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:
1) do not always boil the water
2) can heat water beyond the boiling (212 °F) to a superheated level
3) 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:
1. avoid overcooking water or any other liquid in the microwave oven by not
setting the timer longer than is minimally necessary
2. wait one minute per cup for that liquid to cool before removing the
container from the microwave
3. be careful using spoons, powders, teabags, or anything else that
"disturbs" very hot liquid
4. keep the water away from your face and body until you're sure it's safe
5. 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
(1) the water will evaporate and you'll need to refill the pot with water or
(2) 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).
