines set no taste standard for it. Boiling is a poor way to strip oxygen anyway: a comparison of four common deoxygenation methods found boiling the least effective of them, and the quantities are tiny throughout, under 9 milligrams per litre even in cool tap water. That does not make the old caution baseless, only wrongly explained.
Sources
- dissolved oxygen has no effect on the taste of water Bruvold, W.H. & Pangborn, R.M. (1970), Journal AWWA 62(11), 721-722
- no drinking-water guideline sets a taste value for dissolved oxygen WHO Guidelines for Drinking-water Quality
- boiling is the least effective way to remove dissolved oxygen from water Butler, I.B., Schoonen, M.A.A. & Rickard, D.T. (1994), Talanta 41(2), 211-215
- dissolved oxygen solubility stays under 9 mg/L at typical tap temperatures Water on the Web, University of Minnesota Duluth
- boiling raised water pH by 0.5 to 1.0 units through CO2 loss Cabrera, M. et al. (2021), Molecules 26(12), 3485
- bicarbonate concentration inversely correlated with tea aroma freshness and purity Zhang et al. (2025), Food Chemistry: X 29, 102758
- a three-trial blind test found no detectable difference between reboiled and freshly boiled water America's Test Kitchen, three-trial test (non-academic)