Flash freezing

In physics and chemistry, flash freezing is the process whereby objects are frozen in just a few hours by subjecting them to cryogenic temperatures, or through direct contact with liquid nitrogen at . It is commonly used in the food industry. Flash freezing is of great importance in atmospheric science, as its study is necessary for a proper climate model for the formation of ice clouds in the upper troposphere, which effectively scatter incoming solar radiation and prevent Earth from becoming overheated by the sun. The process is also closely related to classical nucleation theory, which helps in understanding many materials, phenomena and theories in related situations. When water is supercooled to temperatures below , it must freeze. When water is in a conventional freezer, a dynamic phase transition is triggered. The resulting ice depends on how quickly the system is cooled: If the water is cooled below its freezing point slowly, an ice crystal will result, rather than the poly-crystalline solid that flash freezing produces. Flash freezing is used in the food industry to quickly freeze perishable food items (see frozen food). In this case, food items are subjected to temperatures well below the freezing point of water. Thus, smaller ice crystals are formed, causing less damage to cell membranes. Flash freezing techniques are used to freeze biological samples quickly so that large ice crystals cannot form and damage the sample. This rapid freezing is done by submerging the sample in liquid nitrogen or a mixture of dry ice and ethanol. American inventor Clarence Birdseye developed the "quick-freezing" process of food preservation in the 20th century using a cryogenic process. In practice, a mechanical freezing process is usually used due to cost instead. There have been continuous optimization of the freezing rate in mechanical freezing to minimize ice crystal size. The results have important implications in climate control research. One of the current debates is whether the formation of ice occurs near the surface or within the micrometre-sized droplets suspended in clouds.