This guide shows how to make instant, well almost instant, ice cream. There is a scientific reason for conducting this practical which we will get to later.
This is a perfect practical or demonstration for a science club, but be aware that food and drink should not be consumed in the science lab. Because of the cooking involved, it may be more appropriate to conduct this in a home economics/food technology department.
The sugar and milk should be put into a large bowl which is then stood in a saucepan of boiling water. This is to ensure the mixture remains at 100°C and doesn?t overheat. Stir the three beaten egg yolks into the mixture and simmer until you see small bubbles forming at the edges of the mixture. When the mixture can coat the back of a spoon and is of a consistent texture, it is done.
When the mixture is cooled down to room temperature, add a few drops of vanilla extract and the whipping cream. Stir gently until thoroughly mixed.
Now comes the science part!
Pour approximately a cupful of the mixture into the medium sized sandwich bag and seal. Double bagging the mixture may be sensible to ensure none leaks out during the next part.
Half fill the large sandwich bag with crushed ice and add the cup of salt. Then place the medium bag inside the large bag and seal it securely.
Wrap the bag in a tea towel and using your hands, mix it around, roll it and generally let as much of the ice touch the inner bag as possible. It takes around 7-10 minutes for the ice cream mixture to turn into a frozen desert.
Remove the ice cream from the bag once set and add fruit if required.
When ice melts, it takes heat from its surroundings, in this case from the ice cream mixture. Without adding the salt to the crushed ice, the temperature of the mixture would only get down to the temperature of the ice, ultimately zero °C. This is not cold enough to freeze the mixture.
Adding salt to the ice means that the resultant mixture has a much lower melting point than water alone. As the ice melts, because the salty water cannot re-freeze (it has a lower freezing temperature than plain water) it takes more heat away from its surroundings, thereby cooling the ice cream mixture rapidly and taking it down to a temperature well below 0°C.
Typically temperatures of around -24°C can be achieved by mixing ice and salt.