A domestic ice cream maker or ice cream freezer is a machine

Machine

A machine manages power to accomplish a task, examples include, a mechanical system, a computing system, an electronic system, and a molecular machine. In common usage, the meaning is that of a device having parts that perform or assist in performing any type of work...

 used to make small quantities of ice cream

Ice cream

Ice cream is a frozen dessert usually made from dairy products, such as milk and cream, and often combined with fruits or other ingredients and flavours. Most varieties contain sugar, although some are made with other sweeteners...

 at home. Ice cream makers may stir the mixture by hand-cranking or with an electric motor, and may chill the ice cream by using a freezing mixture, by pre-cooling the machine in a freezer, or by the machine itself refrigerating

Refrigeration

Refrigeration is a process in which work is done to move heat from one location to another. This work is traditionally done by mechanical work, but can also be done by magnetism, laser or other means...

 the mixture.

An ice cream maker must freeze the mixture, and must simultaneously stir or churn it to prevent the formation of ice crystals and aerate

Aeration

Aeration is the process by which air is circulated through, mixed with or dissolved in a liquid or substance.-Aeration of liquids:-Methods:Aeration of liquids is achieved by:...

 it to produce smooth and creamy ice cream. Most ice creams are ready to eat immediately, but some, especially those containing alcohol, must be chilled further in a freezer to attain a sufficiently firm consistency.

Some machines, such as certain low-priced counter-top models, require that the resulting mixture be frozen an extra four hours or more (or overnight), depending on the recipe, in order for the ice cream to harden to a desired consistency.

Manual machines

These machines usually comprise an outer bowl and a smaller inner bowl with a hand-cranked mechanism which turns a paddle, sometimes called a dasher, to stir the mixture. The outer bowl is filled with a freezing mixture

Cooling bath

A cooling bath is a mixture used in a laboratory when low temperatures are needed, for example to conduct low-temperature chemical reactions , to collect highly volatile liquids from distillation, or in cold traps...

 of salt and ice: the addition of salt to the ice causes freezing-point depression

Freezing-point depression

Freezing-point depression describes the phenomenon in which the freezing point of a liquid is depressed when another compound is added, meaning that a solution has a lower freezing point than a pure solvent. This happens whenever a non-volatile solute is added to a pure solvent, such as water...

; as the salt melts the ice, its heat of fusion allows it to absorb heat from the ice cream mixture, freezing the ice cream.

This type of ice cream maker is inexpensive, but inconvenient and messy as the ice and salt mixture produces a lot of salty water as it melts, which the user must dispose of, and the ice and salt mixture has to be replenished to make a new batch of ice cream.

Some small manual units comprise a bowl with a capacity of about one pint (500ml) whose hollow walls are filled with a coolant. The paddle is often built into a plastic top. The mixture is poured into the frozen bowl and placed in a freezer. The paddles then are turned by hand every ten minutes or so for a few hours until the desired consistency and flavor is reached.
Nancy Johnson invented the first hand-cranked model in 1847. She then sold the patent to William Young, who marketed the machine as the Johnson Patent Ice-Cream Freezer.

Electric machines

There are three types of electric ice cream machines. Each has an electric motor which drives either the bowl or the paddle to stir the mixture. The major difference between the three is in how the cooling is performed.

Counter-top machines use a double-walled bowl which contains between the two walls a solution that freezes below the freezing point of water. This is frozen in a domestic freezer for up to 24 hours before the machine is needed. Once frozen, the bowl is put into the machine, the mixture is added and the machine is switched on. The paddles rotate, stirring the mixture as it gradually freezes through contact with the frozen bowl. Twenty to thirty minutes later, the solution between the double walls of the bowl has thawed, and the ice cream has frozen. The advantage of this type of electric machine is low cost, typically under $100. The disadvantage of the pre-frozen bowl approach is that only one batch can be made at a time. To make another batch, the bowl must be frozen again. For this reason, it is usually possible to buy extra bowls for the machine, but of course these take up a lot of freezer space.

Small freezer-unit machines sit inside the freezer (or the freezer part of the refrigerator), and operate similar to a food processor

Food processor

A food processor is a kitchen appliance used to facilitate various repetitive tasks in the process of preparation of food. Today, the term almost always refers to an electric-motor-driven appliance, although there are some manual devices also referred to as "food processors".Food processors are...

in slow-motion. The paddles turn every few seconds to stir the mixture enough to prevent large ice crystals from forming. When the ice cream has frozen sufficiently, the paddles automatically stop rotating and lift up. As the mixture is cooled simply through being in the freezer, it takes longer to freeze than other types of ice cream makers, which work by placing the ice cream bowl in direct contact with the cooling element. A disadvantage is that the freezer door has to be closed over the flat cord, which is plugged into the nearest power point outside, though some modern refrigerators have a built-in ice-cream maker as an accessory or a specialized electrical plug for use with certain freezer-unit machines. The advantage of this type of ice cream maker is that no pre-freezing of the appliance is necessary. However, some people feel that this type of machine produces a lower-quality ice cream because of its slow-motion method. It's also possible to get cordless, battery-operated ice-cream makers which can be placed directly in the freezer, though these tend to require expensive non-rechargeable potassium batteries (most rechargeable batteries or regular alkaline cells perform very poorly at low temperature).

More expensive, and much larger, machines have a freezing mechanism built in and do not require a bowl to be pre-chilled. The cooling system is switched on, and in a few minutes the mixture can be poured in and the paddle switched on. As with coolant-bowl machines, ice cream is ready in twenty to thirty minutes, depending on the quantity made. These machines can be used immediately with no preparation, and any number of batches of ice cream can be made without a delay between batches. Some of these machines cannot be moved without waiting twelve hours before use, as moving the unit upsets the coolant in its freezing system; they would normally be kept permanently positioned ready for use, impractical in a smaller kitchen.

Vending machines

One company has developed a vending machine that makes individual servings of ice cream from non-frozen ingredients.