Hello. I have several chemistry questions about tea, water, and oxygen, which are too technical for the rec.drink.tea group. A common warning to tea drinkers is that over-boiling water in a kettle will cause the water to lose its dissolved oxygen, resulting in "flat-tasting" water that is unsuitable for brewing a decent cup of tea. Tea aficionados recommend that water for black tea be heated just to the boiling point, and the water should then be immediately removed and poured over the tea. There is some controversy as to whether "heated just to the boiling point" means the first bubbles are appearing or the water has attained a full rolling boil. The reasoning given for using boiling water for black tea is that the high temperature increases the rate at which the "good-tasting" chemicals in the tea diffuse into the water. Steeping (soaking) the tea in the water for too long will cause bitter "bad-tasting" chemicals to dissolve into the water. The goal then is to maximize the diffusion of the good chemicals into the tea in the 3 to 5 minutes before the delayed bad chemicals begin to diffuse. For oolong and green teas, however, the temperature recommended is "somewhat cooler than boiling" to prevent "cooking" of the more fragile leaves. When pressed, knowledgeable tea drinkers suggest 180-200F (82-93C) as the recommended range for steeping greens and oolongs, with little explanation. This is about the temperature that water attains just after being poured into a room-temperature mug or teapot. As a result, for black tea one should warm the pot with boiling water (and then discard this water) before pouring water into the pot (or mug) over the tea leaves. Pouring water over the tea is imagined as partially reoxygenating the water, especially when the loose tea is used. (Dunking teabags in lukewarm water is considered sacrilege.) Pouring the water over the leaves is also expected to dislodge air bubbles that might otherwise adhere to the leaves (thus lessening the surface area between leaf and water, resulting in a reduced efficiency of the steeping). In theory, some of the above seems reasonable to me, while other parts sound less plausible or just likely to be ineffective. In practice, I (and many others) have found that following these recommendations produce good cups of tea, and ignoring these recommendations produces a poor or only passable drink. As an engineer and tea lover, I want to find a more objective way of evaluating how effective these recommendations are. Some questions I have are: 1) How does the concentration of dissolved oxygen change as water is raised from below room temperature (from the cold water tap) to boiling? 2) How does the concentration of dissolved oxygen change as water is poured over tea? 3) How does the concentration of dissolved oxygen change as the tea is left steeping? (That is, as more acids and other chemicals leach into the water from the tea leaves and as the water slowly cools.) 4) What temperatures will be best for the steeping of various types of tea? How important is temperature for making a good cup of tea? I've speculated on these questions, but only have guesses as to their answers. I'm sure I could easily settle the matter if I had an oxygen probe in my kitchen, but alas I don't, and I'm also interested in an analytical approach. One might also ask if it is really a lack of oxygen (and not, say, a lack of nitrogen) that is responsible for flat-tasting water. For question #1, I've checked a CRC (72nd edition) for information on the solubility of oxygen in water (pg 6-4). I've plotted the equation for solubility vs temperature, but the text states that the equation applies only for temperatures between 273.15K (0C) and 348.15K (75C). (See http://web.mit.edu/rjbarbal/Tea/oxygen.gif; the green graph is the range for which the equation is valid.) Does anyone here know what happens above 75C? It appears that most of the dissolved oxygen will have been lost by the time the water reaches 75C, and that boiling would cause little additional loss. Kettles, however, are usually covered (with perhaps a small opening for a whistling steam release). Would this nearly fixed volume create enough of a pressure change within the kettle to alter the solubility of the oxygen? Also, how would the change in the partial pressure of oxygen (mostly trapped inside the kettle) affect the solubility? Finally, once the kettle lid is removed for pouring the water into the tea pot, how will the sudden change in pressure affect the solubility? How fast can substantial changes in dissolved oxygen occur in these processes? For question #2, I have studied situations similiar to pouring water over tea. Water flowing over a weir will be reoxygenated (due to entrainment of air bubbles into the stream), with the amount of air added to the water dependent on many factors including the height of the water drop, penetration depth of the stream, and amount of splashing. I cannot, without experimentation, design a model appropriate for the scale of water poured into a teapot, but I suspect the reoxygenation is not significant. Does anyone have a different idea about this? For question #3, I'm really not sure how the addition of solutes would affect oxygen solubility, or how effective the slow cooling would be in changing oxygen content. Can anyone offer advice on this? For question #4, I'm really not sure how dependent the quality of tea is on the temperature of the water. The temperature would certainly affect how various substances diffuse from the tea leaves into the water, but so many chemicals are present in tea that evaluating all the reactions is probably not feasible, and of course there is much subjectiveness in evaluating how the tea tastes. I think, however, that it should be feasible to model how quickly some interesting chemicals (caffeine, the pigments, bitter tannins) enter the water. Can anyone provide some direction on this last point? One reason I'm skeptical of the importance of temperature is that any difference is subtle, if noticeable at all. A tea steeped longer at a cooler temperature tastes much like a tea steeped shorter at a higher temperature. In the extreme, iced tea that is made by steeping tea overnight in a refrigerator tastes as good (to me and my fellow tea-loving friends) as iced tea that is made from cooling hot tea that has been prepared regularly. Yet, heating the refrigerator-made tea produces a drink which is far less palatable than fresh hot tea. Why? I'm also curious about the affect atmospheric pressure has on tea. I've heard people state that one cannot brew tea as good in Denver as in Boston, the reason being that in high-altitude Colorado the water boils at a lower temperature and less oxygen is in the air. I'm usually quick to respond that weather maps often show sunny Colorado with a higher pressure than rainy Massachusetts. Having had tea in both locations, I've not been noticed any discernible differences. It's been a long time since I've studied chemistry, so any help or advice on these questions and thoughts would be appreciated. I also apologize for the length of this post, but those of you who drink tea can probably appreciate the complexity of the topic. + Richard <rjbarbal@mit.edu> ---------------------------------------------------------------------- I cannot answer any of your questions, but I can add another. (<:} What effect does the changing concentration of CO2 have on the brewing process? Changes in the concentration of CO2 in the water changes the acidity of the water, and changes in acidity can have profound effects on chemical reactions and equilibria.