In a bowl, add 1/2 cup lukewarm water, sugar and 1/2 tsp yeast and mix it well, cover it and keep it aside for a 15 minutes for fermentation. In a another mixing bowl, Add all purpose flour, corn starch, gram flour, oil, curd, turmeric powder or yellow food color and yeast mixture. Mix it using wire whisker to make a lump free batter. Add more water if needed. The batter should be flowing consistency not very thick and not very thin. Cover it and keep it in a warm place for 2 -3 hrs.