Ice cream maker

[2] In 1843, Nancy M. (Donaldson) Johnson of Philadelphia received the first U.S. patent for a small-scale hand-cranked ice cream freezer.

The salt causes the ice to melt and lowers the temperature in the process, due to freezing point depression.

The sub-freezing temperature helps slowly freeze the edible mixture, making the ice cream.

Counter-top machines use a double-walled bowl with a solution between the walls (typically distilled water and urea) that freezes below 32 °F (0 °C).

After twenty to thirty minutes, the solution between the double walls thaws, and the ice cream freezes.

A disadvantage is that the freezer door has to be closed over the flat cord, which is plugged into the nearest power outlet.

However, some modern refrigerators have a built-in ice-cream maker as an accessory or a specialized electrical outlet for freezer-unit machines.

However, some people feel that this type of machine produces a lower-quality ice cream because of its slow-motion method.

Also available are cordless, battery-operated ice-cream makers that may be placed directly in the freezer, although these tend to require expensive non-rechargeable potassium batteries[citation needed] as both rechargeable batteries and regular alkaline cells tend to perform poorly at low temperatures.

[citation needed] More expensive, and usually larger, machines have a freezing mechanism built in and do not require a bowl to be pre-chilled.

As with coolant-bowl machines, ice cream is ready in twenty to thirty minutes depending on the quantity and recipe.

A high-speed electric motor, geared at approximately 75 rpm, drives a mechanism that simultaneously rotates the canister, counter-rotates the scraper, and holds the churn paddles stationary.