Stinky Chemistry

    Kids, want to see how some basic chemistry can help go a long way with making good food? We'll bet that you have never tried cabbage, but this experiment might change that. First, never buy a cabbage that has yellow spots. Where does the yellow come from? Green vegetables have two types of chlorophyll molecules: type A is bright blue-green and type B is bright yellow-green. Together this makes green. But type A is fragile and will break down and fade if it gets warm. This leaves only type B, causing the yellow spots. The same thing happens during cooking. You have to cook this stuff very carefully in order for it to taste good (yes, it can taste good). 

    Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

    What happens when you cook, and especially if you overcook, cabbage can be envisioned by heating bubble wrap. Stack and lightly tape together some sheets of bubble wrap, about 12" by 12" squares. Using a variety of sheets that have bubbles of different sizes and colors helps the demonstration. Imagine that the bubble sheets are a greatly magnified cross-section of the leaf with a lot of cell layers, each doing different jobs. Some do photosynthesis, some metabolize, some reproduce. In between the cell layers are membranes that keep things protected (one chemical away from others) and also full of liquid ("solvated"). This is a very safe and ordered world, at least until the cook comes along and turns up the heat. The next step can only be done by an adult partner. They need to support the bubble wrap on a heat-resistant, but disposable, surface of some kind (for example, coat a cookie sheet with aluminum foil and fix the bubble wrap stack down on this with one or two pieces of tape).

    Use a hair dryer to heat the bubble wrap from the top and sides. As the wrap shrinks up imagine that, in the cabbage leaf, membranes are bursting, peptic substances are gelling, enzymes are unleashed, and pigments fade. After a few minutes, sulfur-containing gases are released that fill the air with a rotten egg smell. The chaos of this widespread destruction can be controlled, however. First, have your adult partner boil a large pot with a lot of excess water and then add 1 tablespoon each of salt (NaCl) and sugar (sucrose). Then put in some shredded cabbage.

    Notice that as soon as the leaves are added to the hot water they get brighter in color. This is because the gases trapped in the cells are released, letting you see the true chlorophyll colors. Boil for only 2 minutes and drain quickly. If you go 5 minutes, leaked acids start to degrade all of the chlorophyll molecules, making the leaves gray, and sulfurous gases leak out and stink up the room. You can try one pot of each just to see (and smell) the difference. Using lots of water helps to dilute the acids that build up. The sugar helps to preserve the plant cells and keeps the leaves crunchy. We admit that adding something else after draining (maybe seasoned bread crumbs fried in butter) is pretty much essential for making this vegetable taste really good. Just realize that along the way some basic chemistry knowledge helped go a long way with ensuring success!


    Kathleen Carrado Gregar, PhD, Argonne National Labs
    February 2004


    References: The Food Network Alton Brown's Good Eats episode on Head Games