Question: What is used to freeze and harden ice cream?
About the article "Ice cream", section "Cryogenic".

Candidate passages:
Using liquid nitrogen to freeze ice cream is an old idea and has been used for many years to harden ice cream.
The use of liquid nitrogen in the primary freezing of ice cream, that is to effect the transition from the liquid to the frozen state without the use of a conventional ice cream freezer, has only recently started to see commercialization.
Some commercial innovations have been documented in the National Cryogenic Society Magazine "Cold Facts".
The most noted brands are Dippin' Dots, Blue Sky Creamery, Project Creamery, and Sub Zero Cryo Creamery.
The preparation results in a column of white condensed water vapor cloud, reminiscent of popular depictions of witches' cauldrons.
The ice cream, dangerous to eat while still "steaming", is allowed to rest until the liquid nitrogen is completely vaporised.
Sometimes ice cream is frozen to the sides of the container, and must be allowed to thaw.
Making ice cream with liquid nitrogen has advantages over conventional freezing.
Due to the rapid freezing, the crystal grains are smaller, giving the ice cream a creamier texture, and allowing one to get the same texture by using less milkfat.
Such ice crystals will grow very quickly via the processes of recrystallization, thereby obviating the original benefits unless steps are taken to inhibit ice crystal growth.
For similar reasons, good results can also be achieved with the more readily available "dry ice" and authors such as Heston Blumenthal have published recipes to produce ice cream and sorbet using a simple blender.
Cookies and Cream (or Cookies 'n Cream) is a variety of ice cream and milkshake based on flavoring from chocolate cookies.
Often called the "King Kong of Ice Cream" because it immediately rocketed to the top of the World's Favorite Ice Cream Flavors list, Cookies and Cream ice cream uses sweet cream ice cream (often vanilla) and chocolate wafer cookies of any type.
The flavor is widely associated with the Oreo cookie, a kind of cookie sandwich with a sweet, white cream filling commonly used in milkshakes and other frozen desserts such as Blizzards, pies, etc.
Though traditional cookies and cream ice cream is made with vanilla or sweet cream ice cream, there are variations that use chocolate, coffee or mint ice cream instead. 
There is some debate as to who first invented and marketed Cookies 'n Cream ice cream.
In the fall of 1978, Huggs McShane made Oreo ice cream at Gelato Roberto in the Galleria Mall in downtown Portland, Oregon.
He stood in the window of the shop and made ice cream as people in the street watched through the storefront window.
After he had been working there for a while, he decided to experiment with new flavors and went to the grocery to get ideas.
He bought some Oreos, put them in the vanilla mixture as it started to harden in the ice cream maker, and soon Oreo ice cream became a best seller.
Emack and Bolio's based in Boston claim to be the first to put Oreo cookies in ice cream, around 1975.
Bramble & Mike's based out of Florida, claim to be the first to invent "Oreo Ice Cream" in 1972.
Reportedly there was a shortage of chocolate chips, and Oreos were used as a substitute.
John Harrison, the official taster for Dreyer's/Edy's Ice Cream, claims he invented it first for the company in 1982.
Blue Bell Creameries claim they first mass-produced the flavor two years earlier in 1980 after an employee tasted it at a Houston ice cream parlour the year before; however, they do state "Blue Bell makes no claim to have invented it but certainly pioneered the flavor."
Blue Bell Creameries is reported to have trademarked the name "Cookies 'n Cream" in 1978.
South Dakota State University claims the flavor was invented at the university's Dairy Plant.
Another claimant is Steve Herrell of Massachusetts' Herrell's Ice Cream.
In 1983, Cookies 'n Cream became one of the top five best-selling flavors of ice cream. 
Freeze-dried ice cream is ice cream that has had most of the water removed from it by a freeze-drying process, sealed in a pouch, and requires no refrigeration.
It is also known as astronaut ice cream or space ice cream, typically a slab of ready-to-eat dehydrated ice cream.
Compared to regular ice cream, it can be kept at room temperature without melting and is more brittle and rigid but still soft when bitten into.
It was developed by Whirlpool Corporation under contract to NASA for the Apollo missions.
Freeze-dried foods were developed so that foods could be sent on long-duration spaceflights, as to the Moon, and to reduce the weight of the water and oxygen normally found in food.
Freeze drying (or lyophilization) removes water from the ice cream by lowering the air pressure to a point where ice sublimates from a solid to a gas.
The ice cream is placed in a vacuum chamber and frozen until the water crystallizes.
The air pressure is lowered, creating a partial vacuum, forcing air out of the chamber; next heat is applied, sublimating the ice; finally a freezing coil traps the vaporized water.
This process continues for hours, resulting in a freeze-dried ice cream slice.
Freeze-dried ice cream is sold by mail order and is common in science museums and NASA visitor center gift shops, sometimes accompanied by other freeze-dried foods. 
Strawberry ice cream is a flavor of ice cream made with strawberry or strawberry flavoring.
It is made by blending in fresh strawberries or strawberry flavoring with the eggs, cream, vanilla and sugar used to make ice cream.
Most strawberry ice cream is colored pink or light red.
Strawberry ice cream dates back at least to 1813, when it was served at the second inauguration of James Madison.
Along with vanilla and chocolate ice cream, strawberry is one of the three flavors in Neapolitan ice cream.
Variations of strawberry ice cream include strawberry cheesecake ice cream and strawberry ripple ice cream, which is vanilla ice cream with a ribbon of strawberry jam or syrup.
Some ice cream sandwiches are prepared neapolitan-style, and include strawberry ice cream.