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 tales 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 net 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, lee 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, lee 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 i