Ice creams, sorbets and sherbets are typically frozen while being stirred with a paddle so that the mixture forms small, even ice crystals rather than freezes into a solid block. The size of the crystals is one of the most important factors in an ice cream's texture. The smaller the ice crystals, the smoother the ice cream.
Beginning with a thoroughly chilled base and giving it adequate agitation as it freezes are the surest ways to keep ice crystals small. Added ingredients that contain a high percentage of water, such as fresh fruit, form larger crystals and tend to freeze in icy lumps. When flavoring a base with fresh fruit, macerate the chopped or crushed fruit in a few ounces of sugar for an hour or so. The liquid will be drawn from the fruit and form a syrup; both fruit and syrup should be added to the base.
Another important determinant of the final texture is the amount of air incorporated into the base as it freezes. The more air, or overrun, that ice cream has, the less dense its texture will be. Commercial premium ice creams have about 20% overrun, while cheaper brands can have as much as 100%, which means that they are half air. Beginning with a well-chilled base helps keep overrun to a minimum. Filling your machine to full capacity also limits overrun by allowing the ice cream no space to expand as it takes on air.
Butterfat, of course, gives ice cream its rich, creamy texture. Too much butterfat in a base can give it a greasy or chalky texture. In extreme cases, the fat can be churned into little lumps of butter during the freezing process. Expensive brands usually have about 18% butterfat.
A base with a butterfat content between 16% and 24% makes a rich, evenly textured ice cream. Create this balance by using a combination of heavy cream, generally 40% butterfat, and whole milk, which is about 4%. Experiment with different flavors by replacing part of the whole milk or cream in a base with other dairy products--buttermilk, sour cream and creme frai^che work well, as does ricotta, mascarpone or other soft cheeses. Because heating these alternative dairy products can change their flavor and texture, they should be stirred into the custard as it cools.
Egg yolks act as an emulsifier in an ice cream base and add richness, color and flavor. Ice cream made without yolks tends to be less stable and melts at a lower temperature. Sherbet and sorbet recipes sometimes call for egg whites to bind the base together, but this can increase overrun, resulting in a powdery, snowy texture.
The amount of sugar in an ice cream determines not only how sweet it will taste, but also how firmly it will freeze. Sugar inhibits freezing--too much and it won't freeze finely enough to scoop but some of it is necessary for a smooth, creamy consistency. Professionals use a floating instrument called a sachrometer to measure the amount of sugar in a base (measured in degrees beaume). For at-home production, it is more practical to find a formula that works and go from there. And be aware that when adding flavoring ingredients such as chocolate, wine and fruit, you are adding additional sugar.
Alcohol is another ingredient that inhibits freezing. The more alcohol added to a base, the softer it will freeze. Some cooks use a small amount of a flavorless alcohol--vodka, for instance--as a regular ingredient in sorbets to promote a smooth creamy texture. About 1 ounce of an 80-proof alcohol can be added per pint of base without negatively affecting the texture.
Stabilizers, either in the form of natural or chemical additives, are found in commercial ice creams, sorbets and sherbets, but are not really practical or necessary for home use. They thicken the base and allow it to maintain its texture for long periods and to be more resistant to changes in temperature. Gelatin is a common stabilizer in sorbet and sherbet recipes, but I prefer not to use it because it sometimes produces an unpleasantly gummy texture.
No matter what kind of machine you have or what formula you use, all ice creams, sorbets and sherbets will come out of the machine with a texture like that of soft-serve ice cream. They require a hardening period of at least 4 hours in the freezer before they're ready to scoop and eat.
Not all frozen dessert recipes are created equally, but once you find a formula that produces a texture you like, you can vary it to create your own ice cream and sorbet flavors. These recipes will get you started.
CHOCOLATE KAHLUA ICE CREAM
1 1/4 cups heavy whipping cream
3 1/4 cups whole milk
1 3/4 cups sugar
5 tablespoons cocoa powder
1 ounce semisweet chocolate, melted
4 egg yolks
1/4 cup Kahlua
Heat cream and milk with 1 cup sugar and cocoa powder in heavy-bottomed pan over low heat, stirring until sugar and cocoa are dissolved, 5 to 7 minutes. Set aside.
Melt chocolate in top of double boiler over pan of barely simmering water. Set aside.