Liquid Nitrogen being churned as kids are over watching the gases diffusing through the air. (Photo Credit: Alpha Chi Sigma and Explore Your Universe at UCLA. )
Guest post by Steven Du
Creamistry – n. the science of creating ice cream using Liquid Nitrogen and not to be confused by the Ice Cream shop with the same name [4]. Ice cream does not seem complicated to make, but contrary to popular belief it is not as simple as just freezing cream and sugar; rather, this complex process requires slowly freezing cream to allow for small ice crystals to form, which results in a creamy ice cream texture. While the ice cream making process can be long and arduous, liquid nitrogen can enable similar creamy results at a much faster rate.
Liquid Nitrogen is a chemical that boils at a very low temperature, to be precise -312 F; this means that at room temperature, nitrogen is a gas, while at very cold temperatures < -312F, it is a liquid. In its liquid form, nitrogen provides a handy way to make ice cream fast: it can be poured over the ice cream base mixture—which is mostly heavy cream and sugar— to reduce the temperature of your ice cream mixture very quickly. The drop-in temperature reduces the motion of all the molecules and water molecules begin to form small seed crystals and nucleation sites. As you stir, the mechanical energy breaks up the crystals into tiny pieces. By contrast, if you place your ice cream mixture into the freezer, there are no external forces to interrupt the growth of the ice crystals, and the resultant ice cream will feel grainy and coarse.
We mention crystals a lot, but in the ultimate ice cream, we never want to feel the texture of crystals in our mouths; ideally, they are much smaller than the particle size that our taste buds can detect which is about 20μm [1]. However, if the water crystals are too large they can have an adverse effect on ice cream texture, resulting in an ‘icy’ texture that is not so smooth and creamy. Formation of ice crystals starts with nucleation sites or seed crystals that are frozen in the solution of cream and sugar. The more nucleation sites, the more ice crystals that are formed. If frozen too quickly, the initial nucleation sites may develop into much larger crystals and have adverse effects on the overall ice cream texture [4]. But with rapid stirring, large ice crystals are prevented from forming and a creamy and smooth texture of ice cream results, which even rivals the smoothness of store bought ice cream. Most ice cream shops centrifuge bowls filled with ice cream mixtures with a mixer to prevent the formation of large ice crystals, but flash freezing with liquid nitrogen circumvents that, by not allowing enough time for large ice crystals above 100mm from developing in ice cream and creating more seed crystals [4]. Creamier ice creams contain minute ice crystals ranging around 10- 20 μm, when churned slowly or frozen quickly.
The cryogenic technique of using liquid nitrogen to make ice cream is not new; it is used in other industries to preserve samples of cells and tissues, as well as to flash freeze food products for preservation. The use of liquid nitrogen in gastronomy has been used in a plethora of restaurants and dishes in the recent age of modernist cuisine. For example, creating powder from flash frozen herbs with fresh picked herbs, or even creating a foie gras flower from frozen duck liver. Creating ice cream from liquid nitrogen isn’t the only cooking technique. Cryocooking, primarily cryo ‘frying’, is utilized as a technique to freeze a food product and allow it to fry without becoming overcooked. For example, a burger patty may be flash frozen with liquid nitrogen, and deep fried to allow for a perfectly medium rare burger patty. By contrast, if the burger was pre-frozen, it could slowly develop ice crystals in the freezer, resulting in an undesirable texture.
Safety is something we should always discuss with working with chemicals, as liquid nitrogen is a very cold liquid, it should always be handled with gloves and protective eyewear. If liquid nitrogen gets in contact with your skin it can result in severe frost bite.
Overall, liquid nitrogen provides a fun and unique method used to develop dishes that have unique textures. Cold and refreshing new dishes have inundated the food world from frozen caviar pearls to frozen cereal balls that are called “Dragon Breath”. Check out your local Creamistry ice cream shop, for a quick display of this science in action!
References:
- Cook, K.l.k., and R.w. Hartel. “Mechanisms of Ice Crystallization in Ice Cream Production.” Comprehensive Reviews in Food Science and Food Safety, vol. 9, no. 2, 2010, pp. 213–222., doi:10.1111/j.1541-4337.2009.00101.x.
- Halford, B. “Ice Cream: The Finer Points of Physical Chemistry and Flavor Release Make this Favorite Treat so Sweet.” Chemical & Engineering News,Nov 28, 2004: http://pubs.acs.org/cen/whatstuff/stuff/8245icecream.html [accessed Dec 2013].
- “Ice Cream.” C&EN: WHAT’S THAT STUFF? ICE CREAM, pubs.acs.org/cen/whatstuff/stuff/8245icecream.html.
- Kilara, A.; Chandan, R. C.; Hui, Y. H. “Ice Cream and Frozen Desserts.”Handbook of Food Products Manufacturing, John Wiley Online Library, Chapter 74, pp 593–633, Aug 1, 2006: http://www.researchgate.net/publication/227580162_Ice_Cream_and_Frozen_Desserts/file/9c9605151b162a696c.pdf [accessed Dec 2013].
- “Spooky Halloween Treats.” Creamistry. N.p., n.d. Web. 02 Nov. 2017.
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