Put the frozen rice in a heatproof container, sprinkle some water over it, and then throw out the water that pools in the bottom of the container.
Cover the container with plastic wrap and microwave at 600 W for 2 minutes. (I used a microwave lid in the photo.)
Take the rice out of the microwave after it's been heated, and crumble up any lumps with a spoon.
Re-cover with plastic wrap and microwave at 600 W for another 2 minutes. (If you can adjust the temperature on your microwave, set it to 70°C.)
The rice is nice and plump just as it was before being frozen. (I used 16-grain rice in the photo. You can reheat plain white rice in the same way.)
When freezing rice, wrap up small portions in plastic as soon as it's cooked and let it cool down a bit before freezing. This way the rice will be tastier when defrosted.
Story Behind this Recipe
The main carbohydrate of a meal can be noodles or bread or okonomiyaki and so on - it's not always rice, so it's a bother to cook it each time. Therefore I freeze it instead. But I wondered why frozen rice always tasted worse, and tried out various techniques to make it taste better. In the end, a simple method did the trick.
To re-heat 100 g of rice: In Step 3 microwave for 1 minute; in Step 5 for 1 to 1.5 minutes (or adjust the temperature to 70°C). To reheat 300 g of rice: In Step 3 microwave for 3 minutes; in Step 5 microwave for 3 to 3.5 minutes (or adjust the temperature to 70°C). Please keep an eye on the state of the rice while microwaving.