A Nice Hot Cup of Tea

Douglas Adams and George Orwell have both given us a legacy of tea. Both have written of their recommended procedures and methods for making and drinking a great cup of tea. Both emphasize the use of boiling water but neither writer delved deeply into the chemistry of why boiling water is so crucial to a proper cup of tea.

There are 6 main reasons for the use of boiling water, each of these has a separate goal. There are 2 main principles. The first is thermodynamics……the solubility has a temperature dependence. Hotter liquid almost always means higher solubility of solids and lower solubility of gases.

The second is kinetics…..regardless of how soluble or insoluble a compound is, it takes time for something to dissolve, and they dissolve faster at higher temperature.

The 6 principles are:

1 The solubility of caffeine versus temperature

2 The solubility of tea versus temperature

3 The solubility of oxygen gas versus temperature

4 The dissolution rate of caffeine versus temperature

5 The dissolution rate of tea versus temperature

6 The evolution rate of dissolved oxygen from water versus temperature

In #2 and #4 above the word “tea” implies all substances which dissolve in water including tannins, antioxidants, and all other relevant chemicals. I separate out caffeine because it has such a strong flavor and is an important component of all the flavored compounds, maybe the most important. One key reason people don’t like decaffeinated tea or coffee is that they can taste caffeine and they notice when it is missing.

As any chemist has learned, regardless of the true solubility of a compound, making a water solution of that compound can take a long time. I personally have sat for hours stirring a solution, waiting for a highly soluble compound to fully dissolve. One way to speed up the dissolving is to heat the solution. Yes this is changing the solubility, but the rate of dissolving changes quickly as the temperature increases. Tea and caffeine do not suddenly and completely dissolve in hot water. It takes time. It might take a long time depending on the surface area of the tea leaves.

So you should leave the tea bag in the hot water for many minutes to get the tea as strong as possible. And the hotter the water, the faster you will get that delicious cup of tea.

But the key pair of principles I want to discuss here are #3 and #6: the comparison of the solubility of oxygen gas (O2) as compared with the rate at which O2 gas leaves or evolves from hot water. This post states that all oxygen is gone in hot water. That is only true for extremely hot wafer, and only after a very long time.

Any tea connoisseur knows what happens when you put a teabag into “hot water”. The water begins to fizz, and froth, and the teabag is filled with gas and it floats on the top of the liquid. It takes a real effort to displace the gas from the teabag such that it will submerge so you can swing the bag through the liquid using the string.

If oxygen gas were fully gone then the solution would not fizz and froth. This water is actually supersaturated with oxygen. This means that the amount of O2 gas dissolved in the solution is higher than the solubility of O2 in the solution. There are only two ways O2 can leave a solution: evaporating from the top of the liquid, or forming a bubble which floats to the top. Both are slow and difficult. Consequently it takes a long time for O2 to leave a hot liquid to achieve a truly saturated solution.

In my experience it only takes a few seconds of vigorous boiling to remove all the oxygen from water. The fizz/froth test proves that my water has been boiling long enough to eliminate the fizzing. I never see fizzing when I pour boiling water onto a teabag.

Will O2 gas eventually be totally gone from hot water (below the boiling point)? Of course it will. But it might take minutes or even longer. And then the water has started to cool and more gas can dissolve from the air. Nobody wants to wait that long. In my experience we all want our tea now! The quickest way to eliminate oxygen from water is to vigorously boil it for a few seconds.

Why is this so important? Why is O2 in tea a major issue? There are some who say it’s important to the taste, that oxygen in water adds flavor. This is simply wrong, because as soon as a tea bag is added the liquid fizzes and all the oxygen leaves. In practice it is challenging to make a hot cup of tea which contains any oxygen. That requires a subtle balance between temperature and time. When the tea cools down the O2 from the air dissolves into the liquid and maybe cold tea tastes different from hot tea.

But a much more important point is that tea is an extremely healthy beverage due to the antioxidants. Many people drink tea solely for the antioxidants. But antioxidants can react with oxygen. This reaction removes the healthy antioxidants. And that reaction is much faster in hot water than in cold, maybe 10 times faster. By exposing hot (but not boiling) water to tea we are reacting away the precious antioxidants which protect us from radicals. The fizzing reaction can help, it can quickly remove all the oxygen. But if the water is just a little cooler it will not fizz. I haven’t tried but maybe below 90C the water won’t fizz and the oxygen reaction can continue for a long time, perhaps completely removing the antioxidants.

Some would say that antioxidants only react with oxygen atoms and radicals. But O2 forms a few oxygen radicals in hot water, those react away, then by LeChatlier’s principle the O2 just keeps producing more and more radicals until all the antioxidants are consumed. I would enjoy reading a chemical study of the concentration of antioxidants in tea as a function of the temperature of the tea. I have found a few studies of the reaction of O2 with antioxidants but they are not specific to tea.

If we truly want the largest possible dose of antioxidants in a cup of tea then we want as little oxygen as possible. That requires boiling water exactly as Mr. Orwell and Mr. Adams have taught us. This also gives us the most dissolved tea, and the most dissolved caffeine. It’s a double play, a wonderful cup of tea with the largest possible amount of antioxidants.

So carefully boil your water and enjoy a nice hot cup of tea.

Here are a few blogs for tea lovers