Saturday, December 28, 2013

Destructive Myths: The Dissolved Oxygen Hypothesis

Part I: The Argument

There are a lot of myths and unfounded maxims surrounding the proper preparation of coffee and tea. Most are harmless, or, at worst, detrimental to beverage flavor. But one such myth has resulted in a massive waste of energy and water. I refer to this myth as the dissolved oxygen hypothesis.

The dissolved oxygen hypothesis states that, when brewing tea, one should always use freshly drawn water, and never reboil water in the kettle. The justification given is that water that has previously been boiled has less dissolved oxygen (DO). The result is that many tea drinkers are wasting enormous amounts of energy by dumping leftover hot water from their kettles.

The dissolved oxygen hypothesis rests on two premises: (1) that once-boiled water contains more dissolved oxygen than twice-boiled water; (2) that dissolved oxygen improves the flavor of tea. Both premises are demonstrably false.

Boiling itself does not remove dissolved gases. It is the change in temperature or pressure that affects the amount of gas that a liquid can hold (i.e., the solubility of a gas in a liquid). Solubility decreases as temperature increases. Assuming normal atmospheric pressure and composition, water at 0˚C can hold a maximum of ~15ppm DO, while water at 50˚C can only hold ~5ppm. Once the water reaches 100˚C, solubility is zero. Therefore, if you've brought water even close to a boil, you've removed virtually all the DO. What this means is that neither once-boiled nor twice-boiled water contain significant levels of DO, refuting premise (1).

Studies (Faust & Aly 1998, Pangborn & Bertolero 1972) have found that the level of DO in drinking water does not have a significant impact on its taste. It seems likely that the folk belief that DO improves water flavor results from the fact that running water (e.g., streams) is generally preferred to stagnant water (e.g., lakes), and is also higher in DO.

In principle, DO could soften the tannins in tea, just as decanting a bottle of red wine does. However, decanted wine contains much more DO, on account of its lower temperature, and wine is usually allowed to breathe for at least 15 minutes, compared to the 1-5 minutes that tea steeps for. Furthermore, tea drinkers can control the level of tannins in their cup via manipulation of steeping time, water temperature, and water/tea ratio. In short, a well-brewed cup has no need of oxidation.

Therefore, both premises that lead to the dissolved oxygen hypothesis are false. (1) Tea water does not contain a significant amount of DO, and even if it did, (2) there's no evidence that the level of DO has any impact on the flavor of the tea.

All argumentation aside, I simply cannot tell the difference between tea brewed with once-boiled water and tea brewed with twice-boiled water. I've done the tasting blind, more than once. In part, this post is a challenge to any believers in the dissolved oxygen hypothesis: try a blind triangle test. If you succeed in distinguishing tea made from once- and twice-boiled water, let me know.

All this is not to say that water is unimportant. Water is important. Alkalinity is important. Salt content is important. Minimal iron content is super important. Dissolved oxygen is not important.

Part II: Confounding Results

Here's the twist: I can easily distinguish between fresh tap water and water that has been boiled for an extended period of time. In the process of investigating the dissolved oxygen hypothesis, I boiled a small quantity of filtered water for 5 minutes, then refrigerated it until it matched the temperature of my tap water. I then drew some fresh filtered water, and tasted the two, blind.

There was a clear taste difference between the boiled and freshly drawn water. The freshly drawn water had a clean, crisp finish, while the boiled water had an off-putting twang to it. This was not a hard distinction to make.

I repeated the test using a different pot to boil the water, to make sure there weren't any contaminants in the first pot. Same result: the boiled water tasted worse than fresh water.

Given that DO does not have an impact on water flavor, what could explain the flavor impact of the 5 minute boil? Boiling can have a number of effects besides removing dissolved oxygen. It can also remove chlorine as well as concentrate dissolved minerals by reducing the water. But my water filter removes all detectable chlorine, and the short boiling time did not reduce the amount of water by an appreciable amount. Boiling can also remove calcium and bicarbonate ions (by precipitating temporary hardness), but my water is quite soft, and I have never noticed any scaling (which would indicate precipitation of temporary hardness.)

The only plausible explanation I can think of is that boiling the water would also have removed any dissolved CO2. Dissolved CO2 will form a small amount of carbonic acid, which can significantly lower the pH of very soft water (like mine). The fresh water would then have a lower pH than boiled water. Since most consumable liquids are at least slightly acidic, this might explain why the fresh water seemed to have a clean, crisp finish compared to the boiled water. This explanation predicts that less of a difference would be evident if more alkaline water were used, because alkalinity buffers against changes in pH.

Why then wouldn't removing dissolved CO2 from water also affect tea flavor? Well, the amount of dissolved CO2 in tap water is not enough to have a significant impact on the pH of any solution much stronger than pure (soft) water. A full explanation of this phenomenon would require an in-depth discussion of pH buffering, but this is why the pH of a brewer's mash depends much more on the alkalinity than the pH of the source water.

Even if small amounts of dissolved CO2 did affect the flavor of tea, heating water close to boiling will remove virtually all dissolved CO2, just as it removes DO. For brewing tea, coffee, or any other hot beverage, dissolved gases are irrelevant.

Faust & Aly, 1998. Chemistry of Water Treatment, 2nd Edition. p. 114 (
Pangborn & Bertolero, 1972. Influence of Temperature on Taste Intensity and Degree of Liking of Drinking Water. Journal of the American Water Works Association.


  1. What's in your fridge? Maybe its leaving a not-so-nice flavor in your water. As for water sitting in a kettle, it will slowly absorb CO2 over time and produce carbonic acid, which leaves the water tasting generally less nice.

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  2. The water was covered with plastic wrap while in the fridge, so I don't think that's the culprit.

    As far as carbonic acid goes, my hypothesis is that it has a positive flavor impact in small quantities. This is of course based on nothing but intuition.

  3. I did the experiment to test changes in PH with boiling and also to compare distilled water with filtered Lincoln tap water. I let the water come to room temp in glass cups before tasting.
    Filtered water: fresh PH 5.0 clean taste; 1 boil in stainless electric kettle PH 6.0 aftertaste; 2nd boil PH 7.0 same aftertaste; microwaved PH 5.3
    Distilled water: PH 4.5 clear taste; 1 boil PH 4.5 couldn't really taste dif; 2nd boil PH 4.5
    My guess is minerals in tap water react with stainless steel kettle when boiled.

    1. how did the microwaved water taste ?

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  5. Thanks for the excellent discussion of the dissolved oxygen in tea water theory. I came here because Dr. Karl on the same-named BBC program had mentioned as being true, and to me as an avid tea drinker and a fan of evidence it was important to know.
    I should now go and repeat the above boiling experiment using a plastic kettle!

  6. Have to report back on my experiment to test the effect from boiling non-filtered Berlin tap water once or twice on its taste using a plastic kettle.

    Cleaned three fairly similar cups. All three samples were covered with aluminum foil after having been filled to the brim to avoid contamination and placed on the balcony in a thin layer of snow that had fallen to ensure equivalent temperature. The twice boiled water sample was allowed to cool in that manner between boilings.
    The sample let the water come to room temp in three cups
    before tasting clean and mark underside
    The cups were sampled in a blind fashion once cold and once at room temperature.

    Results: No difference was found. We have good water here.

    Criticism: An n of only 1. My sense of taste might not be good enough.

  7. The real problem isn't pouring out old water from your kettle, it's boiling more water than you need.

    If you boil the amount of water you require and not more, you can use fresh water every time and save all that electricity/gas boiling the extra unused water.

    I put markers on my kettle for 1, 2, 3, 4 cups, so that I don't boil more than I need.

    You can boil 250mL 4 times a day and it will take 4x less time to boil than boiling say 1L of water and use the same amount of energy. Boiling more water than you need in the kettle to save time boiling later will probably take as much time as just boiling the amount required every time. It's amazing how quick your kettle will boil if you only put the small amount in that you require for say a cup a tea.

  8. The 2nd claim that the amount of DO present doesn't change tea flavor (extrapolated from results with plain water) is faulty. Firstly, oxygen is pretty reactive and can create oxidizing or reducing environments based on its concentration. Some reactions are comepletely different in oxidizing vs. reducing environments. It seems reasonable that the presence of oxygen could somehow alter the compounds released from tea making them taste better. The oxygen (being reactive) could also assists in the stripping of chemicals from the tea leaves.

    As for your experiment. You should try this:

    Remove two large samples of water from the tap.

    Place each sample in a pot.

    Boil 1 pot.

    Allow both samples to come to room temperature.

    blind taste.

    This will control for most external factors.

  9. This comment has been removed by the author.

  10. Thank you for an interesting post! However, solubility alone does not tell the whole story about the oxygen already present in the water. There are people who do not want oxygen in the water, and they state that boiling is a rather inefficient method of removing oxygen. Even after boiling water on a hot plate for 30 minutes in open air, there might be around 1 ppm (about the same amount in mg/l) oxygen left.
    This is research paper from 1994:
    "Removal of dissolved oxygen from water: a comparison of four common techniques"
    IB Butler, MAA Schoonen, DT Rickard

    Now, if this has anything to do with taste, is a different question ;-)

  11. Thanks for this post!

    You would be correct; provided one actually /boils/ their tea water. The near entirety of tea is brought to 170F, 180F, at times 190F, but rarely 212F (except for pu-erh). And like you stated, the life in fresh tap water imparts a world of difference. In fact, those flavour molecules in tea bond to the rising oxygen as it dissolves from the water giving us their nuance. It offers the same affect as natural carbonization in alcohol due to fermentation.

  12. this is amazing blog, we should not reboil water in kettle

  13. Pots are basically iron, or mixed with chromium (stainless steel around 10% Cr) tin plated, ....or copper(traditional cookware in India where it presents serious heavy metal toxicity hazard especially to children). The latter metals are all active transition metals creating reactive oxygen species in the presence of an oxygen source (like air), low or high pH, and an electrolyte (dissolved minerals in the current case). Interestingly, oxygen-alkali bleaching of wood plup takes place in near boiling conditions where transitions metals are noted as catalytic. I think boiling tea in a pyrex pot is probably best unless you really enjoy experiencing your chemistry.

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  16. This was an awesome post. I'm trying to understand the physics that bring water in its boiling state (a significant physical change) into coincidence with flavonoid extraction and tannin avoidance and plant matter cell permeation.
    While searching I came across the diffused oxygen hypothesis (on Quora also, ��) and felt the skeptic alarms going off.
    Would you be able to recommend any other articles relating to this topic?

  17. While yes, water at boiling holds 0ppm of oxygen, I think you've made a mistake in assuming that the moment water reaches boiling point, all oxygen will leave it. It takes time for the oxygen to leave the water, to leave the oxygen molecules need to encounter nucleation sites, where they will form bubbles, and then leave. So as you heat the water, the water will become supersaturated with oxygen. It will then take time for the oxygen to leave the water, I don't know how long that will be, but one comment above suggests half an hour to get 99% of the oxygen out. So, if takes 3 minutes to boil the water, it's still going to have a high oxygen content when you mix it with the tea leaves. Reboil it while it's still warm, and what you'll have is water that has been at a high temperature for a lot longer than if you've just boiled the water once, and so will have a much lower dissolved oxygen content.

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  19. I don't understand the insistence that DO by itself can not affect taste when both theory and practical experience (as demonstrated in your own experiment) suggests otherwise. If it looks like a duck, and quacks like a duck ... Ouila! You have a duck. DO affects taste. If it doesn't make a difference when used in tea then it's an interaction with the tea and the water, which is an extremely reasonable expectation. As for the wasting of hot water ... I couldn't agree more. One more thing: oxygenate your water or tea, etc with a high powered immersion blender like the cuisinart. 5 seconds on power level 9 is all it would take, in fact will probably super saturate it with oxygen.

  20. Some reactions are comepletely different in oxidizing vs. reducing environments. It seems reasonable that the presence of oxygen could somehow alter the compounds released from tea making them taste better.


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  25. Can you respond to this comment from James Roper, as that effectively disproves your experiment with regards to twice boiling not affecting oxygen levels.


    James Roper March 14, 2017 at 4:26 AM
    While yes, water at boiling holds 0ppm of oxygen, I think you've made a mistake in assuming that the moment water reaches boiling point, all oxygen will leave it. It takes time for the oxygen to leave the water, to leave the oxygen molecules need to encounter nucleation sites, where they will form bubbles, and then leave. So as you heat the water, the water will become supersaturated with oxygen. It will then take time for the oxygen to leave the water, I don't know how long that will be, but one comment above suggests half an hour to get 99% of the oxygen out. So, if takes 3 minutes to boil the water, it's still going to have a high oxygen content when you mix it with the tea leaves. Reboil it while it's still warm, and what you'll have is water that has been at a high temperature for a lot longer than if you've just boiled the water once, and so will have a much lower dissolved oxygen content.

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