Kitchen Theater Science in a Scoop

by Claire Spampinato

Most scientists would not consider eating their experiments, but that happens every day in the Kitchen Theater at Carnegie Science Center. Several times daily, visitors gather to explore the chemistry and physics of popular recipes, while also learning the history, folklore and nutritional value of foods. The balmy days of summer are perfect for trying out one of the Kitchen Theater's most popular recipes, "Freezer Baggie Ice Cream." No ice-cream maker is needed, just a few simple ingredients and a little energy. Even the smallest children can help.

Freezer Baggie Ice Cream

You will need:
* two zip-lock freezer bags: a one-gallon bag and a one-quart bag;
* ice (crushed ice works better than cubes);
* salt (any kind)
* ice cream mix (recipe follows)

Step 1: Prepare ice cream mix.

Step 2: Pour one cup of mix into the smaller, quart-size bag. Seal well, but leave some air in the bag. This air will be mixed into the ice cream mix, giving it a light, smooth texture. In the ice cream business, this incorporated air is called "overrun." In economy brands of ice cream, this overrun can be 50-100 percent, which means that the half gallon you buy is only one quart of ice cream mixed with a quart of air. Premium brands and home-made ice creams have overrun of only 20-50 percent, so you get more ice cream and usually pay more.

Step 3: Place the bag containing the mixture into the larger bag, and alternately add ice and generous amounts of salt (6 TBSP minimum) around the interior bag. You may have noticed that ice cream melts faster than other foods, and here's why: The fat and protein (cream and eggs), along with the sugar in ice cream prevent the normal formation of ice crystals at 32¬ F. Salt works to lower the freezing point of the water so that the ice cream will freeze. Energy is needed for the salt crystals to break into particles small enough to dissolve in water. This energy is obtained by removing the heat from the water, making it colder, and lowering the freezing point at the same time. The same principle applies to salting roads in the winter: the salt lowers the freezing point of the ice and prevents it from freezing above about 20¬ F.

Step 4: Seal the large bag, and begin gently shaking it, passing it on to someone else as your hands get cold. (The ice in the bag may reach 4¬ F, depending on the amount of salt you add.) Step 5: After about 15 minutes, check your mix. It should be frozen and stuck to the sides of the smaller bag. Remove the smaller bag, keeping it closed, and rinse the salt and ice from the outside of the small bag. Squeeze to break up any chunks. Then open and scoop out delicious homemade ice cream.

Basic Vanilla Custard Ice Cream Mix


*3 egg yolks
*3 - 4 c sugar
*1 c milk
*1 c light cream
*1 TBSP vanilla*

Beat egg yolks and sugar until lemon-colored, light and creamy (about 5 minutes with an electric mixer¥longer if by hand). Set aside. In a small saucepan, scald milk (heat to just below boiling). Pour hot milk over sugar-egg mixture. Transfer to a stainless steel bowl or the top of a double boiler. Cook over, not in, a pan of simmering water, stirring slowly and constantly for 10 minutes until custard is slightly thick. Custard should stick to the back of a spoon and you should be able to run a finger through it and leave a track. Remove from heat. Stir in cream and vanilla. Allow to cool in refrigerator, or place bowl or pan in an ice-water bath to quick-cool. Once mixture is cool, proceed with freezing directions. Yield: three bags of ice cream. For chocolate: mix 1 - 4 to 1 - 3 cup cocoa powder with 2 tablespoons of milk until smooth. Add to eggs along with sugar mixture. For mocha: prepare chocolate mixture, and add 1 tablespoon of instant coffee or espresso powder to the scalded milk. For berry: smash one pint berries with 1 - 4 cup of the total sugar. Cook in microwave or boil on stove for 7 to 9 minutes. Cool. Add to chilled mix.

* When making frozen foods, you must add extra flavoring, like vanilla, cocoa or cinnamon, because when your taste buds are cold they have a more difficult time detecting sweet flavors. Conversely, cold salty foods taste saltier than do warm salty foods.

Visit the Kitchen Theater at Carnegie

Science Center to learn about other experiments in cooking, along with a taste and a recipe to take home. This summer, demonstrations include the science behind cheesemaking in "Curds `n' Whey," the way fats work in your body and in your favorite baked goods in "The Thick and Thin of Fat," and the history of peanut butter in "Peanut Butter Inventions."

Demonstration times vary, but a daily schedule is posted in the Science Center entrance lobby. The Kitchen Theater at Carnegie Science Center is sponsored by the Jewish Healthcare Foundation.

Claire Spampinato is an educational coordinator in the Kitchen Theater.
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