By now our regular readers are aware that we think chemistry and economics define cool. But what could be cooler (figuratively and literally) than liquid nitrogen ice cream? You may have seen liquid nitrogen featured on cooking shows that highlight molecular gastronomy, and have wondered what it is. We are most familiar with nitrogen (N2) as a gas comprising 78% of earth’s atmosphere. Liquid nitrogen is made by fractional distillation of liquified air. Liquid nitrogen boils at -321 degrees on the Fahrenheit temperature scale (that’s 77 K or -196 degrees Celcius). This very cold temperature is what makes liquid nitrogen ice cream the smoothest and creamiest frozen treat we have ever tasted. It is also dramatic and fun – so much fun that the UCR Chemistry Department has an annual liquid nitrogen ice cream contest in which graduate students from the different groups compete to make the most delicious and creative flavors. This year’s contribution from the graduate students in the Larive group (Daryl, Greg, Derek, Consuelo, Meredith and Ngoc), Black Forest Ice Cream with Candied Cherries, was an especially creamy dark chocolate ice cream with dark chocolate chips, sweet candied cherries and a cherry sauce drizzled over the top. Yum!
Any standard ice cream base can be used for liquid nitrogen ice cream, which also means that those of you who do not have access to liquid nitrogen can use our recipe with standard ice cream freezers. To keep liquid nitrogen cold, it is transported in a special container called a dewar, which is basically a large thermos bottle. The liquid nitrogen is added directly to the ice cream base while stirring. (Caution – liquid nitrogen can cause severe burns – see the safety precautions below.) Since it is so cold, liquid nitrogen freezes the ice cream very quickly, so quickly that the ice crystals formed are much smaller than in regular ice cream, giving it an unbelievably smooth and creamy texture. The liquid nitrogen quickly evaporates into a colorless and odorless gas. Although it appears to “smoke” it is really just condensing water vapor present in the air to form a fog.
Recipe (adapted from http://kitchensimplicity.com/black-forest-ice-cream/)
Chocolate Ice Cream
2 cups heavy cream
3 tablespoons dark cocoa powder
5 oz. dark or semisweet chocolate chips, chopped
1 cup whole milk
3/4 cup sugar
1/2 teaspoon vanilla extract
1.5 cups water
1 cup sugar
1 tablespoon fresh lemon juice
1 drop pure almond extract
Candied cherries: Place cherries, water, sugar and lemon juice into a medium-sized sauce pan. Bring to a boil and lower heat boiling gently for 25 minutes with frequent stirring until the liquid reaches a syrupy consistency. Remove from heat and add almond extract. Allow cherries to cool in the syrup then transfer to sealed container and store in the refrigerator (for up to 2 weeks) until ready to use. Drain in a strainer before adding to ice cream. Reserve syrup to drizzle over the top.
Chocolate ice cream: In a large bowl (if making liquid nitrogen ice cream you must use a large metal bowl, glass or plastic can shatter from the liquid nitrogen), mix together the liquid ingredients. Add sugar and cocoa powder, stir until dissolved. Slowly and carefully add liquid nitrogen. Stir vigorously until the ice cream is smooth and set. Alternatively, add the mixture to a conventional ice cream machine. Once the ice cream has set, stir in finely chopped dark chocolate chips and candied cherries. Drizzle each serving with reserved cherry syrup and garnish with chopped toasted almonds.
Safety precautions for handling liquid nitrogen and making liquid nitrogen ice cream: Liquid nitrogen should only be handled carefully. Safety glasses should be worn to protect the eyes. It can cause severe burns, avoid direct contact with skin. Do not pour liquid nitrogen into any glass or plastic container as these can shatter. While a stainless steel bowl, like the one we use, will not shatter when the liquid nitrogen is added it will become very cold so handle with care to avoid frostbite. Stir the ice cream with a study plastic spoon or a metal spoon with a handle made of rubber or plastic that will not conduct cold.