May 17, 2006

Zucchini: Freezing and Thawing

Lower the temperature and you slow down the growth rate of microorganisms. That's the principle of refrigeration. But, as you undoubtedly know, food can and does spoil even under refrigeration if you keep it long enough. Freezing, however, stops microorganisms from growing. It doesn't kill them, so if thawing occurs, the microbes will start growing again. (That's why thawed food should be cooked and eaten as soon as possible.)

The main advantage of freezing is that it changes food from its fresh state less than any other method of food preservation. Even so, freezing can change the texture of foods, especially vegetables. This doesn't matter much if the vegetable is to be cooked, which would change its texture anyhow. But freezing vegetables like lettuce or tomatoes that are to be used raw in salads will give disappointing results. (You don't have to take our word for this—freeze some lettuce and see for yourself.)

Frozen food companies have spent a great deal of time and money developing ways to keep frozen vegetables as much like fresh as possible. In the next experiment you will investigate the difference between your home-freezing technique and that of commercial frozen food producers.

Materials and Equipment

• 1 package commercially frozen zucchini
• 1 small young zucchini squash
• paper towels
• knife
• 2 plastic sandwich bags with twist fasteners
• freezer thermometer (optional)

Procedure

Put a package of commercially frozen zucchini into your freezer. Wash and pat dry the fresh zucchini squash. Slice into 1/2-inch slices. Put half of the slices into a plastic sandwich bag. Close tightly with a twist fastener. Put the bag into the freezer. Put the other half of the sliced zucchini into another plastic bag, fasten, and refrigerate. This is your control group, against which the effects of freezing will be compared. (The best experiments are "controlled" experiments—ones in which changes are introduced systematically, so that differences can be linked to the changes you make in the setup.)

The next day, take out the home-frozen zucchini and the package of commercially frozen zucchini. Allow them to thaw completely. This may take several hours. Compare the texture of the home-frozen squash with that of the commercially frozen and the unfrozen.

Observations

Which zucchini is firmest? Which is softest?

Frozen water has more volume (is less dense) than an equal weight of liquid water. If you've ever seen an aluminum can of soda stretched out of shape because it was left in the freezer to cool down quickly and then forgotten, you know what we mean. Plant cells contain a great deal of water. When this water expands as it turns to ice, it can damage the cell walls, so the vegetable loses its crispness.

Commercially frozen zucchini is flash-frozen at about –20° F. How cold is your freezer? (You can measure with a freezer thermometer if you want.) When the freezing process is slow, larger ice crystals form. Which zucchini must have had the largest ice crystals?

Put all the zucchini—including the refrigerated control—into a saucepan with about a cup of water. Bring to a boil and simmer for about 5 minutes. Drain and season with butter, salt, and pepper. Can you still tell the difference between fresh-cooked zucchini, home-frozen, and commercially frozen?

For Further Study

Check out the effect of freezing on meat. Cut a piece of steak in half and freeze one half. Refrigerate the other half. A day or so later, defrost the frozen steak. Compare it in appearance to the unfrozen half. Broil both and taste to see if there is a difference.

A food chemist at a meat-packing plant told us that animal cells are somewhat affected by ice crystals. The cells are enclosed in flexible protein membranes that can be broken down by sharp ice crystals, making the meat somewhat more tender. He also said that the difference was so slight he didn't think the average person would be able to detect it. Food chemists use a special instrument to measure the texture of meat.

Reprinted with permission from More Science Experiments You Can Eat by Vicki Cobb. Copyright © 1979 by Vicki Cobb (www.vickicobb.com). Published by J.B. Lippincott.

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