Relationship between Kinds of plastic wraps and food dehydration

What You Need to Know

Polymers are long molecules made up of repeating units linked together by chemical bonds. Dehydration is excessive loss of water. Permeability is the ability of a material to allow a substance to pass through it without affecting the material. Evaporation is the change of the molecules on the surface of a liquid to a gas.

How Does a Polymer Work?

Polymers are substances whose molecules are made of large numbers of repeating units, much like a train with many cars attached. Proteins and starches such as silk and cellulose (the tough material in plant stems) are examples of natural polymers. Plastics are an example of synthetic (man-made) polymers. Synthetic polymers are formed by chemical reactions in which many separate units are joined into a chain. There are three different kinds of polymers used for kitchen plastic wrap: polyethylene (such as Glad or Handiwrap); polyvinyl chloride (such as Reynolds Wrap); and polyvinylidene chloride (such as Saran Premium Wrap).

What Does This Have to Do with Food Dehydration?

Water in food evaporates. If the food is covered with plastic food wrap, the water vapor collects in the space between the food and the plastic wrap. If the plastic wrap is permeable to water vapor, the water vapor escapes through the wrap, and the food can become dehydrated. The amount of water that escapes depends on how permeable the plastic is. In the diagram on page 69, there is more water leaving through the plastic wrap covering bowl A than there is through the wrap covering bowl B. The food in bowl A will dehydrate faster than the food in bowl B.

Real-Life Science Challenge

To limit permeability to moisture, films of polycarbonate, polyester, or polyethylene plastics are sometimes laminated together. Some also have metallic layers. Military food, packaged in such metallized plastic wrap, has a very long shelf life (five years or longer) if kept cool.

Experiment

Now, start experimenting with different kinds of plastic food wraps, and determine how good they are at preventing food dehydration.

Hints

• Before testing the wraps on actual food, design an experiment to test each type of wrap's permeability to water vapor.
• Design a method for determining the rate of evaporation.
• Using containers of water instead of food will make measuring evaporation easier.

FUN FACT

A helium balloon is actually filled with a mixture of helium gas and air. The balloon is permeable to helium but not to air. This is because the molecules that make up air are bigger than the holes in the rubber balloon, but the molecules of helium gas are smaller than the holes. Over time, usually one day, the helium will pass through the small holes in the balloon, leaving the larger particles of air behind. When the helium leaks out, the balloon is partially inflated because of the air in it, but it does not float.

Many inventions by NASA for its space missions are also part of our everyday lives. One such material (used by NASA to insulate and protect astronauts and their instruments) is now used to make the foil balloons that are filled with a mixture of air and helium. Foil balloons are made from a sheet of nylon coated on one side with polyethylene and a thin coating of metal on the other side. Unlike latex balloons, commonly called rubber balloons, the surfaces of foil balloons have smaller spaces, which keep the helium gas inside for a much longer time.

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