Hygroscopic Chemicals in Food Experiment

Stale Bread

Thinking about getting older and (of course) cake, I thought about an experiment with old cake. When you think about it, cake and bread are very similar, but they age very differently.

To try this, you will need:

  • a slice of white bread
  • a slice of cake

Cut a one inch square from a slice of bread. Put the rest of the slice back in the loaf. There is no use in wasting an entire slice, and the next person that makes a sandwich will go crazy wondering why their bread has a square hole in it.

Next cut a one inch square of cake. Try to get a square with no icing. You can't really put the rest of the slice back on the cake, so to prevent waste, you will have to eat it. If you are a sloppy cake cutter, it might take you several slices to get one that gives you a square that you are happy with. I never was very good at cutting cake neatly.

Place the bread square and the cake square on a plate. You might want to put a "Danger! Science Experiment" sign on the plate, to keep wandering family members from eating them. Feel the texture of both squares. They should both be soft and flexible. Put the plate someplace where it will not be disturbed.

After about 15 minutes, touch both squares again. You should notice a difference now. The cake should still be about the same, but the bread square is probably much harder. The bread is going stale. After 30 minutes test them again, and the difference should be even greater. When the cake begins to show signs of getting dry, the bread should be as hard as toast. This hardness is not just a matter of drying. As water is removed, chemical changes occur in the gluten that gives bread its texture. That is why we can't "fix" stale bread by adding water to it. You can soften it a bit, but once it is stale, you can't get back its freshness.

Why are the two substances so different? Both are made mainly of flour. What is the difference between the taste of bread and the taste of cake? The cake is sweet, right? Don't take my word for it. Have another slice of cake to be sure. That sweet taste comes from the chemical that causes the difference. Sugar.

Sugar is hygroscopic. What in the world does that mean? Hygroscopic means that the substance will absorb water from the surrounding air. As the two squares sit on the plate, water is evaporating from both of them. The sugar in the cake is collecting water from the air, replacing some of the water that evaporated. That is why it takes much longer for the cake to go stale. The hygroscopic property of sugar is also the reason that stale cookies go soft instead of staying crunchy.

Sugar is not the only chemical that is hygroscopic. Another common hygroscopic chemical is table salt. Salty crackers will get soft as they absorb water from the air. Here in Florida, it is not uncommon for the salt in a salt shaker to form a hard lump. It has collected enough water from the air to partially dissolve the grains, letting them grow together.

You could probably see quite a bit if difference in the staleness rate of different kinds of cake. High sugar cakes should stay moist much longer than less sweet cakes. Some very sweet cakes, such as angel food, collect enough water to form droplets of sugar syrup on their surface. You could probably make a very tasty science fair project out of that idea.

Have a wonder filled week.

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