The thin layer of film that forms on homemade instant oatmeal when it cools off in the refrigerator is likely due to the natural properties of oats and starches. Oats contain a type of starch called amylose, which has a tendency to form a gel-like substance when cooked and then cooled. This gelatinization of starch is responsible for the film or layer you observe on the surface of the oatmeal.
Here's a basic explanation of what happens:
Gelatinization: When you cook oats, the heat causes the starch molecules to absorb water and swell. This process is called gelatinization, and it thickens the mixture, giving you the desired oatmeal texture.
Retrogradation: As the cooked oatmeal cools down, the starch molecules start to reorganize and bond together again, a process known as retrogradation. During this process, some of the starch can migrate to the surface of the oatmeal, resulting in the formation of a thin gel-like film.
Moisture loss: The formation of the film can be more noticeable if the oatmeal cools slowly, allowing moisture to evaporate gradually. This can lead to the concentration of starch on the surface, creating the thin layer.
While the appearance of the film might not be visually appealing, it is entirely harmless and doesn't affect the taste or quality of the oatmeal. If you prefer to minimize or prevent the film from forming, you can try covering the oatmeal while it cools or stirring it occasionally during the cooling process to distribute the starch more evenly. Additionally, consuming the oatmeal while it's still warm might prevent the film from developing as rapidly.