Steamy cover up.
Absorption. Evaporation. Latent heat. And why the rice cooker has a lid.
1-Minute NomNom
Why does the rice cooker have a lid? After all, we do want the water to evaporate – that’s how we get rice that is fluffy and not soggy, right?
Only partly right.
Rice grains become fluffy when they absorb water. Different types of rice grains will absorb different amounts of water, but they all need to do so for the starch granules (see item 6 above – don’t worry about the other terms) to swell into fluffy grains. Absorb too much though and the starch granules will burst, releasing all their starch and creating a gooey gum.
The trick is to get the right amount of evaporation. If too little water evaporates, the rice becomes soggy. But if all the water evaporates too quickly, the cooked rice grains will remain fairly hard because they have not absorbed enough water. In some cases, they might even burn before they become fluffy (while slightly harder cooked rice and burnt rice both have their uses in some recipes, soft and unburnt rice are typically preferred).
Cooking rice is hence a delicate balance of water absorption and evaporation.
The lid helps to maintain that right balance. When the water first evaporates, it will rise as steam. Part of it will escape. But part of it will condense on the lid, and subsequently back into the cooking vessel. This prevents water from evaporating too soon.
The lid also plays another important function. During the time it helps to maintain the balance between evaporation and absorption, it also ensures that the temperature will not rise and burn the rice. This is because of latent heat: all the heat energy is used to change the water into steam. As long as there is water present, the temperature stays constant at the boiling point of 100ºC/212ºF.
The combined effect is that just the right amount of time is given, for just enough evaporation, for just enough of the water to be eventually absorbed by the rice grains. In fact, it is these scientific phenomena that underpin the design of the rice cooker.
photos: in order – depositphotos/jakerbreaker; Ansonde; Maryna_Melnyk; FreerLaw; keerati and mayakova