In its gaseous state, nitrogen is odorless, colorless, non-flammable, non-toxic, and relatively inert. Liquid nitrogen was first liquefied at the Jagiellonian University on 15th April 1883 by Polish Physicists, Zygmunt Wróblewski and Karol Olszewski (1). A Short History of the Progress of Scientific Chemistry in Our Own Times (2). Since then, it is produced industrially by fractional distillation of liquid air. At its purest form, liquid nitrogen is abbreviated as LN2 and chemically speaking; the diatomic character of the covalent N bonding in N2 gas is retained after liquefaction (3). Structure of Liquid Nitrogen, Oxygen, and Argon by Neutron Diffraction (4).
LN2 is a relatively affordable cryogenic liquid and typically stored in containers called dewars. These cylinders are insulated, vacuum-jacketed pressure containers equipped with safety valves and rupture discs to release pressure buildup and prevent explosions. The cylinders are easily transportable with special pressurization makes it extremely convenient and useful in a wide range of food production and manufacturing applications, primarily as an economical source of food freezing.
LN2 has an even lower boiling point than carbon dioxide, and it freezes at a much lower temperature than any gas: -210°C about as many degrees below zero as boiling hot oil for deep frying is above zero. And because liquid nitrogen has a boiling temperature of -195.8°C, it freezes anything that comes into contact and dissipate readily into the atmosphere at room temperature.
Due to its cryogenic functionality, LN2 can enable instant freezing and results in the formation of smaller ice crystals in the food – a desirable quality in preserving meat products because smaller ice crystals will have minimal disruption on the muscle cellular and thus preventing water lost upon thawing.
Many food processors utilize LN2 as an instant freezing agent for freezing processes that is traditionally achievable only through mechanical freezing such as blast freezer and refrigerator. LN2 is perfect for lowering the temperature of cooked food such as thick, curry spaghetti sauce from a boiling temperature to ice cold within minutes. This minimizes the multiplication of pathogenic microorganisms in the food. So, in terms of food safety and cost saving, this is a highly recommended solution.
As far as culinary is concerned, the chefs from culinary department of KDU University College Penang, led by the dean, chef Cindy Low said that for many years, the coldest substances chefs had ready access to was only dry ice (frozen carbon dioxide), which sublimates to CO2 gas at -78.5°C. Although there are many wonderful applications a block of dry ice can do, its solid form compared to the fluidity of LN2 prove to have some setbacks when it comes to recipes that require applications of versatility.
But, as with many new cooking products and techniques, the key to mainstreaming is to get chefs to use it in ways that integrate with their daily ingredients. And by far, the most common use for LN2 in the kitchen of KDU University College Penang is the making of fruits sorbet or similar derivatives thereof (i.e. ice cream or granita), it takes chef Aravin, who is a competition chef and lecturer about two to three minutes to whip out something smooth and silky as opposed to the long hours required to mechanically churn out his famous rum and raisin sorbet. According to Aravin, the main advantage of using LN2 is that ice crystals are formed instantly and traditional additives that are added to prevent uneven formation of ice crystals such as sugar, stabilizer or alcohol have become almost irrelevant.
As the usages of LN2 are streaming down from mass food preparation to aesthetical presentation, KDU University College Penang pastry chef Tinagaran is also on a creative high because of its endless possibilities for sponges, frozen creams, frozen mousses and espuma. But a rather common usage in his pastry lessons for liquid nitrogen is in grinding and shattering raw ingredients, particularly colorful fruits, to create powder or dusts. For example, fresh raw raspberries may be dipped in LN2, once frozen they can be pounded or crushed to pieces and then used for a dramatic presentation in a plated dessert dish. According to chef Tina these are some groundbreaking and innovative applications that cannot be achieved with conventional freezing applications. And for those hot kitchen savory chefs that are constantly in their quest for the perfect steaks or burgers, they are definitely not left obliterated, as many top restaurant chefs are now exploring the rapid freezing advantage of LN2 to cook their protein dishes.
So then one may ask, how exactly does a liquid that freezes so fast fit the bill of a grilled steak that requires high temperature cooking? Well, as it turns out, chef Cindy Low and her hot kitchen team are already cooking their meats on different ranges of temperature! And this can only be achieved with the help of another culinary technology called sous vide cooking where the meats are first marinated, vacuum packed and simmered in water bath of 80°C to medium rare, then dipped briefly in LN2 to freeze completely before being deep-fried in hot oil for serving.
But as chef Cindy explained the process, we began to understand the extra efforts she takes to prepare slabs of rib eye steak, indeed, the deep-frying process is essential because it creates a right amount of Maillard reaction induced crust on the outside but does not overcook the interior, which is deliberately frozen. “I believe that LN2 applications will be the standard repertoires in many kitchens, while it is still new for the food and beverage industry, but what’s really important is that it can help us to have better control on our food and deliver better results every time”.
What makes LN2 so versatile is that its viscosity is about one-fifth that of water (5) and has relatively low surface tension (6), “The variation with temperature of the surface energies and densities of liquid nitrogen, argon & carbon monoxide (7),” make the solution flows rapidly into every part of foods that needs to be frozen. To be able to manipulate minus degree temperature without costly mechanical systems will certainly help many food operators to monitor, upgrade and standardize their food qualities to meet the stringent demands from the food industry that is progressing towards sustainability and efficiency.
(1) Tilden, William Augustus, 2009 • (2) BiblioBazaar, LLC. p. 249. ISBN 1-103-35842-1 • (3) Henshaw, D. G.; Hurst, D. G.; Pope, N. K., 1953 • (4) Physical Review. 92:1229 • (5) Foster, S., “Viscosity measuremeants in liquid neon, argon, and nitrogen, cryogenics, vol. 3, 176-177, 1963 • (6) Baly, E.C.C, and Donnan, F.G. • (7) J. Chem. Soc. (London) 81. 907 – 23, 1902.
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