May 2002 Issue
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Kettle or Microwave: Water for Tea

by Diana Rosen




I confess to a marked distrust of microwave ovens that dates to the time I saw a cake-baking demonstration using a countertop microwave oven. Midway through the demonstration, it blew up! Acrid smoke filled the room and the pathetic spice cake batter seeped over and under the pan, coated the revolving tray, and splattered the oven's ceiling and walls thoroughly. Since then, I've used them infrequently, and with extreme caution.

That's an emotional reason for my aversion to the microwave oven.

Lots of people, I have learned, use the microwave to heat water for tea!

I tried it and this is what I discovered: The water is always too hot. Often, it does not boil. Worse, to my palate, it makes tea taste "flat", becoming more colored water than the flavorful, complexly layered beverage that tea can be. This flatness, I believe, is directly related to the way microwaves heat liquid.

That's an assumption.

I have now discovered facts to support my theory. Heating water in a microwave, even using the beverage button, is done by microwaves going in and out of the water. These waves elevate the temperature of the liquid but:

  • do not always boil the water
  • can heat water beyond the boiling (212°F) to a superheated level
  • leave no room for adjusting temperatures--by sight or sound of the water heating--as one can do heating water with a kettle. (This makes it extremely difficult to heat water to under boiling or at low temperatures such as 160-185°F for delicate whites and greens. Even if you boil water and then allow it to cool to the lower temperatures for whites and greens, it is almost impossible to do this with microwaved water.)

The scientific process for boiling is the formation of steam bubbles, called "nucleation."

If a steam bubble forms inside the hot water, water molecules can evaporate into that steam bubble and make it grow larger and larger. That's why boiling only occurs at or above water's boiling temperature.

"If a steam bubble forms" is not, however, an absolute by any means. Even in a cup of water heated in a microwave many degrees above the boiling temperature, nucleation might never happen because it requires a defect in the cup or an impurity in the water to help those first few water molecules to form the seed bubble. If there is no nucleation of steam bubbles, there will be no boiling and therefore no effective limit to how hot the water can become. This is called superheating, or what my gut reaction termed as "too hot."

This superheating, in addition to making questionable brews, also can be dangerous. When water (or previously brewed tea---arrghh!) is heated to the superheating level it can spontaneously "explode". This can happen if the cup is removed too quickly from the oven. There have been many cases reported in the media where liquid in containers have exploded inside the oven, all over counter, or worse, exploded onto hands, faces or most frighteningly, on eyes, causing serious burns and other injuries. These injuries can easily be avoided by taking a few precautions:

  • avoid overcooking water or any other liquid in the microwave oven by not setting the timer longer than is minimally necessary
  • wait one minute per cup for that liquid to cool before removing the container from the microwave
  • be careful using spoons, powders, teabags, or anything else that "disturbs" very hot liquid
  • keep the water away from your face and body until you're sure it's safe
  • do not hover over the top of the cup or container, especially if you are unsure of the temperature of the liquid.

"Using microwave ovens properly" means not heating liquids too long. Opt for the beverage setting, allow the container to rest before removing it from the oven, and test the liquid for heat by using a spoon to take a sip of the water rather than leaning into the cup and taking a sip with your mouth.

Air molecules normally dissolve in water and contribute to its taste. Heating (or freezing) the water causes much of this dissolved air to come out of solution as gas and leave the water. That's the origin of the tiny bubbles that form on the walls of a pot of water as you heat it for the first time (they're not steam bubbles; they're air bubbles).

The hotter you heat the water, the larger the fraction of dissolved air you drive out of solution. By superheating water in the microwave, the dissolved air content may be reduced further than it would in a stove-top kettle. The result, as my tastebuds aver, is "flat-tasting" water.

A stove-top kettle heats water from the bottom of the kettle (where the electrical heating element or gas flame is) to the top. When the water is uniformly hot, bubbles rise to the top and vapor pressure continues causing steam to escape from the kettle, often in a whistling sound to tell you that the water is boiling.

So, which method will you choose for heating water for your tea? I suppose if you really are in a rush, and you really are comfortable with your microwave and you know just how high to heat it and exactly when to turn it off, you will continue. But ask yourself: Should tea preparation be about speed, or convenience, or expertise in the science of microwave energy? Or, should tea preparation be about heating water to release the depth and complexity of flavor in the leaf?

For this tea drinker, I suggest making time for tea, and that includes time for water to heat in a kettle. The worse things that can happen while boiling water atop the stove are:

  • the water will evaporate and you'll need to refill the pot with water or
  • if not taken off the burner, the pot will burn.

Neither will harm you.

More significantly, heating water for tea conventionally will engage you in the timeless rhythm of tea making. What more fulfilling ceremony could there be than to experience fully the sensual anticipation of listening to the water heating, choosing just the right cup or pot to match your choice of tea, watching the leaves steep their essence into the water, inhale their sweetness, and, ultimately, savor the pleasure of tea?

Some of the details about microwaving liquids in this article are from the engaging "Dear Lou" columns on microwave energy written Dr. Louis A. Bloomfield, Professor of Physics at the University of Virginia on the web site, where you can learn more about how and why microwaves heat foods and liquids. He has written a wonderfully accessible book, How Things Work: The Physics of Everyday Life, 2nd Edition (Wiley, New York, 2001).