Secret of ice cream – Freezing point depression

Who doesn’t like ice cream? A soft, slowly melting soft serve, some Italian gelato, a milkshake or a popsicle? Do you prefer chocolate (I do!), fruit, nut, bubble gum flavour? Do you prefer large or small crystals, a cold or a warm ice cream? Starting to get harder to choose? That would be great, because then I have loads to tell you in this post.

I really enjoy ice cream, but also find it highly fascinating. How come some kinds of ice cream can be scooped? How come ice cream is not a big solid rock of ice? Why does ice cream taste so cold sometimes? Why do some sorts of ice cream contain big ice crystals?

I’ve made a start answering these in my post about semifreddo, which is a very easy to make at home ice cream recipe, not requiring an ice cream machine. This post will be more sciency, but will greatly help you in developing your own ice cream recipes!

Freezing point depression

ice cream
Yummy yummy ice creams!

Essential for making ice cream is the concept of freezing point depression. This is a physical phenomena that you will also see in winter time, when salt is sprayed onto roads when it freezes. By throwing salt on the road the water on the road will not freeze. However, the effect is limited, if it gets cold enough (pretty cold in the case of salt) water will freeze. The same happens in ice cream, by adding sugar or alcohol, the freezing point of the water in ice cream lowers.

You might find this surprising, since ice cream is still frozen, it doesn’t just flow away like water or cream would. And that’s correct, however, the trick is that not all water is frozen, only part of it.

Let’s dive into thermodynamics to explain this in a little more detail without using all sorts of formulas. When you dissolve sugar in water the entropy of this solution will be higher than that of pure water. What this means is that there is more ‘disorder’, which makes sense, instead of only having water molecules, all of a sudden sugar molecules float around as well. This will result in a so called lower ‘chemical potential’. This again results in the fact that it needs to be colder to actually freeze.

Sounds complicated? It’s enough to understand that by adding sugar or alcohol to your water, the freezing point goes down. The more sugar you add, the lower the freezing point will become (although you have to make sure that you’re still able to dissolve all this sugar).

Amount of ice in ice cream

When ice cream is cooled to sub zero temperatures, it is the water in the ice cream that will make the ice cream freeze. As we just discussed, the addition of sugar lowers the freezing point of ice cream. However, that does not mean that there won’t be any ice anymore. Thermodynamics is all about equilibria, so also in this case there will be an equilibrium, here between the ice crystals and the liquid water.

When a sugar solution is cooled down part of the water will freeze and form ice crystals. These ice crystals leave the sugar solution. You will be left with a sugar solution + ice crystals. They will still be homogeneously mixed. Because some of the water has formed ice crystals the remaining sugar solution will become more concentrated. If it’s too concentrated, no more ice crystals will be formed. There will be an equilibrium.

The point at which no more water crystallizes depends on the amount of sugar that has been added. You can do something similar as well by adding alcohol or salt to your ice cream. Fat (milk fat for example) doesn’t have this effect. The fat and water don’t dissolve in one another.

If you like you can even calculate how much ice crystals have formed and you can calculate what the new freezing temperature of your sugar solution is. Let’s get back to that later though. You’ve now learned why it’s important to add something like sugar or alcohol to your ice cream.

semifreddo

 

Resources

I’ve used several resources for writing this post, these might also be interesting if you’d like to understand the thermodynamical concept of freezing point depression somewhat more: UC Davis, Bristol University

Semifreddo - The secret of ice cream & the freezing point depression

4 Comments

  1. Great post.

    You say “If you like you can even calculate how much ice crystals have formed and you can calculate what the new freezing temperature of your sugar solution is. If you like you can even calculate how much ice crystals have formed and you can calculate what the new freezing temperature of your sugar solution is.” Please tell me more. I guess, I could simply measure the temperature of some frozen ice cream to determine the freezing point.

    For context, I ‘m making home low-calorie ice cream, frozen yogurts, and sorbets. Over the months the batches are improving, but I am bedeviled by ice crystals, had texture after freezing, and excess coldness on the tongue. Pure erythritol was great for calories and taste, but yielded hard and cold product. Presently, I am using a mixture of erythritol and sucrose (50% each). It’s better, but still too hard and not creamy. I plan to try another fruit alcohol with more freezing point depression next.

    • Hello Mark,

      Thank you for this fascinating comment and question. It took me some time to come up with an appropriate answer, but here it is, I hope it’s useful.

      To start with your first question, calculating the concentrations of ice crystals versus sugar solution. This question requires an answer longer than just a comment so I decided to finish a blog post I had been working on which talks about this exact topic. Using so-called phase diagrams can tell you more about how to calculate these values: http://foodcrumbles.com/phase-diagrams.
      There is also a way to calculate the freezing point depression of a pure solution. It uses the molality of the sugar (or sugar substitute) to calculate the freezing point depression. This molality describes the number of molecules in a certain mass. Therefore the more molecules there are and thus the smaller they are, the more effective they are. Again, it’s a little bit too much for just a comment.

      Then there’s your issue using erythritol for ice cream. I must say that I have never used erythritol but I did some digging on the internet. Erythirtrol is a so called sugar alcohol as you might know. One of the characteristics of erythritol is that it crystallizes more easily than regular sugar, which might be one of the causes of your hard ice cream. This doesn’t necessarily have anything to do with the freezing point depression. At the same time, erythritrol does have completely different freezing point depression then regular sugar due to the different size of the molecule. Maybe this pdf file can help you: http://www.jungbunzlauer.com/fileadmin/content/_PDF/JBL_AR_Erythritol_makes_a_difference_2015-104.pdf.

      Hope that helps and thanks for the questions!

      • Thank you very much. I look forward to reading your fascinating post and learning. It may be a bit of a challenge as I didn’t pay enough attention in high school chemistry and was tossed out after I set off a potassium permanganate and sugar bomb. I guess I can always forget my pride and ask my high school kids.

        Nonetheless, I grew to be an epidemiologist and one day was confronted by a hospital where the water system was contaminated with ethylene glycol antifreeze due to a cross connection in the pipes. the contamination had happened at night and we needed to decide quickly if the water was safe to drink and for the hemodialysis machines (one patient had died during the night as the molecules of ethylene glycol passed the semipermeable membrane.

        My team went to the chemistry laboratory where we learned they were unable to measure levels of ethylene glycol. the head tech had an idea. He filled a beaker with distilled water and another with ethylene glycol. He put both in an ice bath and we watched in fascination as the water began to ice over and he inserted a thermometer and it showed 0 C. Soon afterword, the contaminated tap water froze well below 0. Hence, we didn’t allow people to drink the water.

        the hospital toxicologist told us that he air conditioning fluid was marked with uridene green so people would know if there was contamination. Alas, he explained that uridene green is a relatively large molecule so it didn’t pass the dialysis semipermeable membrane so nobody thought the fluid was contaminated until the patient slipped off into a deep sleep and died.

        This was in New York City, where the city lab could test for anything and they provided us with hourly tests as we flushed out the building, which took about 12 hours.

        Forgive an old man for his stories. Based on the article you attached I have followed its recommendation to use 1/3 erythritol to 2 parts sucrose. Erythritol is great stuff. It is absorbed into the body, so it doesn’t cause osmotic diarrhea like other sugar alcohols. Also, is cannot be metabolized by the body, so it is excreted unchanged in the urine and contributes 0 calories to the body. Also, it can’t be metabolized by oral bacteria, so it is not associated with dental caries.

        Since my goal is produce the lowest possible calorie ice cream, erythritol seems good for the job.

        If I may, I’ll let you know how my freezing point determinations on commercial products works out and use this as a target for my home brew ice cream. If I can figure out how to use the formula in your posting, I’ll compare this with my experimental results.

        Best Wishes,
        Mark

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