How Ice Cream
Works The U.S. 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 U.S. 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 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 (果汁冰糕), 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
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 serapes 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 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. 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 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. Once the ice cream has come out of the ice cream maker, the process is finished.