Rheology and particle size changes during idli fermentation.

Changes in rheological properties and particle size during fermentation of idli (a fermented food made from rice and black gram) were investigated; dimensional parameters of batter formed using a specially developed automated idli processing unit and the effects of rice:black gram ratio (2:1, 3:1 and 4:1) on idli quality were also studied. Shear stress and shear rate values obtained for different idli batters during fermentation (23 h at ambient temp.) showed good agreement with the power law model with yield stress. Yield stress values were in the range 13-43 Pa and reached a max. value at 7 h of fermentation. Flow behaviour indices were in the range 0.287-0.605, indicating strong non-Newtonian behaviour. Consistency index was observed to increase with increasing rice:black gram ratio at all fermentation times. Particle sizes in batters ranged up to 2000 mum; approx. 80% of particles were <782 mum and mean particle size for all batter types as a function of fermentation time ranged from 500 to 600 mum. No defined trend in particle size with respect to fermentation time and rice:black gram ratio was observed; however, a small increase in mean particle size occured between 4 and 13 h of fermentation. Height of batters showed little variation between batter types while diam. decreased with increasing ratio rice:black gram ratio. A rapid vol. increase was observed in batters after 4 h fermentation; rate of vol. increase progressed steadily until approx. 13 h and then slowed down.