Cooked white rice can turn yellow when refrigerated due to a natural process called "retrogradation." Retrogradation refers to the rearrangement of starch molecules in the cooked rice as it cools down and is stored in the refrigerator. Several factors contribute to this phenomenon:
Starch structure: Rice primarily contains two types of starch molecules: amylose and amylopectin. Amylose is a straight-chain molecule, while amylopectin is a branched molecule. When rice is cooked, these starch molecules absorb water and swell, causing the rice to become soft and fluffy.
Cooling process: As cooked rice cools down, the amylose molecules start to recrystallize and reorganize. During this process, the amylose molecules may migrate to the rice's surface, which can cause the rice to appear slightly yellowish.
Amylose-leaching: When rice is refrigerated, some of the amylose molecules can migrate from the interior of the rice grains to the surface. This migration can lead to the yellowish appearance of the rice.
Oxidation: Exposure to air during refrigeration can also contribute to the color change in rice. The surface of the rice can undergo minor oxidation, leading to a yellowish or brownish tinge.
It's important to note that the yellowing of rice doesn't necessarily indicate spoilage or a safety concern. Cooked rice can still be safe to eat even if it changes color. However, if the rice exhibits any unusual odors, tastes, or signs of mold, it's best to discard it to avoid any potential foodborne illnesses. To prevent or minimize the yellowing of rice, consider storing it in an airtight container and consume it within a few days of refrigeration.