Phase change. Solutes and solvents. Sugar and salt. And why freezing point depression is key to making smooth ice cream.
1-Minute NomNom
If we want to make ice cream that is as smooth as possible, freezing it at 0°C/32°F would be a bad idea.
At 0°C/32°F, the water and ice in the ice cream mix are undergoing a phase change, and they are “melting and refreezing at the same rate“.
In this state of limbo, the ice crystals that form are likely to grow bigger and bigger, creating a grainy texture in the ice. We have seen this before: when we take ice cream in and out of the refrigerator several times – it is just not as smooth after a while.
Of course if we had liquid nitrogen, the crystals would be the smallest (read why and how in the 1-Minute NomNom “Freeze!“). But what do we do if we are doing this at home and do not have liquid nitrogen?
Fortunately, ice cream recipes call for the use of sugar, which can contribute to a scientific phenomenon known as freezing point depression. Freezing point depression lowers the freezing point to below 0°C/32°F. This happens when we dissolve a solute in a solvent because…
“As soon as you add a substance (or solute) that dissolves in the liquid portion (or solvent) of the food, the solute physically interrupts the structure and changes the freezing point.” (Source: Exploratorium)
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In the case of ice cream, the solute is the sugar that dissolves in the water (i.e. solvent) in the ice cream mix. Different concentrations of sugar will depress the freezing points to different degrees (pun intended).
Ice cream under microscope (From Clarke, 2003, “The Physics of Ice Cream” Physics Education 38 (3)) – h/t http://www.icecreamnation.org/science-of-ice-cream/
For example, mixing in about 50 grams of sugar into 100 ml of water can depress the freezing point to about -3°C/27°F. The water and ice crystals are not in a state of limbo but will freeze quickly, forming much smaller crystals, and giving us a smoother ice cream texture.
But what if your freezer or ice cream maker can only be set at 0°C/32°F – where do we get the lower temperatures that will freeze our ice cream?
Making Thai traditional ice cream by rotating the tube in tank filled with ice and salt
Why not use the same scientific phenomenon again?
We can mix salt with water, freeze them, and use that to freeze our ice cream. Freezing point depression is even more pronounced with salt as the solute. At just 9 grams of salt in 100 ml of water, the freezing point is lowered to -5°C/23°F.
Perfect for making yummy ice cream.
Like this? We are sugar and ice and everything nice too so like me to discover more? All you need is a minute a day to explore the world’s marvels through the phenomenon of food!
