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Introduction to How Ice Cream Works

The U.S. ice cream industry sells more than a billion gallons of ice cream each year, dispensing cones, gallons, pints, sundaes and other desserts through grocery stores and ice cream shops. In fact, eight percent of all the milk produced in the United States ends up in a f­rozen dairy product [ref].

Although ice cream can be easy to make at home, it is actually a very complex substance. In this article, we'll learn how it's made, what goes into it and who invented it. We'll also learn how to quickly make ice cream in your kitchen.

Ice Cream or Frozen Dessert?

Not just any frozen treat can be called ice cream. In fact, the U.S. Department of Agriculture has specific rules that define what can and can't be labeled "ice cream." To bear the "Meets USDA Ingredient Standard for Ice Cream" stamp, it has to contain at least 10 percent milk fat, and a minimum of six percent non-fat milk solids. A gallon has to weigh at least 4.5 pounds.

The range of milk fat (sometimes referred to as butter fat) used in ice cream can go from the minimum 10 percent to a maximum of about 16 percent. Most premium ice creams use 14 percent milk fat. Higher fat content leads to better, richer taste and a creamier texture. Ice cream makers don't go higher than 16 percent because it would be costly and very high in calories. An ice cream with this much milk fat would also taste so rich that people would probably eat it in smaller amounts, which would be bad news for people who sell ice cream for a living.

Other frozen desserts, such as sorbets, low-fat ice cream, and frozen yogurt, are not technically ice cream at all. Frozen custard is ice cream that has at least 1.4 percent egg yolk solids, and "soft serve" can be any frozen milk-based dessert that has not gone through the hardening process -- more on that later.

In terms of specific ingredients, the recipe for ice cream is simple. But in scientific terms, it's complicated stuff. Ice cream is a colloid, a type of emulsion. An emulsion is a combination of two substances that don't normally mix together. Instead, one of the substances is dispersed throughout the other. In ice cream, molecules of fat are suspended in a water-sugar-ice structure along with air bubbles. The presence of air means that ice cream is also technically a foam.

In addition to milk fat, non-fat milk solids, sugar, and air, ice cream also contains stabilizers and emulsifiers. Stabilizers help hold the air bubble structure together and give the ice cream a better texture. Although gelatin was originally used as a stabilizer, xanthan gum, guar gum, and other compounds are used today. Emulsifiers keep the ice cream smooth and aid the distribution of the fat molecules throughout the colloid. Egg yolks were once used, but ice cream manufacturers now tend to use other chemical compounds. These stabilizers and emulsifiers make up a very small proportion (less than one percent) of the ice cream.

­In the next section, we'll find out how you make ice cream.

Ice cream mix

Making Ice Cream

Whether it's being made in your kitchen with a hand crank, at a local homemade ice cream shop with a stand-alone ice cream maker, or in a factory that cranks out thousands of gallons of ice cream every day, the process of making ice cream is basically the same. The only difference is the scale of the operation.

First, you need ice cream mix. You can buy commercially made ice cream mix that is set to a certain milk fat content. Ice cream factories usually make their own mix by combining milk, cream and sugar in a 3,000 gallon vat, with the proportions and mixing controlled by computers. The mix is then pasteurized, or heated, to kill any harmful bacteria. If you were to make your own mix at home, you could pasteurize it by cooking it in a double boiler, or use an egg substitute or pasteurized egg product. This step is important, because otherwise people who eat your homemade ice cream could get sick due to salmonella contamination. According to the Centers for Disease Control, those most at risk include the elderly, very young children, and people with compromised immune systems [ref].

Photo courtesy Ed Grabianowski and Deborah Hanny Electro Freeze batch freezer

The next step in production is adding flavor to the mix. There are thousands of varieties of ice cream, so just about any combination of flavors is possible. From vanilla to cinnamon, chocolate to triple chocolate fudge brownie, it all gets blended into the ice cream mix. In a factory, this step takes place in vats that hold hundreds of gallons of ice cream, while giant steel paddles do the mixing. In your kitchen, a large bowl and a food mixer will work, or even a wooden spoon and muscle power if you want some exercise. Solid chunks such as pieces of fruit, chocolate chunks, marshmallows, and candy are added later.

The next step is where an ice cream making machine comes into play. The mix has to be simultaneously frozen and whipped. In a factory, this happens in a giant tube surrounded by pipes. The pipes contain chemicals such as ammonia that freeze the tube, but the ammonia never comes into contact with the ice cream. The ice cream mix is pumped through the tube, where it gets cold very quickly. A dasher, or blade, turns inside the tube. This whips the mixture, introducing the air bubbles that help give ice cream its structure. The dasher also scrapes the sides of the tube, clearing off ice crystals that form there. This prevents large ice crystals from ruining the flavor and texture of the ice cream. All the elements of this process are carefully monitored and controlled by computers. Most homemade ice cream shops use a batch freezer for this step, where the same process happens on a smaller scale.

Photo courtesy Ed Grabianowski and Deborah Hanny A single batch in the Electro Freeze takes 15 to 20 minutes and produces two gallons of ice cream.

This step can be accomplished at home with a rock salt/ice mixture for freezing and a hand or electric cranked dasher to mix and scrape off the ice crystals.

Photo courtesy Ed Grabianowski and Deborah Hanny With some creativity and experimentation, the flavor options are almost limitless.

Once the ice cream has come out of the ice cream maker, the process isn't finished. At this point, the mixture is frozen, but still soft. Large chunks of candy and other goodies are now added. Then the ice cream is placed into containers. Factory machines pour it straight into cartons or buckets, or it can be extruded into shapes that have wooden sticks placed into them for individual treats.

Photo courtesy Ed Grabianowski and Deborah Hanny After being drawn from the machine, the ice cream is placed into a -20 degree Fahrenheit freezer to harden.

Now the ice cream needs to be reduced to a very low temperature, zero degrees Fahrenheit or below. Factories make it even colder since they need the ice cream to stay frozen while it is packaged and loaded onto trucks. It needs to be very cold to freeze the ice cream quickly and prevent the formation of large ice crystals. This process is known as hardening. "Soft-serve" is often simply ice cream that has not gone through this process.

In the next section, we'll take a look at the business of ice cream.

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Photo courtesy Ed Grabianowski and Deborah Hanny Sweet Jenny's Ice Cream in Williamsville, NY

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Ice Cream Business and History

 

In 1999, retail sales of ice cream in the U.S., the worldwide leader in ice cream production, topped $4 billion. In 2002, more than $20 billion was spent on frozen desserts. The leading states in ice cream consumption are California, Indiana, Pennsylvania, Texas and New York. Americans ate an average of 21.5 quarts of ice cream per person in 2004 [ref].

With that much money to be made, the ice cream industry can be secretive and underhanded. Deborah Hanny, owner of Sweet Jenny's Ice Cream in Williamsville, NY, protects her recipes carefully. Her shop has been photographed by men in suits and she once caught someone in her upstairs office hurriedly trying to copy down her recipes.

Ice cream making secrets are seldom passed down from generation to generation these days. So where do people in the ice cream industry learn their craft? At ice cream school. Pennsylvania State University offers a week-long "Ice Cream Short Course" intended for industry professionals. The course teaches the science and technology used to make ice cream. They also offer Ice Cream 101 for ice cream hobbyists who just want to learn more about their favorite frozen treat. The University of Guelph, Ontario's Dairy Science and Technology school, also has a long history of teaching ice cream science.

The earliest reports of people enjoying flavored ice desserts come from the Romans and the Chinese. Marco Polo returned from his famous expedition with fruit-flavored ices, reporting that Asians had been making them for thousands of years. These delicacies became popular in France in the 1500s, but only among royalty. Over the next few centuries, the process of making them evolved from hauling mountain ice to salt/ice freezing methods. Cream was introduced as an ingredient, and by the 1700s, people were enjoying a dessert that was very similar to today's ice cream.

The hand-cranked ice cream freezer was first developed by Nancy Johnson in 1846. Ice was readily available from large warehouses in that era, so ice cream became a treat that could be enjoyed by almost anyone, not just the rich. Ice cream making was revolutionized again in 1851, when Jacob Fussel started the first wholesale ice cream manufacturing operation in Baltimore, Maryland. Fussel's dairy business had excess cream and he couldn't figure out what to do with it. He tried using it to make ice cream, and before long his ice cream business outsold the rest of the dairy.

The business slowly evolved for the next few decades. Manufacturing methods and ingredients improved, while refrigeration technology became cheaper and more efficient. By the 1920s, home refrigerators and freezers became more common, which gave the ice cream industry another boost. Sugar was rationed in the U.S. during World War I, but the ice cream industry convinced the government that ice cream was an "essential food." Ice cream factories were allotted sugar rations and production continued.

Ice cream increased in popularity until the Depression years caused a drop in sales for virtually all non-essential goods. Sales increased in the years leading up to World War II before leveling off in the post-war years. The rise of the giant supermarket created demand for cheaper, mass-produced ice cream, but quality suffered. The 1960s saw a resurgence in "premium" ice cream, while the following decades saw the market fragment into low-fat varieties for the health-conscious, including frozen yogurt, fruit bars, ice milk, fat-free ice cream, and dozens of other varieties. However, ice cream still makes up about 60 percent of the market share among frozen desserts [ref]. While ice cream is enjoyed worldwide, it has become an American tradition.

In the next section, we'll learn how to make ice cream in five minutes -- without using an ice cream freezer!

Photo courtesy Ed Grabianowski

Five-minute Ice Cream

There are many recipes out there for making your own ice cream at home, but did you know that you can make your own ice cream in five minutes using two Ziploc bags?

Here's what you'll need for this experiment:

1 tablespoon sugar

½ cup milk, cream, or half and half

¼ teaspoon vanilla extract (or other flavoring)

6 tablespoons salt

Enough ice to fill the gallon-sized bag halfway

1 gallon-sized Ziploc bag

1 pint-sized Ziploc bag

Photo courtesy Ed Grabianowski Fill the gallon-sized bag halfway with ice. Add the salt.

Ordinary table salt will work, but salt that has larger crystals, such as kosher salt or rock salt, will work much better. Mix the salt around in the ice and set aside.

Photo courtesy Ed Grabianowski Pour the milk, sugar, and vanilla extract into a bowl or other container and mix.

Photo courtesy Ed Grabianowski Carefully pour the mixture into the pint bag.

Photo courtesy Ed Grabianowski Close the bag, making sure it is completely sealed.

Photo courtesy Ed Grabianowski Put the pint bag into the gallon bag.

Make sure the pint bag gets buried in the ice. Seal the gallon bag. Shake the bags vigorously for five minutes. You might want to use a towel to hold them, since they will be very cold and slippery from condensation.

Photo courtesy Ed Grabianowski Remove the pint bag, open it up, and grab a spoon.

Tips:

  • Milk will provide a less rich, lower calorie ice cream, while using heavy cream will have the opposite effect.
  • This method will make a small amount of ice cream, about enough for two people to enjoy. Experimenting with other methods can allow you to make more. One version uses two coffee cans of differing sizes instead of plastic bags.
  • Flavor combinations are almost limitless. Chocolate syrup is a basic option, while various flavor extracts available in your grocery store's baking section can lead to more exotic variations. Try combining mint extract with chocolate, or adding small chocolate chips.

For more information on ice cream and related topics, check out the links on the next page.

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Lots More Information

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Sources

  • Arbuckle, W.S. "Ice Cream." 4th Edition. Van Nostrand Reinhold, 1986. 0-87055-479-4.
  • Damerow, Gail. "Ice Cream: The whole scoop." Gelnbridge, 1991. 0-944435-09-2.
  • Fleisher, Paul. "Ice Cream Treats: The inside scoop." Carolrhoda, 2001. 1-57505-268-7.
  • "Ice Cream in a Bag." Ag, Ziplocs, and You, Illinois Farm Bureau. http://www.agintheclassroom.org/resources/agziploc.pdf
  • "Ice Cream Sales and Trends." International Dairy Foods Association. http://www.idfa.org/facts/icmonth/page2.cfm
  • Jaspersohn, William. "Ice Cream." MacMillan, 1988. 0-02-747821-1.
  • Penn State Ice Cream Short Course Information. http://conferences.cas.psu.edu/IceCream/default.htm
  • "Safe Handing of Milk and Dairy Products." Clemson University Cooperative Extension Service. http://hgic.clemson.edu/factsheets/HGIC3510.htm
  • "Standards for Ice Cream." United States Department of Agriculture. http://www.ams.usda.gov/standards/Icecream.pdf
  • Stogo, Malcolm. Ice Cream and Frozen Desserts: A commercial guide to production and marketing. John Wiley & Sons, 1998. 0-471-15392-3.
  • "What's the difference?" Newsday. http://www.newsday.com/features/food/ny-icecream-difference,0,1076538.story?coll=ny-foodday-print