You can keep freshly cooked rice in a thermos and it is generally safe as long as you follow the best safety practices above. However, there are some concerns about the rice and the reasoning is the same behind the general advice not to leave rice at room temperature for more than a couple of hours—spores.  According to England’s National Health Service (NHS), uncooked rice can often contain some spores that are immune to heat. After cooking the rice, the spores can turn into bacteria at room temperature, which may then release toxins and cause food poisoning [3]. To guarantee your rice remains safe, you must ensure the content of your thermos remains above 140°F (60°C) the entire time, which can be done if preheat the vessel with boiling water beforehand. However, it could be a slightly risky choice, as thermoses are better at maintaining hot temperatures with liquidy and dense meals, and regular cooked rice is neither.