Appliance Science: How rice cookers cook rice without burning – CNET

One of the great things about technology is how it frees people from mundane tasks. Cooking rice is one of these. If you have ever cooked rice manually, you’ll know what a hassle it can be. You have to keep a close eye on it: get distracted and it will boil dry, burning the rice at the bottom and ruining the rest. Fortunately, you don’t have to worry about this in most modern kitchens, because a cheap gadget called an automatic rice cooker handles this for us, turning down the heat when the rice is done to keep it warm without burning. How does it do this?

The short answer is by paying attention to the temperature. The long answer is by paying attention to the temperature in a fuzzy, casual sort of way. I’ll look at the short answer in this column, and the long answer in the next one.

The earliest automatic home rice cookers came out in the late 1950s from companies like Toshiba. Commercial models had been around for a while before this, using a variety of techniques to measure the doneness of the rice, but the first home models relied on the fact that water conducts heat better than rice does, and that it boils at 100℃ (212℉) at sea level.


The Oster 20, a simple, low-cost rice cooker


When you use a rice cooker, you put water and rice (usually in a 2:1 proportion) into a metal bowl, which has a heating element and a temperature sensor below it. These are usually on springs to press them against the bottom of the cooking bowl to make sure they can conduct the heat well. The bowl is thin and made of a metal like aluminum that conducts electricity well.

When the automatic rice cooker is turned on, the heater starts heating the bowl, which conducts the heat into the water and the rice. Because this mix is mainly water at this point, it heats up until it starts to boil. Once it begins boiling, the heat is carried off in the steam that rises from the bowl. So, the temperature of the dish at the bottom of the bowl won’t go much higher than 100℃ (212℉), because the water at the bottom of the bowl will boil, turn to steam and carry off the heat.

Meanwhile, the rice is cooking, absorbing the water into its structure, which is mainly composed of starch. Starch is mostly composed of long strings of sugar molecules tied together, with a few other chemicals thrown in. When they are heated, these long sugar strings grab onto passing water molecules in a process called hydrogen bonding: the hydrogen in the water weakly bonds to the oxygen atoms that are poking out of the sides of this chain.  Because of this, the rice grains absorb the water, and become less well connected to each other, a process called gelatinization. If you kept adding water, this would eventually turn into a gooey paste. However, you don’t want that to happen (unless you are making rice pudding or porridge) which is why you only add a certain amount of water: enough for the rice to absorb and swell, but not enough to break the structure down completely.

When the rice has absorbed all the water, the heater is still pumping heat into the bowl along with the rice, but it doesn’t have any loose water left to remove it by boiling, so the temperature of the bowl quickly rises.

That sudden temperature bump triggers the mechanism inside the rice cooker to turn the heat down or off, because the rice is done. The first rice cookers (and many cheap models today) used a bimetallic switch, where two metals expand at different rates when heated, triggering at just over 100℃ and releasing a latch, producing the familiar “clunk” that lets you know the rice is done.

That’s the short version. The long version has to consider a couple more issues: water boils at lower temperatures at altitude, and the simple switch only allows you to have one trigger for doneness: you can’t adjust it for different types of rice that may absorb different quantities of water, or for different uses such as making porridge. Automatic rice cookers that can handle these use a dfferent approach, called fuzzy logic. We’ll discuss this in my next column. 

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