If you are living in cold country, the batter will not ferment properly. You can keep the batter inside the oven and switch on the oven light. Or, you can keep the batter near heater vent.
One of our Readers Mr. Ramaswamy sent the following tips for Fermentation of Idli batter:
The fermentation is caused by air-born wild yeast. Urad and Fenugreek seeds draw the wild yeast from air. So keep the washing to the minimum.
The tap water in developed nations are mostly has Chlorine and/or Fluorine. The common table salt is iodized. These halogens (Chlorine, Fluorine and Iodine) kill the wild yeast. Use non-iodized salt such as ‘Kosher salt’ and Use spring water, boiled or filtered tap water to avoid Chlorine/Fluorine.
I use an electric hand blender to infuse air and make it frothy to accelerate the fermentation process, before keeping it in the oven.
The ideal temperature for fermentation is 30 to 32.2⁰C. If the temperature is below 30⁰C, it will take longer to achieve acceptable level of fermentation. If the temperature is higher than 32⁰C, the batter may become sour. Acceptable level of fermentation is when the batter has doubled in volume
Keeping the batter inside the oven and switch on the oven light (40 w bulb is ideal) and keep it over night.