Any chemists about? (water solvency & O2)
Offline I have a question that I'm hoping someone with knowledge in chemistry can answer.
I've been hearing on science podcasts and from tea buffs that one should not boil water fully (when making tea) because it drives off oxygen, and so it produces less robust tea. I should note that on the science podcasts nobody has said anything definitively on the subject, only that it's a hypothesis. Tea buffs will also talk about using the same water when it's refrigerated will produce better results than if left to warm to room temp before heating, and other such things.
While it's true that using water that is cooler than boiling temp does produce a fuller-bodied tea, I've seen some information lately that seems to contradict the oxygen hypothesis. Namely that some of the very good quality teas will take fully boiled water, and that boiling the water and allowing it to cool produces the same results. I've found the later to be possibly/probably true. I've seen a few other things as well, but those are what stick out in my memory. I can understand heat driving off volitile compounds and extracting more of some chemicals than others, but I'm beginning to be skeptical about oxidation during infusion. Oxidation plays a big part in production of the dry leaves (the darker the tea, the more oxidized), but the infusion?
As a tea buff myself, I plan on doing some experiments with measuring the oxygen content of water and tasting, but I'm wondering if anyone can give me some ideas about the science behind this? Namely, does the process of extracting water soluble compounds involve any kind of oxidation? Would oxygen play any role?
If anyone needs to know more about the compounds in tea, here are some articles about it:
http://www.fmltea.com/Teainfo/tea-chemistry%20.htm
http://lpi.oregonstate.edu/infocenter/phytochemicals/tea/
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I'm asking the question around elsewhere as well, and I got a partial answer from one scientist (although I don't believe he is a chemist) that noted the effect of brownian motion in tea, that the tea leaves will move around in water just under the boiling point, except that it does not occur when the water is over-boiled. Searching around, I did find one small mention that oxygen may be involved in brownian motion in water, but I can't find any details, sources, justification, or anything else. This would indeed make some sense, but I could also imagine this having to do with the consistency of the heat throughout the water (not enough variation in the fully heated water to cause a significant amount of brownian motion).
So perhaps the question can now be extended to: Does oxygen in heated water play a significant part in brownian motion?
The oxygen would not play a significant role in the extraction process. That process is simply solvation of the tea compounds. Also, the brownian motion would be even higher at boiling point, since the convection currents would be much stronger. I don't think this would be a factor unless you used a very short extraction time (~ a few seconds) and even then would be minor in relation to the temperature term. You did not mention the extraction time. How long do you leave the bag in for?
Since you said that the same result can be obtained by cooling the tea, there are two possibilities for the taste change that I would consider. First, the temperature and/or oxygen content are affecting the binding of your taste receptors to the compounds that you taste. Secondly, there is a reversible oxidation of a certain compound that has a tipping point in its change in gibbs energy in the temperature difference range you are talking about. This is unlilkely, though.
Yes, the oxygen concentration will be higher at the lower temperature. But, unless you have a way to perform the extractions in an oxygen-free environment with degassed water, you can not isolate temperature and oxygen content. You should repeat the experiment of cooling the hot tea and see if that giveas the reported results.
Hope this helped
Perhaps a simpler solution is that differences in temperature change the infusion process itself. Brewing coffee at different temperatures changes the flavor supposedly because the amount of oils extracted at 100 C is more than at 80 C. Optimum temp for coffee, at least at Dunkin Donuts, is between 80 and 85 C.
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There can't be a difference in the extraction process. He stated that the same result is obtained when the hot tea is cooled.
He said that high quality teas weren't affected. Maybe at some point the quality of the teas overwhelms the other factors. To do a proper experiment, he would have to find a tea that brews to a different quality depending on how the water is handled. Then he could begin to isolate different factors that the temperature might be influencing.
I think he'll be drinking a lot of tea.
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Different temperatures very definitely affect teas differently. The lighter the tea (eg, green tea), the lower you want the water temperature. Too hot of water can and will make the tea bitter, sour, astringent, and overall flat.
What some people are alluding to, however, is that if you, for example, had one pot of water that was heated to 180 F, and another that was fully boiled and then cooled down to 180 F, the first should produce tea that is more flavorful and robust. They go on to say that you should never ever use double-boiled water, even if it is then cooled down to the proper temp.
For green teas it's definitely water temp, but for the more oxidized teas (oolong, black, pu'erh), it's supposed to be mainly about oxygen, with the notion being that you should stop heating the water just prior to boiling for black and pu'erh tea.
The high quality teas that I'm referring to are the kind of super high quality that people don't usually get outside of Asia. Now that's fine for green tea, but the others that oxygen is supposed to be the main factor?
You could say that cooled water would regain some oxygen, but as an aquarist I know that only happens with a significant amount of disturbance of the surface of the water (which causes the exchange of gases until equilibrium is reached).
My thought was the same as Monkeyfishfrog's, that oxygen shouldn't play a significant role in the extraction process, particularly when you're only steeping the tea for 1 or 2 mins (and sometimes only seconds), although I could be wrong.
The brownian movement thing is interesting. The scientist I got a partial response from is from the International Society of Tea Science (they do studies for people in the industry and such), and he noted that if you brew black tea with water below boiling that the leaves will float, over-boiled and they will all sink, and if the water is just at boiling then some will sink and some will float. Doing some research on this they attribute this to brownian motion, and note that some will rise back to the top. Strange, huh?
I'm actually going to do some testing of my own. I had to get pet supplies anyway, so ordered a test kit to measure oxygen levels in water. So I figure I'll do some experimentation, but I plan to do a writeup and hope to be able to back up my findings with some science as well. I wish I could test more things about the tea, as that would be the most revealing, but perhaps I can, at least, check the water hardness to see if that's a factor at the different levels of heating. The hardness of water can very definitely affect tea, sometimes dramatically.
Thanks for your thoughts so far, guys. It can really help to have other perspectives. You've given me some additional things to consider while experimenting.
Sounds like fun.
Maybe you could artificially alter the oxygen content by bubbling O2 in the water. You could open up a new tea parlor where you charge extra for oxygenated tea.
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From a strick chemical POV, higher temps will give you more diffusion and better bang for your tea buck. The hotter the solvent, the more flavor you should extract - what the temperature actually does to the flavor, I cannot say, and this may be an important consideration.
As a very simply approximation, we can look at Fick's law and the Stokes-Einstein eqution.
S-E is a good indicator of diffusivity:
D=kT/(6Piv)
Where the only values that matter are T, temperature and v, viscosity. One obviously gets more diffusion in a more active, hotter, medium.
There are plenty of other diffusivity coefficient equations that take into account other factors such as hydroden bonding and polarity, but the E-S is a great first approximation.
You will get MORE out of your tea bag the hotter the solvent is. But like I said, I cannot say what that heat may do to the flavor of the tea.
I'm an engineer, and not a botanical chemist.
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Brewing supply shops (the kind that deal supplies for making beer and wine) sell a lot of different kinds of devices for aerating liquids. Some of them are as cheap as aquarium pumps, and some are a lot more elaborate. So if you're serious about trying out this experiment, that might be a place to get the equipment.
The claim is that aerating your beer wort before adding yeast makes the yeast thrive more readily (and produce more alcohol) because more oxygen (?) is in the wort. It supposedly also improves the flavor. I'm not sure how scientific it all is, and I've never had trouble making beer from unaerated worts, but brewers do have some thousands of years of tradition backing them up.
Can you even taste oxygen?
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@Yellow#5:Interesting. That will definitely help to give me some background on the process as well as some reading to do.