Keeping it cold

One of our recent Christmas gifts was an ice-cream maker. It’s been very welcome, what with all those long hot sunny days we haven’t had.

Making ice-cream is pretty straightforward. Just bung in the ingredients and stir the mixture while having it freeze. That’s what the machine does. Stirring makes sure large ice crystals don’t form, and also aerates the mixture to give it that gorgeous texture that every summer day needs – if we every get any.

Now, the difficult thing with a benchtop ice-cream maker is obviously getting the mixture to freeze. That means cooling the bowl, and cooling isn’t an easy job for nature to do – the second law of thermodynamics tells us that. There are several solutions – one is to make the machine a mini-freezer in itself, and give it a pump, coolant and power supply. That’s going the way of the commercial machines. You could put the machine in the freezer – but then you need a power supply to turn the mixing paddle – so you need some decent batteries or a power lead coming out of your freezer.

The most elegant solution in terms of keeping things tidy is what’s done with our machine. The bowl has two layers – an inner container (which has the mixture in) placed in an outer container – and between the two is some kind of of liquid or gel that acts as a coolant. The two containers are sealed together, so the coolant doesn’t escape. The idea is that you put the bowl in the freezer for several hours before you want to use it – this takes the bowl down to the correct temperature – then you bring it out.

Just how the coolant works I’m not sure, but it will have at least two purposes – first to be an insulator, slowing the passage of heat from the outside into the bowl when ice-cream making is in progress, and, second, to have a high thermal inertia. It will take a lot of heat for it to increase its temperature significantly – but also, it will need a long time in the freezer for it to cool down significantly. Also, the coolant may undergo a phase change when cooled and use latent heat to keep the bowl cold. In this case, the coolant requires heat to melt, and takes this from the contents. Thus it can bring the contents down quickly in temperature.  A mix of water and some impurity like salt would do this as it will freeze/melt at a lower temperature than zero degrees C, thus keeping the bowl below zero as it melts.

This kind of cooking is about taking heat away, rather than providing it, and is just as much to do with physics as is heating things up.



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