There’s nothing quite like perfectly cooked rice to go with just about any dish and fill you up, but how does a cheap, basic rice cooker know when the rice is ready? Perhaps, like me, you’ve always assumed there was some sort of simple computer doing a calculation based on weights or temperature. Or perhaps you’ve never thought about it! The way it really works is one of the most effective and elegant designs you’ll find in your kitchen. There’s no computer, no calculation, just a few basic principles put together brilliantly.

The first foundational fact we have to establish is what we want from our cooked rice. This one is pretty simple – we want it to have been heated enough to cook the grains of rice and simultaneously to have absorbed enough water to become soft and edible – this is called gelatinisation. And for perfectly fluffy rice, you really want all the water absorbed. So, assuming you put in the correct ratio of water to rice, all the rice cooker needs to do is heat it up, keep it simmering until all the water is absorbed or evaporated, then turn off when it’s cooked.

Credit Roboblow Universe

Now for the physics. The boiling point of water is 100°C, which means that under normal circumstances, it is impossible to heat water above that temperature. Once water gets to 100°C, any extra heat energy added to it is absorbed by the molecules as they phase change from liquid to gas – that is, as the water boils off and becomes steam, all the water left behind stays at 100°C. This concept is called the latent heat of vaporisation.

So, water can’t get above 100°C. How does that help us? Well, water can’t, but cooked rice can. Once all the water added to the rice cooker is absorbed by the rice (meaning the rice is cooked), the extra heat energy being added to it by the heating element starts to raise its temperature above 100°C. So, all a rice cooker now needs to do is sense when the contents of the cooker start heating up above that level.

Surely here’s where a thermometer comes in, right? Surprisingly, no. Here we encounter our second handy dandy physics concept: the Curie temperature (named for Pierre Curie, a physicist who studied multiple fields of physics alongside his wife, Marie Curie). The Curie temperature, or Curie point, of a metal is the temperature at which it ceases to be magnetically attracted. When a regular magnet is heated above its Curie temperature, it is permanently broken, rendered non-magnetic. However, for metals that are ferromagnetic (that is, attracted to magnets because they contain iron), they will regain their magnetism once cooled to below their Curie point again.

This leads to the brilliant simplicity of the design of a basic rice cooker. If you have one at home, you’ve probably noticed that in the middle of the circular heating element at the bottom of the cooker, there is a little metal disc on a spring. When this is depressed by the weight of the pot, you’re able to turn the heating element on to full power by pressing the switch down from ‘Warm’ to ‘Cook’.

That disc, which is made of an alloy with a Curie point of just above 100°C, is held in contact with the bottom of the pot by a magnet, which is being resisted by a spring. So, by combining the two basic principles we’ve looked at, we can see how the humble rice cooker effortlessly cooks rice perfectly, every time. Water and rice are added to the pot and the heating element gets the water and rice up to 100°C until the rice is cooked and all the water is either absorbed or evaporated. Then, as the pot subsequently begins to heat up above 100°C, the little disc touching the bottom of the pot loses its magnetism and the spring that had been resisting it pushes it down, flicking the switch back from ‘Cook’ to ‘Warm’ with a satisfying thunk. And there you have it – delicious, nutritious rice, cooked by science!

If you want to see how this works in more detail, check out the YouTube video where I learned about this for the first time!