Ragi dosa batter bubbles after refrigeration due to the fermentation process continuing, albeit slowly, even at low temperatures. The bacteria and yeast present in the batter produce carbon dioxide, causing the bubbles.
Ragi dosa batter, like other dosa batters, contains a mix of grains (ragi in this case) and lentils. These ingredients naturally harbor microorganisms like bacteria and wild yeast. When you grind the ingredients and mix them with water, you create an environment conducive to fermentation. Fermentation is a process where these microorganisms consume the carbohydrates in the batter and produce byproducts, including carbon dioxide gas.Even when refrigerated, the fermentation process doesn't completely stop; it merely slows down. The cold temperature inhibits the activity of the microorganisms, but they are still active to some extent. As they continue to consume the carbohydrates, they release carbon dioxide. Since the batter is a thick liquid, the gas gets trapped, forming bubbles. The longer the batter sits in the refrigerator, the more bubbles you'll likely see. This bubbling is a sign that the batter is fermenting and developing its characteristic sour flavor. The extent of bubbling also depends on factors like the ambient temperature during the initial fermentation period before refrigeration, the specific microorganisms present, and the sugar content of the ragi and lentils.
If your ragi dosa batter becomes too sour after refrigeration, try adding a pinch of baking soda just before making the dosas. This will help neutralize some of the acidity and improve the texture. Be careful not to add too much, as it can affect the taste.