When idly batter is kept isolated in your kitchen at room temperature,the glucose molecules are fermented by the action of the microorganisms like yeast and the batter releases CO2 gas and raises from its usual volume. 
     On a summer day,this reaction is faster
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When idly dough is kept at room temperature on a summer day, fermentation occurs due to which CO2 is released from the dough which makes it turn fluffy and also secretion of ethyl alcohol takes place in this process. This is fermentation which is anaerobic respiration by the cells in dough.
This not only happens in dough but this process helps to make the bread and cakes fluffy.
That is why when idly is made with fermented dough, it's so fluffy and porous due to CO2.
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