Reheating leftover rice and then eating it, or stir-frying leftover rice and then eating it — many people believe this is both hygienic and economical. In fact, reheating leftover rice makes it difficult to digest, and over time, it may cause stomach problems.
Let's first look at how the main component of rice, starch, is digested and absorbed in the body. First, salivary amylase in the mouth hydrolyzes starch into dextrin and maltose.
Second, in the small intestine, pancreatic amylase and disaccharidases secreted by the pancreas continue to break down dextrin and maltose into monosaccharides for absorption by the intestinal mucosal cells. When starch is heated above 60°C, it gradually expands and eventually becomes a paste-like substance; this process is called "gelatinization." The body's digestive enzymes can more easily hydrolyze the molecular structure of this gelatinized starch. However, after the gelatinized starch cools, the starch molecules rearrange and expel water, causing a "syneresis" phenomenon, which is called the "retrogradation" of starch. Even when reheated, retrograded starch molecules cannot return to the molecular structure they had during gelatinization, no matter how high the temperature. The retrograded structure of starch reduces the body's ability to hydrolyze and digest it. Therefore, long-term consumption of this cold, reheated rice can easily lead to indigestion and stomach problems. Consequently, for people with weakened digestive function, such as the elderly, young children, or patients, especially those with gastrointestinal diseases, it is best to avoid or eat very little of rice that has been cooled and reheated.