More on Please Explain: Salt

Friday, January 14, 2011 - 05:17 PM

It's almost impossible to answer every question during a Please Explain segment, and today's discussion of salt left us wondering about one question in particular—Elliott from New York asked: Why do you put salt in an ice cream maker to keep the ice from melting…then put salt on the sidewalk to make ice melt?

I did a little research and found the explanation. Salt lowers the freezing point of water, which is normally 32°F (0°C). When you spread sodium chloride on a sidewalk, the freezing point becomes about 15°F (-9°C). 

When you make ice cream, the ice needs to stay below a freezing temperature for a long enough time to allow the milk or cream to freeze. So you add salt to the ice in order to keep its temperature well below freezing, even after it has melted.

Here's a simple recipe for homemade ice cream that requires no special equipment:

1/2 cup milk
1/2 teaspoon vanilla
1 tablespoon sugar
4 cups crushed ice
4 tablespoons salt
1 quart size zip-top bag
1 gallon size zip-top bag

Mix together the milk, vanilla, and sugar, the pour into the small bag and seal, making sure there's as little air in the bag as possible. Put the small bag inside the large bag and add the ice, then add the salt. Seal the bag with as little air inside as possible. Wrap the bag in the towel and shake and massage it for 10-15 minutes. The ice will melt but will remain below freezing, and the milk will turn into ice cream!

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Comments [2]


My understanding of the salt/ice dilemma is actually that salt in fact causes the ice to melt, just like it does on the sidewalk. The idea is that water surrounds the ice cream a lot better than solid ice would, and salt allows that water to be at the correct temperature. Salt, of course, also raises the boiling point of water, making it a good tool in making pasta!

Jan. 26 2011 10:16 AM
Pierre Laurent

Sorry that I caught your show on salt late and was not able to participate. I used to get terrible cramps in my feet, gastrocnemius and tibialis anterior at night. One of my Patients (I am a Massage Therapist) recommended a pinch of salt with a small glass of Shweppes w/Quinine a day. It works! Pass it along and greetings from France! Great show! Keep up the interesting work. Pierre L.

Jan. 15 2011 02:13 AM

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