This Week's Experiment - #299 Broccoli Science
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 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 broccoli 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 vegetables 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 chlorophyll 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.)
From Robert Krampf's Science Education Company
PO Box 60982
Jacksonville, FL 32236-0982
904-388-6381
krampf@aol.com
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