a) Due to fermentation idli becomes soft and fluffy
b) Saccharomyces cerevisiae is the fungus which is commonly known as baker's yeast. The yeast cells are added to the batter of idli and dosa. The fungus performs anaerobic fermentation. The fungus utilizes the fermentable sugars and convert them to ethanol and carbon dioxide. The carbon dioxide gas causes the dough to rise and become soft and fluffy. The carbon dioxide bubbles gets trapped in the dough and when baking is done, this bubbles break and forms pores.
