Using dry ice to make ice cream (molecular gastronomy)

Homemade ice cream is a wonderful treat and really rewarding activity. But it can take time to freeze your creations. To eliminate the need for expensive ice cream machines and to reduce cooling time (and effort) why not make it easier, fun and more spectacular by incorporating some science?

Ingredients

  • 600mL Ice cream mix/anglaise (preferably refrigerated/chilled) – or for easy ice cream that requires no prep scroll click here.
  • 2kg dry ice (see below for more information on dry ice including stockists and safety requirements)

Equipment

  • Food processor or stick blender with food processor attachment
  • Safety glasses
  • Tongs
  • Electric beaters or wooden spoon
  • Metal bowl
  • Sieve
  • Clean tea-towel

Method

  1. Put on safety glasses.
  2. Dry ice usually comes as pellets; using tongs add dry ice pellets to food processor and blend/pulse until pellets are converted to a snow/dust like consistency.
  3. Put dry ice “dust” through a sieve to remove any larger lumps.
  4. Pour ice cream mix into metal bowl.
  5. Place bowl onto folded tea towel to insulate and to help create a non-slip surface.
  6. Add 1/3 cup dry ice dust to ice cream mix and stir (either by hand using a wooden spoon or using electric beaters). You will notice that the ice cream gets very cold and freezes where the dry ice touches it. It will also bubble and produce “smoke” – this not dangerous and is actually condensation (basically clouds).
  7. Once “clouds” are no longer being produced, add more dry ice. Continue this process until you reach the consistency of ice cream you desire.
  8. Allow to rest for a few minutes to ensure all dry ice has disappeared and then serve.

SAFETY

  • Dry ice is very cold (minus 78.5 degrees Celsius).
  • As such it should not be handled with bare hands or eaten.
  • Keep away from pets and kids.
  • Do not tip down drains as water in the drain will freeze and as it does so will expand, which may result in damage.
  • Do not enclose in airtight containers.
  • Use in a well ventilated area.

Scientific stuff

Dry ice is actually frozen carbon dioxide. Unlike water that can express all three phases (solid, liquid and gas) – on Earth, carbon dioxide has a gas phase and solid phase, but no liquid phase. So it changes from a solid directly into a gas (a process called sublimation). This results in no liquid phase (and no liquid at all) and therefore it is termed “dry” ice.

Dry ice is colourless, slightly acidic and has a zesty odour. It is mainly used to refrigerate food or lab samples and for flash freezing.

Adding dry ice to water increases the speed of sublimation and can make the water acidic. As the carbon dioxide changes to gas some of it dissolves into the water and produces carbonic acid (a weak acid). This change can be seen if an indicator such as universal indicator or a pH meter is used.

As dry ice is constantly changing from a solid to a gas it doesn’t last very long. At the time of purchase you may have an esky full of pellets while 12 hours later you won’t. This means three things:

  1. you should never put dry ice in an air tight or enclosed space as it will cause an explosion (due to built up pressure as gas takes up more space than a solid)
  2. you should only keep and use dry ice in a well ventilated area (as carbon dioxide can cause hypercapnea – too much carbon dioxide in the blood)
  3. if using the dry ice sometime after purchase, you should buy more than you need to counteract sublimation rates of your dry ice.

For more experiments you can try with dry ice, click here.

Stockists

Dry ice can be purchased by the kilogram from the following businesses. It is suggested you call ahead to ensure they have stock and take your own esky so your dry ice is well insulated.

Australia wide – http://www.boc.com.au

South Australia – http://www.dryicesupplies.com.au

New South Wales – http://www.mriceman.com.au