This week we will take a look at the science of cooking green
vegetables. Now, many of you may be saying "Ick!", but even if you
don't like to eat them, it is still interesting to watch them cook.
You will need:
* a pot
* water
* fresh broccoli or some other green vegetable
* baking soda
* a timer or stopwatch
Fill the pot about half-full of water and place it on the heat. Cut two
or three small pieces of broccoli and rinse them. Wait until the water
starts to boil and then add the broccoli and rinse them. Wait until the
water starts to boil and then add the broccoli and rinse them. Wait
until the water starts to boil and then add the broccoli and start your
timer. Watch the broccoli carefully and notice what happens to the
color. After a few seconds, put the lid on the pot. Boil the broccoli
for 15 minutes. Then remove it and place it on a plate. Look at the
color again.
When you first added the broccoli to the water, it turned bright green.
This is not due to any chemical changes. Instead, the hot water causes
tiny air bubbles between the plant cells to expand and escape. This
gives you a much clearer view of the green chlorophyll that gives the
broccoli its green color. It is similar to comparing a dry pebble to a
wet one. If the water is in direct contact with the colors, there is
less scattering of the light and you see them better.
After the broccoil has cooked, the color has changed again. Now the
green is much duller. In fact, it probably turned olive green. This is
because the chlorophyll changed its chemical structure. The heat makes
it easy for the chlorophyll to lose magnesium. The magnesium is
replaced by hydrogen from natural acids in the plant. This chemical
change causes the color to change.
Empty the pot and start with the same amount of fresh water. Repeat the
experiment, but this time, add some baking soda to the water. Again,
the broccoli turns bright green when it enters the boiling water, but
even after 15 minutes, it is still bright green.
Why did we add baking soda? The hydrogen that caused the color change
came from natural acids in the broccoli. Adding the baking soda will
neutralize the acids, causing the color to stay bright green.
Wonderful! Now we know how to make perfect broccoli, right? Wrong. Look
carefully at the broccoli cooked in the soda water. It is very soft and
mushy. The soda does more than neutralize the acids. It also weakens
the cell walls, making the broccoli fall apart. This works with other
green vegetable too. Normally, this is a bad thing, but it is a vital
part of "mushy peas," a favorite dish when we visit New Zealand.
So, how can you cook the broccoli and keep it nice and green? First, be
sure that the water is boiling before you add the broccoli. The enzymes
that help the chlorphyll break down are destroyed by boiling, so you
want the water to keep boiling as you add the vegetables. Add
vegetables a few at a time, so the water does not cool too much. Use a
lot of water. This will help keep it from being cooled too much by the
vegetables and will also dilute the acids. Don't put the lid on the
pot. That lets the acids boil away instead of having them condense and
drip back into the water. Cut the broccoli into small pieces. That will
help it cook faster. Do all that and you should get nice, tender
broccoli that is still a nice shade of green. Add some butter and
cheese sauce and you have a wonderful reward for your success with the
experiment. (OK, it's not chocolate ice cream, but it still tastes very
good.)
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these
experiments on my web site.
