WHAT IS LIQUID NITROGEN?
Nitrogen makes up nearly 80% of the earth's atmosphere by volume and the element was discovered in 1772. Liquid nitrogen is a cryogenic liquid that is created by fractional distillation, (separating chemical compounds by their boiling points), of liquid air. Nitrogen has a low boiling point, lower than oxygen, so it can be distilled easily. Liquid nitrogen has a boiling point of around -196 degrees C and freezes at -210 degrees C. It also boils upon contact with warmer temperatures, this is called the Leidenfrost effect http://en.wikipedia.org/wiki/Leidenfrost_effect, as it boils it evaporates into nitrogen gas. Inhalation of too much nitrogen can cause dizziness, nausea, loss of consciousness and death, so it is VERY important to work with liquid nitrogen in well-ventilated areas.
STORAGE OF LIQUID NITROGEN
Liquid nitrogen is stored in specially designed containers called dewars. Dewars are insulated, pressurized containers equipped with safety releases and rupture discs that help prevent pressure build up and explosions. As highlighted above, the canisters should be stored in a well ventilated area.
SAFETY
As well as adhering to correct storage methods, there are other important safety issues to remember. The extremely cold contents of the canister means that any uninsulated metal piping will be at LOW temperatures, so do not touch with bare flesh to prevent freezing and possible tissue damage. When using liquid nitrogen, protective gloves, goggles and face shields are recommended.
ICE CREAM
Heston Blumenthal brought liquid nitrogen to the attention of most people with his scrambled egg ice cream, although he certainly was not the first to use it in the kitchen. We associate liquid nitrogen with ice cream making and this is probably the most common use of it but really the possibilities are endless. For a quick 'nitro' ice cream, take a bowl with some ice cream base of choice and stir with a spoon/spatula whilst pouring in (slowly) liquid nitrogen. Once the correct consistency is achieved you can serve the ice cream immediately. Be careful not to add the nitrogen too quickly and ensure the ice cream is not consumed at too low a temperature to prevent frozen burn to the tongue or other areas of the mouth.
'Egg & Bacon' at The Fat Duck including 'nitro scrambled egg ice cream'
Advantages of making ice cream with this method is the lack of traditional preservatives and anti freezing ingredients that normally ensure a smooth and consistent product. Sugar, of course is added to ice cream, not just for its sweet flavour profile but to prevent ice crystals forming and ensuring a 'scoopable' product. Sugar derivatives and alcohols do the same thing but are now not needed as liquid nitrogen ensures a smooth and consistent product every time.
OTHER APPLICATIONS
Obviously, being able to rapid chill a product has huge benefits to the safety conscious. Being able to chill a stew or custard in seconds significantly reduces the chances of bacteria development in foods.
Powders and fine preparations can also easily be achieved, in particular tricky high fat products such as nuts. When grinding nuts or other high fat products, less than satisfactory results are often achieved due to friction increasing temperature and leaching out oils. Now, almonds say, can be frozen in liquid nitrogen and blitzed to produce a superior result.
Vegetables and fruits can also be frozen to tenderize them. Ice crystals are formed which prick tiny holes in the membranes of cells inside these ingredients. Once thawed, the fruits & vegetables are tenderized but still retain some of their original texture and flavour. Fruits and vegetables once frozen to a low enough temperature can also be broken or shattered to produce a different presentation to finishing a dish or for other applications.
Liquid centres! This is an area we are looking closely at, as the possibilities are seemingly endless. Imagine freezing something that normally can't be frozen? e.g. vodka in a normal freezer won't work, but in nitrogen it freezes hard. Now take this frozen 'ball' and encase in chocolate or ice cream and allow to defrost. What would happen? Well, we wil be working on this in the next couple of weeks and will post our results here.
CONCLUSION
As long as the medium is respected and all of the important regulations and safety measures are in place then liquid nitrogen is an extremely exciting and useful tool for the modern chef.
RESOURCES
http://www.restaurantecalima.com/ - Chef Garcia's restaurant is at the forefront of low temperature cooking.
http://www.en.wikipedia.org/wiki/Liquid_nitrogen/ - Useful wiki page
http://cookingissues.wordpress.com/?s=liquid+nitrogen - Nitrogen articles on Cooking Issues
Happy experimenting!!
