Does Bouncing Your Tea Bag Actually Do Anything Substantial?

 https://www.forbes.com/sites/quora/2012/12/04/does-bouncing-your-tea-bag-actually-do-anything-substantial/#460b0f3e62f5

This is a fairly complicated question to answer, one which I spent a considerable amount of time researching about fifteen or so years ago.
First, tea bag tea largely obeys first order kinetics, so the rate of dissolution slows down as the concentration of tea rises. This is governed by the Noyes-Whitney equation ( http://en.wikipedia.org/wiki/Dis... )

The idea of exposing it to fresher (unsaturated) water and thereby making it dissolve more tea would be correct if Cb, the concentration in bulk solution, ~= or approaching Cs, the concentration at the surface. Tea, however, contains a series of layers - the boundary of the leaf and the boundary of the tea bag both matter.
At the instant you dip the teabag, there is no water inside the bag. This is the first problem: the tea bag material is porous, but is also relatively hydrophobic, so it takes a bit of time and energy for water to diffuse across the boundary of the bag. Different papers have different wetting coefficients, and this is the first reason you might want to "dunk" the tea bag, to encourage water to enter the bag.
At the boundary of the leaf, you are faced with another hydrophobic barrier: the slightly waxy cuticle of the tea leaf. Large, orthodox-style leaves have more of this boundary intact, and so the water will take time to diffuse into the leaf. CTC tea is processed in a way to crush the outer layer, allowing slightly better wetting characteristics (and a handful of other properties conducive to a redder, more flavorful brew). There is also a significant amount of variation in how the leaf was fired (dried) after fermentation; you want to remove moisture so it doesn't go moldy or spoil, but the more moisture removed from the leaf, the more "closed" the cell pores will be and the harder/slower to brew. So tea from the same plant might vary a lot depending on how long it spends in the firing kiln; it's like french fries: some are crispy, and some are just barely cooked. It's easier to fire a uniform CTC than an orthodox leaf.
Inside the leaf, wetting of the leaf increases Cs. Diffusion and brownian motion into the leaf decrease Cb. Swelling also increases A. So as the leaf expands, the rate of brewing can change a lot. At the instant the leaf hits the water, Cb=Cs=0 and dm/dt is slow. As the leaf swells, Cs rises, and dm/dt goes up to diffusion limited rate - how quickly water can get in and lower Cb. As the leaf gets fully extracted, Cb rises and dm/dt slows. This bell curve will vary a lot depending on how fine the leaves are, and the exact properties of the leaf. For most tea bag tea, it will look more or less like a straight line and less like an "S" on a semilog plot of concentration versus time - an even extraction over most of the time.
Once the leaf is wet, the Noyes Whitney equation takes over, and the speed of diffusion becomes rate limiting. However, tea compounds are far more soluble than you would think, so in general, Cs is generally >> Cb until most of the tea is brewed out of the leaves.
So, to answer the question, does dunking matter? There are a couple of competing hypotheses:

1. Dunking mixes the tea, reducing the concentration around the leaf, encouraging dissolution.
2. A wetted teabag on the surface of hot water will, because the hot water rises and the heavier and slightly cooler tea solution falls, set up a circulation loop where the concentrated tea will fall to the bottom of the glass, keeping "fresher" water nearer to the leaves.
3. A teabag on the bottom of the glass will be in a stronger solution and will inhibit less desirable components from leaching out of the tea as fast, and will taste better.
4. Hard water will affect the ability of tea to dissolve, whereas soft water will penetrate the leaves more readily, so the water hardness matters more than the brew time. (Conversely, hardness can increase pH, and tea compounds are more soluble in high pH solution.)
5. Tea leaves will swell and fill the bag, and the "pores" won't open, preventing the tea from brewing correctly. Boiling water can even cause the bag to burst from swelling.
6. Boiling water will "burn" the tea components (or conversely, water that isn't hot enough won't "dissolve" the tea components.

1 and 2 can be observed experimentally with simple home experiments - try it yourself! 3, however, turns the logic on its head - there is a claim that the "bitter" parts of the tea are "slower," and that's why if you leave a teabag steeping for a long time or at too high a temperature, the taste is bitter. 4-6 raise uncomfortable issues - does the bag matter? What about the water?
I have made many attempts to design experiments to change all of the observed factors above, and tried to measure various properties controlling for all the other variables. In general, I have observed the following:

1. Tea leaves do swell in hot water. If you don't allow the tea bag to expand with it, you will reduce A (inside the leaf) in the Noyes Whitney equation, and the rate of dissolution will be lower. However, both Pyramidal and Constanta (pic above) bags perform nearly identically, and "pillow" bags perform noticeably poorer (except with fine "dust" grade tea), when using CTC or broken orthodox leaf. Orange Pekoe and larger sizing tends to perform better in a Pyramidal bag. A correctly constructed and filled bag should not burst, and is more dependent on the machine used to fill and the choice of paper than any inherent difference.
2. Once the bag and tea is wet, diffusion takes over. In almost all circumstances, dunking versus teabag at bottom or top of cup does not matter, because for "weak" cups of tea (1:100 is rule of thumb for black tea, generally 2.2 g of tea in teabag to ~240 ml ( 8oz 240g) of water) tea is nearly diffusion limited all the way to 1:10 ratio, ten times stronger than a consumer normally drinks. NB: this assumes the bag is squeezed at the end in the same way in both instances, as if you leave the teabag completely undisturbed the concentration is very high inside the teabag, and can be "pulled" out to some extent with the spent bag causing significant statistical variance. This is the essential secret of multiple brew of loose leaf - there is considerable flavor left in the layer close to the surface of the leaves, left undisturbed because the leaves are not squeezed.
3. Dunking gives you something to do. As with "a watched pot never boils," the perception of time decreases if you are dunking versus watching it float. Why does dunking seem to impact the color, then? It doesn't - if you let the bag steep idle, the tea will diffuse into solution around the bag, and a quick swirl will darken the rest of the cup to approximately the same level as constant dunking.
4. All other factors controlled, green tea and black tea in a teabag brew at the same rate. Dissolution is temperature sensitive, and about a +10 degree change in water temperature will roughly double D, the dissolution constant, and halve brew time. If you brew a green tea at 80C for 2 minutes, you will not observe a (statistically significant) difference in taste versus 1 min at 90C or 30 seconds at boiling. It's not a perfect doubling, and leaf and bag wetting do modify this a little bit, but in essence, you cannot "burn" tea - you extract more with longer times and hotter temps, and any tea will become "bitter" if steeped too long. Bitter is somewhat concentration dependent - so doubling the amount of leaf will increase the "bitter" or "scalded" perception nearly linearly.

Within statistical error, under almost all testing conditions, I cannot find a difference between dunking and not dunking under controlled circumstances, so do it how you want. There is almost as much statistical noise in the wettability of the paper and the leaf from one bag to the next as there is in dissolution rates, and small changes in manufacture usually matter more than anything the consumer on the teabag side can control. The tea taster responsible for blending the tea bag can usually "blend" these variations away from year to year by using tea from a number of places around the world.
The act of brewing, the ritual, deeply influences the perception of an individual. If I'm rushing and dunking, I'm probably a little stressed and am not going to feel the relaxing properties of tea - and I'm going to gulp it down and get a quick buzz. If I am slow, meditative, I'm less likely going to get a heavy buzz from the tea until I drink a whole lot of it, just like I can drink more alcohol if I sip it slowly versus pound it down. I'm going to see the tea as calming, relaxing, because I'm moving more slowly and enjoying the ritual. There are even people who insist dunking will "bruise" the leaf and make it taste different, which is utter nonsense. You just have to look at how tea is made to realize that if "bruising" were possible, all tea would be black and blue.
Snobs turn their nose up at teabag tea, but that's just silly: the reason teabag tea is usually "gross" is because it sits unprotected on the shelf for ages and goes stale. I had a relative who complained that the "new box of Lipton" wasn't right, didn't have that nice "Orangey Pekoe" taste - when I discovered she had her "old" box for years and it sat mixed together with the orange flavored "Constant Comment" in her drawer.  Fresh tea matters - and fresh tea bag tea from Orthodox BOP, dust, and fannings is indistinguishable to me from fresh Orthodox loose tea from the same field graded OP.
There's also a perception that the tea used in teabags is what is leftover from making good tea, and this is largely a myth. Dust and fannings from conventional manufacture don't tend to stay in a teabag, or worse, block the pores of a teabag and cause it to explode. Teabag tea is made specifically for teabags, and varies in quality as much as wine from grapes does. The tea leaves must be prepared in a way that reduces the size of the leaf but does not produce an excess of fannings or dust. In many cases fannings and dust are used to help fire the drying kiln, and are "waste."
It's also a myth that CTC=teabag tea=cheaper. CTC is actually fairly intensive compared with automated, orthodox-style knives. CTC was developed to reduce the significant, back-breaking labor of getting a uniform crush on the leaves after withering, because the tea would not be red if the portion of the cell that contains the catechins, and the portion of the cell that contains polyphenol oxidase, couldn't be opened up and combined. Orthodox manufacture tends to produce a lighter, more yellowish tea, whereas deeper red brews are generally more preferred by most tea drinkers, particularly when adding milk.
(Think red versus white wine: red wine typically leaves the skins in, and white wine typically takes the skins out, and both can be produced from the same red grape. http://antiwinesnob.com/wine-art... )