Reheating leftover rice in a microwave or hot wok easily kills the living bacteria inside.
But it does absolutely nothing to the heat-proof toxins they've already left behind.

The danger of leftover rice stems from how the grains were treated in the hours before
reheating. The culprit behind what is commonly known as "fried rice syndrome" is a
soil-dwelling bacterium called Bacillus cereus. Uncooked rice frequently harbors spores
of this bacterium. When rice is boiled, the active bacteria die, but the tough, dormant
spores easily survive the high temperatures of the cooking process.

A scanning electron microscope view of Bacillus cereus, the pathogen responsible for
fried rice syndrome.

The critical window for bacterial growth opens right after the rice finishes cooking. If a
pot of rice is left sitting out on a counter at room temperature to cool slowly - or if a rice
cooker is left unplugged but closed for several hours - the surviving Bacillus cereus
spores germinate. The warm, moist environment of freshly cooked rice acts as a highly
effective incubator.

As the bacteria multiply, they produce a heat-stable emetic toxin. When people become
severely ill from eating leftover rice, they are typically suffering from intoxication caused
by the chemicals the bacteria manufactured while sitting on the counter, rather than an
active infection.

Safe preservation of leftover rice relies on rapid cooling. Spreading freshly cooked rice in
a shallow container allows the heat to escape quickly, dropping the temperature below
the danger zone before the spores have a chance to germinate. When a container is
moved into a refrigerator within an hour of cooking, the spores remain dormant. Rice
that is chilled quickly and stored below 40ÿF (4ÿC) does not accumulate these bacterial
toxins, allowing the starches to be reheated safely later.

Full story:
https://www.quora.com/Is-reheating-boiled-rice-really-dangerous

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