急凍的原理(10點)

急凍全寫係急速冷凍，係一種將新鮮食物急速降溫而盡量保持食物鮮度而同時又可以防止細菌滋長既技術．急凍並唔係就咁將食物放入冰箱就叫急凍（就咁將食物放入冰箱可以叫慢凍囉），最主要原因係冰箱唔夠凍．一般急凍程序要係負攝氏四十度低溫下進行．你將D死蝦就咁放係冰箱兩日，只能減慢細菌生長速度，而且對於保持肉質一定無幫助，兩日後攞出黎煮一定會梅．．．


就算本身係急凍蝦，如果已經解左凍，急凍既效果已經消失．如果未解凍，就可以擺係冰格，可以繼續保持急凍既效果．

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Freezing is the process of cooling a liquid to the temperature (called freezing point) where it turns into a solid. Melting, the process of turning a solid to a liquid, is the opposite of freezing. For most substances, melting and freezing temperatures are equal. For example, the melting point and freezing point of the element mercury are the same. Rapid cooling by exposure to cryogenic temperatures can cause a substance to freeze below its melting point, a process known as flash freezing.

For some pure substances, such as pure water, the freezing temperature is lower than the melting temperature. The freezing point for water is only the same temperature as the melting point when nucleators are present to prevent supercooling. The freezing point of water is 0°C (32°F, 273 K). In the absence of nucleators water will supercool to −42°C (−43.6°F, 231 K) before freezing. But in the presence of nucleating substances the freezing point of water is the same as the melting point. Nucleating agents, such as dust, are commonly present in the environment, which is why rain water and tap water will normally freeze at the melting point of water.

http://en.wikipedia.org/wiki/Freezing

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Supercooling (急凍) is the process of chilling a liquid below its freezing point, without it becoming solid.

A liquid below its freezing point will crystallize in the presence of a seed crystal or nucleus around which a crystal structure can form. However, lacking any such nucleus, the liquid phase can be maintained all the way down to the temperature at which crystal homogeneous nucleation occurs. The homogeneous nucleation can occur above the glass transition where the system is an amorphous—that is, non-crystalline—solid.

Water has a freezing point of 273 K (0 °C or 32 °F) but can be supercooled at ambient pressure down to its crystal homogeneous nucleation at almost 231 K (−42 °C).1 If cooled at a rate of the order of 1 million kelvins per second, the crystal nucleation can be avoided and water becomes a glass. Its glass transition temperature is much colder and harder to determine, but studies estimate it at about 165 K (−108 °C).2 Glassy water can be heated up to approximately 150 K (−123 °C).3 In the range of temperatures between 231 K (−42 °C) and 150 K (−123 °C) experiments find only crystal ice.

Droplets of supercooled water often exist in stratiform and cumulus clouds. They form into ice when they are struck by the wings of passing airplanes and abruptly crystallize. (This causes problems with lift, so aircrafts that are expected to fly in such conditions are equipped with a deicing system.) Freezing rain is also caused by supercooled droplets.

An equivalent to supercooling for the process of melting solids is much more difficult, and a solid will almost always melt at the same temperature for a given pressure. It is, however, possible to superheat a liquid above its boiling point without it becoming gaseous.

http://en.wikipedia.org/wiki/Supercooling

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