Science | Science Experiment of the Week 9 | 299 - Broccoli Science

Green Vegetables, Chlorophyll, Chemical Structure - The science of cooking green vegetables.

This Week's Experiment - #299 Broccoli Science

These experiments are from Robert Krampf - The Happy Scientist

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.)

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