Liquid nitrogen, if you’re not familiar with it, is a liquid that is commonly used to preserve tissue samples, remove warts, help keep MRIs cool, or cool high performance computers. The reason for this is its incredibly low temperature: -196 C (compare this to ice, which is 0 C, or dry ice, which is only -78.5 C…).
And this is what makes it perfect for creating ice cream in a matter of minutes… the rapid freezing also results in smaller ice crystals, meaning a much smoother taste. See below for some easy to follow instructions.
You will need a food processor, spatula, access to liquid nitrogen and suitable PPE!
1 cup milk
1 cup double cream
5 tablespoons sugar
1 teaspoon vanilla
1 cup of strawberries (or other fruit to taste)
1 litre liquid nitrogen
Mix all the ingredients in the food processor and whisk for a few seconds. Start gradually pouring in the liquid nitrogen whilst whisking, keeping a spatula on the side of the bowl to break up any chunks of cold mixture. Eventually the mixture will freeze into a smooth, ice cream texture.
Caution: If you go too fast, or the bowl is too small, the rapid boil off of liquid nitrogen will cause your ice cream to bubble over the side of the bowl! (see above picture of the aftermath…)
Safety note about liquid nitrogen: Because it is so cold, and will expand to about 70 times its volume when it warms up, there are 3 major safety concerns when mixing liquid nitrogen and food:
Wear appropriate gloves and clothing when handling, to avoid cold burns. If this does happen, hold the affected part under cold water for a few minutes.
Make sure to perform the activity in a well ventilated room. If you are unsure there are calculators online that will let you determine the drop in relative oxygen content if your dewar of liquid nitrogen were to all boil off at once.
Never eat or drink anything where the liquid nitrogen has not boiled off. This is simple to check as the ‘vapour’ that boils off when you add it to the ice cream will have stopped. Also, the ice cream would be too stiff for the food processor to mix if you had cooled it to -196 C.
Every year as part of British science week Loughborough University opens its doors to the community to host a range of STEM based activities. This year there are several activities organised by my group and in coordination with the UG reps:
Roughly the same cost (weight for weight) as a pint of milk, it’s a common feature in science fiction films: the nitrogen dewar in the background that might at some point be used to freeze that alien chasing you down the corridor…
But how much liquid nitrogen would it actually take to do this?
Hint: Assume the creature weighs about 50kg and has a heat capacity of 2000 J/K/kg. Liquid nitrogen has a temperature of 77K and latent heat of 199 kJ/kg. For arguments sake, let’s say the creature becomes vulnerable at 250K…