Beets isolated on white background
The Toxic Reason You Shouldn't Try To Save Leftover Beets
When continuously heated and reheated without being cooled correctly, the nitrates in beets can become nitrites, which can convert into nitrosamines and be carcinogenic.
Before you toss out your beets, know that nitrates and nitrites are not carcinogenic. The main concern is their potential to become nitrosamines, according to PubMed Central.
Even then, only some nitrosamines are considered carcinogenic, and continuously reheating them causes them to break down and become potentially harmful.
It's crucial to let your cooked beets come to room temperature before you put them in a sealed container, as the trapped steam can continue to cook them, even in the fridge.
Since it's not recommended to reheat beets after the initial cooking, you can reuse them in a salad or pickle them to extend their shelf life.