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Photos by Maya Chavez-Akin.

Making ice cream with cryogens stronger than water ice is a fairly common chemistry demonstration stunt. The ideal way to do it is with liquid nitrogen, which is poured directly into the ice cream mixture, with stirring, and causes it to set up in about 10 minutes. Liquid nitrogen, however, can be rather difficult to get your hands on. Most major cities have a supplier that will sell it to you, but very often they have large minimum orders and/or require that you own an expensive dewar flask into which they may safely dispense the liquid nitrogen. At -196 C, liquid nitrogen is also fairly dangerous to handle.

Dry ice is a much more accessible cryogen; it’s available at several major grocery stores in the Austin area, for instance, and I imagine the same is true in other parts of the United States. It sublimes at -78 C, and is thus vastly more effective at freezing stuff than water ice at 0 C. You can make ice cream, just as with liquid nitrogen, by adding dry ice directly to the ice cream mixture. However, because dry ice is frozen carbon dioxide, this procedure results in carbonated ice cream. Which can be quite delicious. But say you don’t want carbonated ice cream?

This procedure borrows from a common technique in the organic chemistry laboratory for cooling reactions to sub-zero temperatures. Instead of using ice water to cool to 0 C, you make a bath of dry ice in some volatile solvent that will not freeze at dry ice’s sublimation temperature of -78 C. Obviously, you can’t use dry ice in water because the water will freeze solid. In the laboratory, acetone and isopropyl alcohol are common coolants. Acetone, however, can be dangerous if handled improperly, and isopropyl alcohol in sufficient quantities to make a large bath can be rather expensive.

I have discovered, however, that denatured ethanol, which is available in hardware stores everywhere, is reasonably priced and makes a good bath with dry ice. Denatured alcohol is also much safer to handle than acetone. Depending on the denaturant, it is also the least toxic of the various hardware-store solvents. In any case, done with reasonable care, this procedure involves no significant risk of contact between the ice cream itself and the coolant. And although denatured alcohol is quite flammable, the dry ice temperature of -78 C is well below its flash point at 13 C, which means that, once the bath is cool, there is no danger of the alcohol vapor igniting from a stray spark. To err on the safe side, however, you should be sure to work in a well-ventilated area.

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Tools:

  • Hammer
  • Wire coat hanger
  • Paper bag
  • Insulated container for temporary storage of dry ice
  • 1 gallon round personal cooler for bath (I used Igloo Legend 1 gal)
  • Food-grade stainless steel bain-marie pot (I used Polar Ware Model 1Y 1.25 qt)
  • Electric hand mixer with beaters to fit inside flask (I used Cuisinart HTM-7L SmartPower)

Materials:

  • Dry ice (about 5 lbs)
  • Denatured alcohol (1 gallon)
  • Heavy whipping cream (1 cup)
  • Half-and-half (1 cup)
  • Sugar (3/8 cup)
  • Vanilla extract (1 tsp)

Step 1: Add ice cream ingredients to flask

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Measure out your ice cream ingredients and add them to the stainless steel flask. Start with 3/8 cup sugar, then add 1 cup half-and-half followed by 1 cup heavy whipping cream. Finally, add 1 tsp vanilla extract. This recipe gives a basic (but tasty) vanilla. You should feel free to add alternate flavorings, colorings, and/or mix-ins to your own taste.

Note that the recipe given here is actually half of what I used. As written above, the recipe gives slightly more than a pint of ice cream. I tried to make two pints, which proved to be slightly beyond the capacity of my stainless steel flask and led to some overspill, which is undesirable because it contaminates the bath. If you follow the given recipe, the level of the mixture in your flask will be well below that shown in the photographs.


Step 2: Break up the dry ice

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The dry ice will probably come frozen into a solid brick in a plastic bag. Put the bag into a brown paper bag, rest the bag on a solid work surface, and whack it with a hammer for 30 seconds or so to break up the brick of dry ice inside. You don’t want any chunks larger than an average lime.


Step 3: Transfer dry ice to an insulated container

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CAUTION

Although the container shown in this photograph has a screw-on lid, you should in no case store dry ice in a container with an airtight lid. In a closed container, warming dry ice will rapidly build up very high gas pressures as it sublimes into gaseous carbon dioxide. An airtight container of dry ice is, in effect, a bomb. If you must use a container with an airtight lid, DO NOT IN ANY CASE SEAL THE LID WITH DRY ICE INSIDE. A stryrofoam beverage cooler is really a much better choice.

Remove the plastic bag full of crushed dry ice from the brown paper bag. Cut or tear off a corner of the bag, and dump the dry ice out into an insulated container for temporary storage. This is to prevent it subliming away too rapidly.


Step 4: Pour coolant into bath

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If you use the proportions given here, your flask will be about halfway full of ice cream makings. You want to make sure the level of your bath is slightly higher than the level of the ice cream in the flask, when the flask is in place. If you use the same model flask and bath containers that I did, a total bath volume of 10 cups is ideal. Making an allowance of 10% for solid dry ice in the bottom, you should add 9 cups of alcohol to the bath.

If you are using other containers, use the following method to determine the volume of alcohol to use for the bath: Suspend your flask in place over the bath using the wire triangle, then pour water into the bath until its halfway up the side of the flask. Remove the flask, measure out how much water is inside the bath, subtract 10% to account for solid dry ice volume, and use the resulting volume of alcohol. Make sure to wipe the water out of the bath before adding the alcohol.


Step 5: Slowly add dry ice to bath

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The slowest part of the process is cooling the bath. Although dry ice can be safely handled with bare hands if you hot-potato it, good technique calls for the use of gloves. So please use them.

Now, if you just dump the dry ice in all at once, the bath will boil over violently and you’ll get alcohol everywhere. So you have to add the dry ice a little bit at a time. Start with a grape-sized chunk, drop it in, and observe the effervescence that results. Then add another piece. Go slowly, and gradually you will get a feel for how much dry ice you can add, and how fast. As the bath cools, each piece will react less violently. Once the bath has equilibrated at -78 C, additional pieces of dry ice will plunk into it without any appreciable reaction. At this point, add additional dry ice until the bath is about 10% solid dry ice by volume.

CAUTION

The chilled dry ice/alcohol bath is very cold. If the chilled coolant contacts your skin it can cause almost instant frostbite. Use due care in handling the cold bath.


Step 6: Suspend flask in bath

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A simple wire triangle improvised from a coat hanger serves well to hold the flask in place in the bath. Just slip the hanger over the lip of the flask and squeeze the two sides of the hanger in to secure it in place. If you want, you can make a proper wire triangle by twisting together three lengths of coat hanger wire at their ends, in a triangular arrangement, using pliers.

Now slowly lower the flask into the cooling bath until it is resting with the three legs of the wire triangle on the rim of the bath. The bath will likely boil up during this operation, so use caution to keep it from spilling over.

CAUTION

While it is not difficult to carry out this process without getting coolant in the ice cream mixture, you should be very careful to make sure this does not happen. If the ice cream mixture is contaminated with any amount of coolant, however slight, you should err on the side of caution and discard it without eating it.


Step 7: Stir

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Insert the beaters of your hand mixer into the flask and blend on the lowest speed. You’ll be able to feel the ice cream thickening after about five minutes. Continue until the ice cream is the consistency you want. Ten minutes of blending gave me a nice thick smooth-serve type ice cream. If you use the smaller recipe given above, yours will probably take less time to thicken.


Notes and ideas

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Once you’re finished with the cooling bath, cover it loosely with something (NOT airtight, see warning above) and set it aside overnight to return to room temperature. Then pour the alcohol back into its original container. It can be reused as a coolant indefinitely.

If your ice cream spills over into the cooling bath, you can remove some of the contaminants from the alcohol, before storing it, by pouring it through a coffee filter. Many of the ingredients in the ice cream mixture are insoluble in denatured alcohol, and will settle out at the bottom of the bath as white solids. Filtration removes these easily.

The heavyweight of all cryogens is liquid helium. At -269C, it is only a couple of degrees above absolute zero. It is very dangerous to handle and very expensive, so I’ve never known anyone to use it on something as trivial as making ice-cream. Still, it would be quite a stunt…

Sean Michael Ragan

I am descended from 5,000 generations of tool-using primates. Also, I went to college and stuff. I write for MAKE, serve as Technical Editor for MAKE magazine, and develop original DIY content for Make: Projects.


Related

Comments

  1. Anonymous says:

    Denatured Alcohol is essentially moonshine. It contains high amounts of methanol which is highly toxic to animals and is added to prevent people from drinking the stuff. If you do not want to go blind, do not use Denatured Alcohol.

    1. Sean Michael Ragan says:

      Actually, that’s not necessarily correct. A wide variety of denaturants are used to treat ethanol; it’s not always methanol. That’s why I say “depending on the denaturant.” I refer you to the Wikipedia article on denatured alcohol:

      http://en.wikipedia.org/wiki/Denatured_alcohol

      And even if your denatured alcohol happens to contain methanol, as I make quite clear in the article, the alcohol does not come into contact with the ice cream ingredients at any point in the process.

      1. Toaste says:

        Denatured alcohol in the United States is virtually guaranteed to contain methanol, usually around 10%, although I hear SLX (available at Home Depot) is formulated with about 50% methanol.

        10mL methanol is enough to cause permanent blindness. This means, worst case, someone uses SLX to make small batch for him or herself and gets a mere 20mL splash into the food. They figure it’s no big deal, eat it, and go permanently blind.

        Since simple dry ice handling (unlikely to even cause injury even without gloves) merits a big fat caution box, please consider adding a second one warning experimenters to keep denatured alcohol away from anything they plan on eating, on pain of blindness or death.

        Secondly, I advise a stronger caution on the handling of the resulting -78C ethanol/methanol blend. Liquid nitrogen and dry ice to some extent protect idiots by sublimating as they come into initial contact with skin. Chilled alcohol does not, and even a small splash will cause severe frostbite.

        1. Sean Michael Ragan says:

          These are good suggestions, thank you. I’ve added the boxes you suggest.

    2. jim moser says:

      @anonymous 1…moonshine isn’t necessarily poisonous. Depending on who made it and how, it can be quite tasty.

      But don’t use denatured alcohol. If (and it probably will) contains methyl alcohol, it’s bad juju just to have it around. That stuff accumulates in your fatty tissues and even though you won’t be handling a lot of it, it’s a good idea to avoid it altogether. Instead, use ethanol (sold in these parts as ‘Everclear’) also known as grain alcohol. Not nearly hazardous as the other solvents. You could also use a high proof liquor or even vodka.

      Or as anonymous 2 said, rock salt and ice is a great way. All natural and you get some good exercise, too.

  2. Joe says:

    I feel like even mentioning acetone as a possibility is a bad thing. It’s an organic solvent that responsible people try their best not to ingest or even get on their skin. I know it’s in nail polish remover, but it’s an unnecessary exposure nonetheless.

    Also, it’s convenient to say that the bad stuff won’t come in contact with the ice cream ingredients as you’ve done with your comment on methanol above, but in practice you can’t always guarantee this.

    1. Sean Michael Ragan says:

      I’ve made it quite clear that, although acetone is commonly used as a coolant in the chemistry laboratory, it is too dangerous to use in the kitchen. I say these things specifically to deter people who might consider using the common method.

      But no, I can’t “always guarantee” that anyone who tries this isn’t going to accidentally get coolant in the ice cream mixture, no more than I can guarantee any other outcome. I can only assure you that it is very easy NOT to get coolant in the ice cream, and point out that, if you are concerned about the possibility, you should not attempt the project.

    2. oskay says:

      Not that you should go around drinking the stuff, but acetone is actually synthesized (in small quantity) by humans. It is certainly safer than methanol, which is one of the most dangerous easily accessible chemicals– you’d need to drink 2-5 times as much acetone to screw you up forever, and it’s got a distinct scent. (Methanol smells like ethanol, i.e., like very ripe fruit.)

      Also, I second the idea of directly added dry ice. Just be sure to sift it through a wire mesh before it goes in to restrict the particle size.

  3. Anonymous says:

    Rock salt and ice with a hand crank dasher can get you better results with only a little extra time and I do mean a little. However, eating your chemisrty lab version of desert is probably way more rewarding.

  4. http://www.bigroom.org/wordpress/ says:

    “You can make ice cream, just as with liquid nitrogen, by adding dry ice directly to the ice cream mixture. However, because dry ice is frozen carbon dioxide, this procedure results in carbonated ice cream. Which can be quite delicious.”

    This suggests you’ve gotten this to work – I tried this once myself but found that any bits of dry ice that were even as large as, say, a pea or corn-kernel would end up insulated by a rock-hard shell of ice and rattle around in the mixture rather than subliming away and chilling/carbonating the mixture as well as I’d hoped.

    How finely did you have to pound grind the dry ice, and how much did you add?

    1. Sean Michael Ragan says:

      I have not actually done it myself, but here’s a tutorial describing the process:

      http://chemistry.about.com/od/dryiceprojects/a/dryicecream.htm

      Theo Gray has a “Dry Ice Cream” project in his book “Mad Science,” but it’s slightly different: He blasts the stirring ice cream mixture with CO2 from a food-grade CO2 fire extinguisher. The project is also hosted on Popular Science’s website:

      http://www.popsci.com/node/3027

  5. Stuart says:

    This is great. I’m curious as to the texture differences between this and conventionally-frozen ice cream. Any thoughts?

  6. ventifact says:

    Hand-held electric mixers have pretty sparky comutators.

  7. Sean Michael Ragan says:

    “And although denatured alcohol is quite flammable, the dry ice temperature of -78 C is well below its flash point at 13 C, which means that, once the bath is cool, there is no danger of the alcohol vapor igniting from a stray spark. To err on the safe side, however, you should be sure to work in a well-ventilated area.”

    1. https://me.yahoo.com/a/cMvUKhs.qOt.n9tpwcroBOJMx3hW#a1b91 says:

      All this talk about not drinking the denatured alcohol made me think … is there any danger from inhaling the vaporized alcohol created by the dry ice? Now, I’m talking about an accidental whiff, not huffing the stuff. Huffing CO2 on it’s own is an asphyxiation danger. I’ve used dry ice to cool liquor before, and it creates an alcoholic vapor that smells much stronger than taking a whiff from the liquor bottle. I don’t know how the science works, but it seems like a stronger smell would indicate there is more particularized alcohol in the vapor. Any thoughts?

      Cool project. You’re probably tired of people picking holes in it.

      1. Thanks!

        I can’t say for sure, but I wouldn’t worry terribly much about alcohol inhalation during this process. At dry-ice temperatures, the vapor pressure of the alcohol (s) is very low. It seems to me there should be even less alcohol in the air than if you were, say, using the alcohol at ambient temperature to clean something. But there might be something to the mechanical volatilization you suggest. Good ventilation is called for, in any case, and not just for the alcohol; as you point out, dry ice does present some asphyxiation hazard simply because of the possibility that the subliming CO2 will displace the oxygen in a closed space.

  8. Zeke Shadfurman says:

    Aside from being cheaper than the alcohols listed, do rum or vodka freeze at -78c? (or however cold dry ice is)

    1. Jared Bronski says:

      Pure ethanol freezes somewhere south of -110c. Why not use Everclear or similar
      strength neutral grain spirits, and not have to worry about toxic solvents
      coming in contact with your tasty tasty ice cream.

      I think that most vodkas/rums would be too low a proof and would probably freeze,
      or at least turn into the worlds most potent slushy.

      1. Sean Michael Ragan says:

        Everclear was my first choice, and it should work great. Expense is the only problem. The 2 L or so required for this project are going to cost around $50 from the liquor store, versus $13 for a gallon of denatured alcohol.

  9. Lauren says:

    Great article! I will definitely make this :) I was researching some cheap alcohol to use, for those interested, I found the Trader Joe’s Vodka of the Gods is sold in a gallon jug and is only $10. One question though, is a mixer absolutely necessary, or can I just use a spatula and some arm power? Provided the cooling bath is stable of course…

  10. Bud says:

    Store-brand car antifreeze is cheaper than denatured alcohol, You do need to remember to add water in equal parts to the antifreeze. The back will tell you how cold the protection is good for. -80 is pretty common even for the cheap grocery store automotive aisle stuff. If you were wanting to do this in your ice cream freezer that’s probably a cheaper way to fill that big bucket. Ethylene glycol is even less likely to catch fire, like 220 C or something. In those big buckets that make a gallon at a time, the dry ice is what gets pricey; 3 bricks bare minimum. It makes a big heat sink, but not infinite. It’s got to suck the energy out of your ingredients. Make it count. Have the ingredients ready to make 4 gallons and be really sure to not get any ethylene glycol in your ice cream. It will be on the outside of your cylinder. It could drip/melt into your receiving container. You don’t want to poison yourself or your friends and family. Wear INSULATED gloves handling the cylinder. A cheap pair of jersey gloves won’t work. Put the gloves on. Slap a solid surface. If you can’t tell a BIG difference between slapping it with the gloves on and your bare hand, you need better gloves. A thick pair of Thinsulate gloves, from Walmart, cost me $12. There’s no reason to be injured doing something like this.

    1. Bud says:

      Oh, yeah, a gallon of cheap ethylene glycol antifreeze is about $12, so you get 2 gallons, since you dilute it, 50/50, with water.