Freezing point of caramel

Not too long ago I got a question from someone who wanted to know the freezing point of caramel. Thinking about it, caramel in ice creams are just about always soft… This got me interested, because answering this question allows me to explain some great physics!

The short answer is actually pretty simple, though might not be satisfactory. There is no one freezing point of caramel, since there are so many different caramels, each will have a different freezing temperature. There is an easy reason as to why not all caramel become solid in the freezer, it’s because of the sugar. Water-based products with a high sugar content will not freeze until temperatures well below the freezing point of water.

The long answer involves a few more scientific analyses, in which I will immerse just now.

The long(er) answer – phase changes

Let’s start with some basics: phase changes of materials. As you could read in my post on the behaviour of gases, materials can change phases, often caused by a change in temperature. The three basic phases are: gas, liquid and solid. The main difference between the three phases is freedom to move of the molecules. In a gas the molecules can move very freely from one another, whereas in a solid they can only move in a fixed spot.

When freezing water, it changes phase from liquid to solid (ice cubes). In order for water to freeze, ice crystals have to be formed. The molecules in these ice crystals cannot move freely anymore.

Freezing of a mixture of molecules

When water freezes, very neat ice crystals can be formed. All water molecules will snuggle next to each other very neatly. Now what happens when there’s a sugar molecule floating around in that water? This sugar molecule will make it harder for the water molecules to position themselves properly in this crystal type structure. As a result, the water molecules surrounding the sugar cannot freeze well.

Freezing point depression

This results in a phenomenon called the freezing point depression and this is what causes caramel to have a freezing point below zero. Ths phenomenon can be theoretically explained using terms such as entropy, enthalpy and chemical potential. However, for the sake of simplicity, I will save that for another post.

The freezing point depression is essential in making a nice ice cream. If it weren’t for the freezing point depression, your ice cream would always be icey.

Ice Cream Florence
A good ice cream isn’t rock solid, it’s airy and eats away easily, partly thanks to the air, but also thanks to the fact that not all water has actually frozen.

Freezing caramel

So let’s zoom in on a caramel. Caramel always contains sugar, that’s what makes it a caramel. The amount of sugar and moisture, do differ per caramel. Also, caramels might contain some sort of dairy, complicating things a little more when talking about freezing the caramel.

Let us start with a very simple caramel, one that probably isn’t even a caramel: a sugar solution with 40% sugar. Even if the sugar has been caramelized, this will be a very liquid and watery caramel. But, let’s keep it with this simple example. When you cool this sugar solution down to -3°C, none of the water will have actually frozen yet. Upon further cooling more and more water will freeze, however, you will have to go below -18°C to get all water frozen.

As long as not all of the water in the sugar solution freezes, it doesn’t become rock hard so it will seem as if it isn’t frozen yet.

Not all water freezes in caramel

When you freeze a caramel, some of the water in the caramel will certainly freeze only a few degrees below zero, however, not all. There is a scientific way to explain this, using phase diagrams. However, I will keep those for a separate post and stay with the ‘simple’ version.

When you freeze a caramel, the sugar will prevent a lot of the water from freezing and forming a crystalline structure. However, from a certain temperature onwards, part of the water will freeze. This water will ‘leave’ the sugar solution, leaving you with a liquid sugar solution on one hand and water crystals on the other. Since part of the water has left the sugar solution, this will become even more concentrated. Therefore, this won’t freeze anymore. When cooling down further, more water will start freezing, but as a result the still liquid sugar solution is even more concentrated. This will continue going on until either everything is frozen or when the sugar solution is so concentrated that it has simply become too thick to freeze.

An example of one caramel

As mentioned above, each caramel is different and thus will behave differently in the freezer. I’ll use one of my own caramel recipes to estimate its freezing point.

Once the caramel is finished my estimation is that the caramel contains:

  • 200g – 62% sugar (all I put in there)
  • 70g – 22% of milk fat (from my cream and butter)
  • 2g – 1% of protein (from my cream and butter)
  • 50g – 16% of water (all from the cream and the butter, the water added at the start will have just about all evaporated to get the nice brown caramel)

Water and fat don’t dissolve in one another and don’t influence each others freezing point. Milk fat we know is already solid in the fridge (think of butter), so when we freeze the caramel, we can assume all the milk fat has turned solid. This will make it firmer but still flexible.

Of the water/sugar mixture we see that only 20% is actually water, the rest is sugar. This is a very high sugar content. If there wouldn’t be any fat and proteins the sugar might have caramelized. And there’s another thing we have to take into account. Caramelization of the sucrose will actually lead to degradation of the molecule, it will disappear and split into fructose and glucose and participate in all sorts of reactions to become brown. I don’t know how much sugar there’ll be left at the start, but even if it’s only 50%, there is still a lot of sugar left to bring down the freezing point to well below zero!

 

I hope this has answered the question on the freezing point of caramel. I’m afraid it’s not one specific temperature, but it sohuld provide you with a reason as to why a lot of caramels don’t become rock hard in the fridge!

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