Uniform heating and drying of salt-solution diffused rice grain (preconditioning) is an essential prerequisite for obtaining good puffing quality of rice when roasted on hot sand bed or in hot air. Possibility of uniform heating of rice grains inside an externally heated agitated cylindrical vessel (developed mechanized rice preconditioner) has been concluded by estimating the mixing indices using tracer grain technique. Effects of initial positions (top, middle and bottom layers in the bed moisture contents [15.59 ± 0.88, 20.34 ± 0.68, 25.30 ± 0.77, 29.78 ± 0.27% wet basis] and mass [50, 100 and 200 g] of the tracer grains) on mixing indices have been studied while agitation continued up to 90 min at 12 ± 2 rpm. The mixing indices with the designed agitated system attained the maximum value of about 1.0 (i.e., uniform distribution of tracer particles throughout the bed) within 5 min and remained almost unaltered thereafter up to 90 min of mixing. Sample size of the tracer or its initial position on the bed showed no significant change on the mixing trends, and uniform distribution of tracer grains at different locations along vertical and lateral directions in the bed have been observed for the entire period of mixing. Moisture contents did not show any effect on the mixing behavior of the grains in the bed.
Importance of preconditioning of rice before puffing is fully recognized by various authors to produce good quality puffed rice. This is a slow and uniform heating process of the grains. But, this is usually practiced traditionally by soaking the parboiled milled rice with water, adding 2.5–5.0% common salt, tempering for about 6–8 h for diffusion of salt and moisture into the kernel, and following it by slow heating (70–90C). Uniform heating of the grains are a major prerequisite as nonuniform heating of grain at this stage severely impairs the quality of product with less expansion ratio in addition to rough and blistered surface. Besides, traditional method of production of puffed rice is a tedious and laborious process which can easily be replaced by a mechanical system to increase the quantum of production, decrease the cost and improve the quality of the product through uniform heat and mass transfer in the grain. The uniform heat and mass transfer in the grains can be ensured by providing continuous agitating system to constantly turn the grains and expose each grain to the contact of the heated surface. Therefore, conducting mixing study of the rice grains inside an agitated cylindrical vessel will be very much useful in order to ascertain whether conditioning of grain is proper by uniform and thorough mixing of grains and uniform exposure to heat. This study will have further practical implications in designing and development of a proper mixing system for commercial production of good quality puffed rice.