This is the view from the eye piece of a microscope of a slide prepared from the supernatant water of dosa batter. The picture on the right shows the bacteria that actually cause fermentation of cereals.
Dosa, idli or dhokla batter ferments due bacteria, yeast or fungi present in the air. They convert carbon elements in the batter to alcohol, lactic acid and carbon dioxide. This fermentation is healthy and does not cause poisoning when the product is consumed. The bacteria etc are killed when we cook the dosa, idli etc. Vitamins of the B group in the batter increase as a result of fermentation.
Fermented Indian cereals are a good source of easy to digest proteins are are low in calories. Hence ideal for weight conscious or those who may be sick and need to eat light food.
Jalebi, anarase, khaman dhokla, kurdaya, idli, bhatura dough, bread are all examples of fermented cereal products.
These microorganisms are present everywhere and begin their action when conditions are right (soaking, grinding, temperature etc).
The bacteria are usually of the lactobacillus variety.
Now u have a glimpse of what made your idlis soft and fluffy and tasty!