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There seems to be a recommended minimum temperature for making ice cream of about -30C. But, the temperature of liquid nitrogen (-200C), which is often used for the same purpose, is much lower.

So, what is the harm in making ice cream using an outer freeze wall below -30C?

bobuhito
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3 Answers3

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During freezing, you care about two parameters - crystal size and overrun. While people making ice cream at home will frequently tell you that smaller crystals equal softer ice cream, that's not exactly correct, especially when you can control the two parameters over a wider range. In reality, smaller crystals make smoother ice cream, while more overrun makes softer ice cream.

In the context of commercial ice cream freezing with a conventional freezer (not liquid nitrogen), if your ice cream freezes too quickly, it will be frozen before it has been properly frothed by the dasher, and you will end up with too dense ice cream. This is why commercial freezers are made on purpose in the -23 to -29C range. They could build them colder, but that is not desirable. This seems to be true over a wide range of technologies, since both batch freezers and continuous freezers operate at these temperatures. The amount of overrun is then controlled by dasher speed and dasher design.

I don't know the exact physics of liquid nitrogen ice cream and why it works well. I could imagine different scenarios: maybe people consuming it prefer low-overrun ice cream, maybe the boiling nitrogen introduces sufficient aeration, or maybe the dasher breaks it up into very fine pieces because it is so incredibly brittle. Possibly somebody with first-hand experience will comment on this. But it is a different principle than conventional ice cream preparation where you want to give your ice cream time to get churned properly before it freezes too much.

rumtscho
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  • I see. But, could the churning just be done faster to get back the optimum aeration and eliminate this concern in the end? – bobuhito Apr 17 '21 at 19:26
  • I don't think so - at least a whole industry has decided to not pursue this. After all, there is some very complex physics involved in ice cream bases, so I don't think that you can randomly change parameters such as speed without getting a ton of unwanted consequences. They could have other reasons, sure - such as the avaliability of convenient refrigerants, or backwards compatibility, or a small number of producers - but my hunch is that it won't be easy to get optimal aeration just through added speed. – rumtscho Apr 17 '21 at 19:28
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    "I don't know the exact physics of liquid nitrogen ice cream and why it works well." I think that you might be onto something when you're talking about the boiling: not only is it providing aeration, but it's also probably boiling away before it can fully lower the temperature of the ice cream down to the temperature of the liquid nitrogen. If you look at the recipe for it in FuzzyChef's answer, you'll note that it involves adding small amounts of liquid nitrogen into the ice cream mixture and stirring until the liquid nitrogen boils away until you reach the desired consitency. – nick012000 Apr 18 '21 at 03:10
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    @rumtscho This argument neglects cost: it's entirely possible (and at least somewhat plausible) that the industry knows how to use liquid nitrogen well, but that it's vastly cheaper to operate at warmer temperatures, at little or no difference in quality. – Konrad Rudolph Apr 18 '21 at 08:38
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    @KonradRudolph I am sure that the industry knows how to use liquid nitrogen :) When listing arguments, I was thinking more of incremental reduction of the temperature of a conventional freezer to maybe -40 or -60, not a jump to liquid nitrogen technology. And yes, cost can be a factor there too. Seeing how much people are willing to pay for a container of "premium" ice cream, if these temperatures would make noticeably better ice cream, producers would likely have started used them by now, with the cost being offset by the price. – rumtscho Apr 18 '21 at 09:01
  • Konrad, Runtscho: I think there's something to that, because even professional equipment like the Brrrr run a narrow pipe filled with N through the cream, instead of filling the walls of a drum with N. And when you see the Brrr in action, you see a LOT of N outgassing. – FuzzyChef Apr 18 '21 at 19:11
  • @bobuhito churning adds heat. Faster churning adds more heat, in a non-linear way, because power increases as the square of velocity. That works against the goal of keeping things cold. – Lawnmower Man Apr 18 '21 at 21:02
  • For LN2, definitely the nitrogen provides aeration and frothing. The trick is very fast mixing - normally this adds heat you don't want, but the LN2 is so cold that it doesn't matter and fast mixing helps incorporate the rapidly evolving gas into the cream. – J... Apr 19 '21 at 17:47
  • @J... the fast mixing is also compatible with the theory of easy breaking up in the moment just after freezing. I have never worked with liquid nitrogen myself, for ice cream or otherwise, and I would love to see this examined in detail, by somebody with the equipment and knowledge to do so. I doubt that many low-temperature physicists are paid for that, though. – rumtscho Apr 19 '21 at 17:53
  • @rumtscho Not paid for, certainly, but plenty of us have, erm... taken advantage of opportunities when they arise ;) I'm sure someone somewhere has written a paper about it. The cream does want to freeze almost instantly, so yes, vigorous mixing is needed to spread the cold around and to prevent forming a big frozen lump at -55C sitting happily self-insulated in a bath of room-temperature cream. – J... Apr 19 '21 at 17:57
  • @J... that's certainly a new form of frozen lolly! The next time you end up with the lump, I would love to see a picture, just out of curiosity. – rumtscho Apr 19 '21 at 18:07
  • @rumtscho Unlikely to be forthcoming - I had pretty free access to cryogenics twenty years ago. Not so much any more. – J... Apr 19 '21 at 19:12
  • @J... (and rumtscho) I've made LN ice cream by hand a few times; it was smooth and creamy. You're limited as to how fast you can beat it then, especially when it suddenly stiffens. So by hand it's not a quick process - typically one person drizzles in the LN slowly while another stirs. A former colleague also worked in a cafe that had a system for making LN ice cream to order; I believe a powerful machine was used. If I could get my stand mixer to the LN I'd fancy giving that a try, whisking fairly fast and pouring slowly. – Chris H Jul 24 '22 at 08:16
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You can make ice cream with liquid nitrogen; in fact, this makes truly excellent ice cream if you have the equipment for it.

The last is the key, though. Home ice-cream makers are designed to operate and produce optimal creaminess at -10C or -20C. If you were to somehow fill one with liquid nitrogen, the custard would freeze solid before it agitated correctly, and the plastic mixer blades would break. So you can, and should if you can, make ice cream at a lower temperature, just not using your standard ice-cream machine.

FuzzyChef
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    And for today's useless trivia -- the ice cream churn mixing blade is called a "dasher", although for some reason, when you search for that term on the internet, it returns a lot of "dishers" (the scoops for serving ice cream) – Joe Apr 17 '21 at 19:01
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    Just to confirm, you are saying that if my ice cream maker does not break, the lower the wall temperature the better? There is no minimum as far as taste and texture go? – bobuhito Apr 17 '21 at 19:12
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    @bobuhito -- the custard would freeze solid. That's not ice cream. That's just ice. – Pete Becker Apr 17 '21 at 22:16
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    @PeteBecker It wouldn't freeze solid because you're not adding enough liquid nitrogen to do that. Look at the first recipe linked to in this answer; you stop adding liquid nitrogen once your ice cream reaches the desired consistency. – nick012000 Apr 18 '21 at 03:11
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    You don't need any fancy equipment to make it other than the dewar to store the LN2. I've had it made at events using just a metal mixing bowl, and stirring implement. Depending on how much/how fast the LN2 was added the result could be anything from softserve to something similar to dipndots. – Dan Is Fiddling By Firelight Apr 18 '21 at 05:26
  • Bobuhito: that's correct. The faster you freeze the custard, the smaller ... and thus creamier ... the ice cream will be. – FuzzyChef Apr 18 '21 at 18:42
  • Dan: as long as that stirring implement isn't plastic ... – FuzzyChef Apr 18 '21 at 18:44
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    Bobuhito: theoretically, you could make an ice cream machine that ran at 5K and agitated at 200RPM, and make super-creamy ice cream in 15s. If you build one, make a video. – FuzzyChef Apr 18 '21 at 19:03
  • @FuzzyChef I think that depends on the size of the batch, because at some point, the thermal conductivity of the ice cream will limit the process. – Lawnmower Man Apr 18 '21 at 21:10
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    I've made LN ice cream and the key is to add it gradually, stirring all the time with a very strong wooden spoon. It stiffens up locally where the LN hits it but that soon mixes in. I've always fancied trying a stand mixer and LN, but transporting a Kenwood by bike (the LN is in work) always seemed like too much trouble – Chris H Apr 19 '21 at 08:20
  • I've tried pre-cooling my ingredients before adding them to my home ice cream maker. A thin layer of ice formed on the walls, causing the blades to get stuck. I ended up having to manually scrape the walls until everything had had warmed up enough that the blades were sweeping the walls faster than the ice was forming. – Brian Apr 30 '21 at 16:46
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The ice cream dasher is inconsequential when it comes to preparing ice cream. I run a stainless steel 7 inch dasher with liquid nitrogen pumped in by holes in the top of the mixer. The key is to not run the dasher for more than 13 seconds. To get the optimal crystal size without too little overrun, I use 1 cup of liquid nitrogen for every 10 cups of liquid milk. Mix for about 10 seconds and then you will have perfect ice cream every time. I am an ex Blue Bell employee and have experience with making ice cream under commercial conditions.

rumtscho
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randy kepic
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    Hi Randy, jpa is correct - we insinst on neutral language here. I cleaned up the tone, while keeping in the information you provided, it seems very useful. – rumtscho Apr 19 '21 at 07:02