Nitrogen is a natural gas. It can be liquified in a plant and is used as such in molecular gastronomy most often to deep freeze foam pieces or to make ice cream.
The air we breathe is composed of approximately 78% nitrogen in it’s gaseous state, against 21% oxygen and 1% of various other gases. To produce liquid nitrogen, the air is first liquefied and the nitrogen is then separated by distillation. Since the boiling point of nitrogen is -195.8°C (-320.5°F), liquid nitrogen is extremely cold, it's a liquid said to be "cryogenic".
The extreme cold of liquid nitrogen makes handling it very dangerous because it can cause very serious burns that irreparably destroy skin or eyes. Professionals who use it must undergo training to properly learn about its reactivity and thus take appropriate precautions. If a drop of liquid nitrogen comes into direct contact with the skin, the warmth of the skin will cause thermal shock, so that a thin layer of nitrogen gas will immediately revert to gas and the skin will not be burnt. However, if a drop of liquid nitrogen falls on clothing or jewelry, it will freeze instantly and the frozen clothing will inflict a "burn" to the skin it is in contact with. Failing to be completely naked, one keeps hands bare, and protects the body with an apron that covers the clothing. That way, there can be no direct contact between the fabric which the nitrogen can reach, and the body. There are aprons and gloves specially designed to withstand the cold of liquid nitrogen. The feet should be protected in the same manner. The eyes, not exhibiting the same properties as the skin would be burned instantly on contact with a drop of liquid nitrogen: the use of goggles is therefore mandatory.
Any substance occupies a greater volume in its gaseous state than in its liquid state. Thus, liquid nitrogen, which tends to return to its gaseous form under the effect of ambient heat, generates enormous pressure if stored in an airtight container. Even small amounts should never be sealed because the container would explode violently. Containers specially designed to withstand the immense pressure are called "dewars".
Under the effect of ambient temperature, liquid nitrogen evaporates and becomes gaseous. Although not toxic in itself, nitrogen still poses a danger for asphyxiation. An increase in the proportion of nitrogen in the air due to the evaporation of liquid nitrogen can render the proportion of oxygen insufficient to our breathing and cause asphyxiation. It is therefore imperative that laboratories or kitchens where liquid nitrogen is used be very well ventilated.
Nitrogen is liquefied for various industrial uses that require the extreme cold: in the treatment of metals, plastics and rubbers, for example. It is used primarily as a cooling technique for the preservation of biological samples, in particular.
Cooling with liquid nitrogen is one of the methods used to freeze foods. When food is frozen, the water it contains forms ice crystals that damage its cell membranes. As a result, the texture, taste and nutritional properties are altered. When using liquid nitrogen, cooling is much faster and the ice crystals that are formed are so small that the integrity of the food is preserved.
In cooking, liquid nitrogen is used as a coolant. It is not an ingredient and so it is never ingested; it cools the food, then evaporates. It is only after complete evaporation of the liquid nitrogen that the food can be ingested. Foods that are cooled with liquid nitrogen are extremely cold as they have been in contact with this cryogenic substance, and should be left to warm up before being touched and ingested, in the same way that foods dipped in boiling oil must cool down before being touched. The denser the food, the colder it will be and therefore the longer it will need to warm up. This is why chefs typically use liquid nitrogen to make meringues or frozen mousse, that is to say, they dip very low density confections.
Some chefs use the cooling properties of liquid nitrogen to make especially unctuous ice creams. The creaminess of the ice cream is obtained thanks to the small size of the ice crystals formed during cooling with liquid nitrogen. It is also possible to create amazing appetizers like frozen meringue or mousse. Liquid nitrogen finally makes it possible to freeze alcohol to make original cocktails, which is not possible with traditional freezing techniques.
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