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How Ice Cream Works
The US ice cream industry sells about a million 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 US ends up in a frozen dairy product.
Ice Cream or Frozen Dessert
Not just any frozen treat can be called ice cream. In fact, the US 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 sorbet(加果汁的冰水), 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(乳状液). 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(凝胶) 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.
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(用巴氏法灭菌), 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.
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 and 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 mining 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.
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.
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(挤压出) into shapes that have wooden sticks placed into them for individual treats.
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.
We’ll learn about the ice cream industry in the next section.
Ice Cream Industry
In 1999, retail sales of ice cream in the US, the worldwide leader in ice cream production, topped $4 billion. In 2002, more that $ 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.
With that much money to be made, the ice cream industry can be secretive and underhanded (秘密的). 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 I01 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.
Eight percent of all the milk produced in the US ends up in a frozen dairy product.