Why is tea made with microwave-heated water so lousy compared to tea made with water boiled in a kettle?
Because a proper cup of black tea must be made with water that's come to a rolling boil. A kettle is designed to heat water evenly to 100 C. Heat at the bottom of the kettle creates a natural convection current: The hot water rises and the cool water falls in a cyclical fashion, which uniformly heats the contents of the kettle to a boil.
But microwaves don't heat water evenly. Microwave ovens shoot tiny waves into the liquid at random locations, causing the water molecules at those points to vibrate rapidly. If the water isn't heated long enough, the result is isolated pockets of very hot or boiling water amid a larger body of water that's cooler.
Why is water temperature so important to good-tasting tea? When tea leaves meet hot water, hundreds of different compounds that contribute flavour and aroma dissolve and become suspended in the water. Black tea contains two kinds of complex phenolic molecules, also known as tannins: orange-coloured theaflavins and red-brown thearubigins. These are responsible for the colour and the astringent, brisk taste of brewed black tea, and they are extracted only at near-boiling temperatures.
Overheated water results in bad tea, too -- and this is also easier to do in a microwave than in a kettle. The longer water boils, the more dissolved oxygen it loses -- and tea experts say that dissolved oxygen is crucial for a bright and refreshing brew.
-- Washington Post-Bloomberg