The lively discussion that followed my post on how the British really cared about their tea reminded me of an issue that I had idly thought about some time ago when I visited relatives in New Zealand. They are Sri Lankan and thus, like the British, take their tea seriously so that they make sure that the ‘tea things’ (tea kettle, tea pot, tea, sugar, milk, cups, strainer, and spoons) are located in prominent and easily accessible places in the kitchen so that no one dies due to tea-deprivation. Their tea kettle is a powerful electric one that heats water very quickly, not the wimpy one that I have that you heat on the stove.
When I make tea, I only put sufficient water into the kettle that I needed for my tea, by first filling a cup with water, then pouring that water into the kettle, and repeating depending on the number of cups of tea I was planning to make. But I noticed that when they made tea they would hold the kettle under the faucet until there was much more water than they needed, boil and use it, and leave the remainder in the kettle to cool. Then when they needed to make tea again, they would throw away the water that remained in the kettle, fill it up again, and repeat the process.
This struck me as wasteful of energy (and water) and I asked them why they did this. It had never occurred to them to measure the water into the kettle, presumably because their powerful electric kettle heated the water up so fast, unlike my stove top one. Also electric kettles require a certain amount of water in the kettle to completely cover the heating coil at the bottom, though they used far more than the minimum required. But what intrigued me was their claim that one should never use water that has been boiled more than once to make tea. The video I linked to in my previous post also had that guy sternly telling the viewer that you should never do that but always use fresh water from the tap.
I had never heard of this theory, possibly because I grew up in a home in Sri Lanka that had domestic help and so never made tea myself. It appeared mysteriously out of the kitchen whenever I needed it. When I asked my relatives why they this was important, they couldn’t say, apart from some vague suggestions that it was bad for you or that it affected the taste of the tea. It seemed to be one of those pieces of folklore that are handed down from generation to generation and are followed unthinkingly. Off the top of my head, all I could think of was that since any dissolved gases that might affect the taste would boil off the first time anyway, boiling again would have no effect.
I meant to follow up on this question but forgot and this discussion made me look it up. There are some sites that recommend this and suggest all manner of scary things that can happen to those who used re-boiled water but Karl Smallwood seems to have gone into this issue on great detail (you can check his sources at the bottom of his post). He says that unless you are boiling down a massive quantity of water to a tiny amount and thus hugely magnifying the concentration of naturally occurring impurities in the water that do not boil away, there is no benefit to avoiding re-boiled water.
As for the taste question, there seem to be no studies that suggest that taste is affected by boiling water more than once.
While I could not find any studies on the idea of twice boiled water changing the taste of tea, we can at least look at the mechanism involved to get an idea on whether it is likely or not that such a taste shift is occurring.
For starters, many claim the taste difference comes about due to twice boiled water having less dissolved oxygen in it, referencing the fact that, as the water temperature increases, the solubility of oxygen decreases. This latter fact is absolutely true. The former is not.
You see, at 100˚C (boiling point) the concentration of dissolved oxygen (assuming normal atmospheric pressure) will ultimately be near zero, whether it’s once or tenth boiled water. How long you boil the water does come into play somewhat, but even then the differences are minimal, with dissolved oxygen levels at 1 atmosphere and 0°C on the order of 15 ppm, compared to approximately 5 ppm at 50°C and near 0 ppm at 100°C.
“But my tea tasting palate is extremely refined and able to detect even the smallest of changes of dissolved oxygen levels,” you say as you idly clean your monocle.
It doesn’t matter.
Once the water is allowed to cool back down, its dissolved oxygen levels will once again begin to rise to normal levels given atmospheric pressure and temperature. (CO2 levels, which can affect the taste, will also return to normal.) Given this, the second time you bring the water back up to boil, things like the oxygen level are not going to be any different than the first, assuming equal boiling times and/or temperature levels.
That’s not to mention that the idea that more oxygen = better tasting gets into the aforementioned debate on what does or does not taste better when making tea- everyone’s got their own preferences. And if you’re curious, there have been studies (Pangborn & Bertolero 1972, Faust & Aly 1998) that have indicated that dissolved oxygen actually doesn’t noticeably impact the flavour of water, though it should be noted that those studies weren’t dealing with brewing tea, and it is possible (even probable) that dissolved oxygen could be interacting with elements of the tea to change the flavour (similar to what happens with wine). But either way, whether once, twice, or thrice boiled, you’re getting essentially the same dissolved oxygen levels, assuming you boil/steep the same way each time.
I recall the time when I was in a hotel restaurant for breakfast and overheard the woman at the next table order orange juice. But she asked the waiter whether the juice was from concentrate or not. He confidently said that it was not but she was not convinced. She asked to speak to the head waiter and he too said that it was not but seemed to waver a bit. She then asked for the manager and he said that they did not actually squeeze the oranges in the hotel but it came from a supplier and that he thought it was not from a concentrate but could not guarantee it.
While I was impressed by the woman’s dedication to avoiding orange juice from concentrate, I could not see why, if she could tell the difference, she did not simply taste it and check for herself. If she could not tell the difference, then why did it matter, since I doubt that orange juice from concentrate is actually harmful to you?
People who think that they have very discriminating palates when it comes to wine, tea, coffee, orange juice, and the like often cannot tell even major differences when subjected to double-blind taste tests. No amount of science is going to persuade such people that they may be imagining that they can detect subtle flavor differences. But clearly simply knowing some facts about what they are drinking affects their enjoyment. So as with whether to add milk and sugar to tea, we should let people do what they want since it is pretty much harmless either way. If they think it affects the taste, so be it.