Why to not microwave water for brewing tea

Related to a Quora question I recently answered the age old question of why you shouldn't microwave water for brewing tea, as follows.

I really wanted to express that as "Science says don't microwave water for brewing tea" but couldn't bring myself to use that as a title.  If "Science" could speak as a person does it would say do whatever you like; good luck with the outcomes, and enjoy that nice frothy tea.

All of this came up last year, discussed in every online tea group, related to an article saying it's healthier to microwave tea, that it retains more beneficial compounds.  Maybe, but probably not, to any significant degree, and ruining the taste of the tea is a good reason not to do it.  Or the feel instead, I guess I'm claiming, but the individual parts of our sensory experience tend to not be distinct.

I might also mention that I'm an industrial engineer, and I did like those basic classes this subject relates to, but it wasn't an area of specialty.  This probably ties to Chem 201 scope content, and that was awhile back.  But I loved some of those basics courses, learning about different materials, and metalurgy, how physics applies related to types of movements, and heat transfer.  The equations and actual problems not so much, but solubility curves and phase diagrams aren't nearly as problematic.

That Quora link and content follows, only edited slightly for clarity, and filled in a quite a bit, but this is essentially just a longer version than what appears there.

What should you never microwave? What's the science behind it?

This isn’t really along the lines the question implied but a microwave shouldn’t be used to heat water for making tea. It’s universally accepted in tea circles that using this approach makes horrible tea, but it’s not as clear to everyone why.

My take--which is only a guess, really--is that the water is heated very gradually, and in a way that doesn’t provide a heated contact surface (nucleation site) for dissolved air to transition to a gas state and leave the water.  So you end up with more dissolved air than is stable at that temperature (supersaturation of air in water), and frothy tea.

The two factors really come into play, gradual heating, and also the lack of  a nucleation site, and it's hard to tell which is more of a main cause, or to what extent they are connected or separate.  But that doesn't matter.  First a related tangent on word use.

Theory versus hypothesis

I had described this as a theory, but an online comment pointed out that it's really a hypothesis.  It's good to be clear, and this is already running really long, so why not add one more tangent.  This sums up the difference:

A hypothesis is either a suggested explanation for an observable phenomenon, or a reasoned prediction of a possible causal correlation among multiple phenomena. In science, a theory is a tested, well-substantiated, unifying explanation for a set of verified, proven factors.

Clearly the idea isn't tested and well-substantiated, so the question turns to whether the "in science" part applies here, or if this is common use.  All the equations later in this post implies there is science being explained, so maybe.  Google's definition (cited from the Oxford dictionary) fills us in on where conventional use blurring the distinction comes into play:

the·o·ry  (noun)

1. a supposition or a system of ideas intended to explain something, especially one based on general principles independent of the thing to be explained.   

   "Darwin's theory of evolution"

   • a set of principles on which the practice of an activity is based.

     
     

     "a theory of education"
   • an idea used to account for a situation or justify a course of action.

     
     

     "my theory would be that the place has been seriously mismanaged"
2. 
   

   synonyms:

   hypothesis, thesis, conjecture, supposition, speculation, postulation, postulate, proposition, premise, surmise, assumption, presupposition; More

All the same it is a hypothesis, if one is more specific.  

I was more into accepting English as a living language and just going with common usage before it became so common for "literally" to mean "figuratively," which is also a definition accepted by most dictionaries.  Looking up that issue turned up the clearest definition of the relation of theory to hypothesis:

Hypothesis, theory, law mean a formula derived by inference from scientific data that explains a principle operating in nature.  Hypothesis implies insufficient evidence to provide more than a tentative explanation. ⟨a hypothesis explaining the extinction of the dinosaurs⟩.  Theory implies a greater range of evidence and greater likelihood of truth ⟨the theory of evolution⟩.  Law implies a statement of order and relation in nature that has been found to be invariable under the same conditions. ⟨the law of gravitation⟩.

Reference sources and discussion

Sources and details are always nice too, aren’t they? This reference theme tea blog rejects a media claim that microwaving tea is better, for health reasons, but they don’t go into that interesting part about why frothiness occurs, why the tea doesn't feel right (in the World of Tea blog article):

Microwaving Your Tea, The Controversy Explained and Refuted

Related to completeness I might also mention one of the sources that said it was a good idea to microwave water for tea (which they really should have tried out before writing about it):  http://www.foodandwine.com/news/healthiest-way-brew-your-tea-microwave-it


A number of different sources were repeating publication of the same study results, the original source cited in that, so that wasn't really ideas "Food and Wine" were coming up with.  They probably should have checked out results by making a cup of tea though.

The problem, why it doesn't work

That theory about frothiness being due to dissolved air I have turned up background sources on before, and I’ll go through another version of that again here.

Snopes has an article claiming you can, under the right circumstances, actually super-heat water, but I don’t think that’s a standard problem, or very likely to actually happen,  it’s just interesting:

FACT CHECK: Superheated Microwaved Water

Here is a reference that gets into it, but it’s a discussion, and some of the comments about superheating seem completely wrong to me:

Why when I put a teabag into microwaved water does the water fizz?

Note that in that initial study they are discussing microwaving the bag with the water for tea together (just strange, really), and it seems likely this would change things quite a bit, versus this case where the bag is added after, which does cause the water to fizz.

Here is the comment in that last question discussion that matches my take on this.  Again that input and my identical best guess isn't really the last word, maybe just what I happen to think, but it definitely adds up related to the background issues and the effect you see:

What you see is the escape of dissolved gases in the liquid.

Water in a cup in a microwave heats fairly uniformly; by contrast, if you heat on a stove, you hear the water "sing" because you get local boiling of the water that is right next to the wall being heated. During the "singing", dissolved gases are driven out (and they don't really re-dissolve - but the water vapor does, to a large extent). In this sense, conventional boiling is an outgassing operation.

The hot water is supersaturated; the presence of the tea bag creates nucleation points so the dissolved air can escape. That's the fizzing you see.

Seems obvious, doesn't it?  I'll paste the related comment I made in a Facebook group discussion, nearly a year ago now:

It seems likely the tea picks up a frothy feel because boiling [using a kettle] provides a hot surface contact to allow essentially all of the dissolved air to separate from the water, but the gradual, relatively even heating by microwave doesn't, so in the end the water is somewhat super-saturated with air.

That's an effect not completely unrelated to carbonation in soda. It's not exactly the same thing, of course, since you end up in a similar place but in a different way, but in both cases the liquid is holding more gas than really stays dissolved at that temperature, just carbon dioxide in the case of soda and air in the case of microwaved tea water...

It's still not satisfying to me related to clearly explaining why, that background, but this reference on solubility of gasses at different temperatures does work for me related to pushing the explanation a step further.  It's an “Engineering Toolbox” site reference, so if it’s formulas, tables, and graphs that you’re after this is the right place:

Air Solubility in Water

That’s for oxygen, not air, but air is mostly nitrogen, then oxygen, and on from there, so if you checked solubility of those two (and maybe also carbon dioxide) you’d be getting the whole story. Wikipedia explains: air is a mixture of about 78% nitrogen, 21% oxygen, 0.9% argon, 0.04% carbon dioxide, and very small amounts of other gases.  There is an average of about 1% water vapour.

Lots less air can be dissolved or stay dissolved as temperatures go up. It’s my understanding that the part that’s still missing in that reference is that it’s not just about gradual heating, that the contact surface of a pan (nucleation site) heated to much higher than water boiling point provides a driver for this deaeration.  This is which is why you see bubbles of air evaporating out on that surface before actual water vapor transitions phases (although again that’s my take, and I’m bored with looking up these references).

That top part of that last chart graphic is something else altogether, about how much air can be dissolved at different pressures, with water able to hold about twice as much dissolved air at two times typical atmospheric air pressure. I’m not sure when that would come in handy to know. I lived at well above sea level before (at around 8000 feet in the Colorado mountains), and air would hold slightly less dissolved air there, because there was less air to breathe too, much lower pressure.  So I guess your tea would be slightly less frothy if you microwaved the water there, but it would still be a bad idea.

Taking this the next step:  how much air is in the water  

That I don't know, but we can start in with estimations.  

That graph showed air solubility (actually that was just oxygen), and related calculations on that same page run through air solubility in water, at 25 C, or 77 F.


All their calculations for amount of dissolved air under those conditions follow:

Oxygen dissolved in the Water at atmospheric pressure can be calculated as:

co = (1 atm) 0.21 / (756.7 atm/(mol/litre)) (31.9988 g/mol)

= 0.0089 g/litre, or ~ 0.0089 g/kg

Nitrogen dissolved in the Water at atmospheric pressure can be calculated as:

cn = (1 atm) 0.79 / (1600 atm/(mol/litre)) (28.0134 g/mol)

= 0.0138 g/litre, or ~ 0.0138 g/kg

Since air mainly consists of Nitrogen and Oxygen - the air dissolved in the water can be calculated as:

ca = (0.0089 g/litre) + (0.0138 g/litre)

= 0.0227 g/litre, or ~ 0.023 g/kg

I had originally left out the oxygen part in showing this, and had totally overlooked that there is much more oxygen solubility in water than nitrogen.  That's not relevant at all here, really, but still interesting.  It would mean that oxygen solubility curve in the graphic shows off a lot more about what's really going on with both combined.

Volume of air dissolved in the water

It might have worked to just say that dissolved gas (air) is "only a little," and move on.  But I'll go further here, and consider exactly how much, in terms of volume.

.023 grams of those two gasses per kilogram (per liter of water) isn't much.  Per this Wikipedia reference air weighs 1.22 kg / cubic meter (at sea level and 15 degrees C), with this reference going further related to pressure differences (we can skip that part). 

That also works out to 1.22 grams per liter (really, the metric system is amazing).  It we take .023 grams (the amount of air dissolved in a liter of water) and multiply it by 1 liter volume / 1.22 grams (convert that mass amount of air to volume) we get .019 liters.  That's how much air is dissolved in a liter of water, which of course varies by temperature and pressure factors; it's just a standard range amount.  Not much.

If it's a 250 ml cup (around 8 ounces) you might only be seeing a fourth of that much air bubbling out, .005 liters.  Maybe it's more intuitive to express that as 5 cubic centimeters, or around a third of a cubic inch.  Only a little, and even less since all of this is about how much was in the water initially, not what's left after microwaving.

That's assuming there is air is still in the liquid, that gradual heating without a hot contact surface not cause it to extract out is the problem, which leads back to all this just being one possible explanation.  The whole point here is to explain how it works, to what extent it really could happen, and get into details, but it's not exactly a demonstration that some form of supersaturation is happening.

Beyond the tea bag making the water fizz when you drop it in; that does counts as evidence.

How much carbon dioxide is in Coke (soda)

Maybe you were just wondering that, for comparison, or maybe not.  Obviously there is a lot more carbonation in soda (or beer) since the foam from pouring those over ice can be of more volume than the liquid, and the drink still isn't completely flat.

Per this reference there is 2.2 grams of carbon dioxide in a 355 ml Coke per can.  Carbon dioxide's density is more like 1.9 grams per liter, slightly more dense than air, but it still works out to around a liter of that gas (carbon dioxide) that's compressed into roughly a third that much Coke.  Note that 2.2 grams estimate is just a good guess;  different sources site numbers like that but Coke doesn't confirm any specific amount, since to them it's a trade secret. 

This is why soda feels so fizzy in your mouth, and that dissolved air in tea (from microwaved water) may or may not be the cause for that odd feel.  There is almost no gas in it compared to soda.  To put numbers to that a 355 ml Coke should contain more than a liter of carbon dioxide, and I estimated a 250 ml cup of tea (around an 8 ounce cup) might only contain .005 liters of dissolved air, that one third of a cubic inch.

But again, those tiny bubbles fizzing when you put a tea bag in a microwaved cup of water don't amount to a lot of air volume, but it seems quite possible the feel of air leaving the liquid could contribute a lot of negative "feel" effect.  Texture plays more of a role in a tasting experience for teas than one might expect, even without any "outgassing" as a factor.  It's my guess that this is what makes the microwaved water tea experience so awful.