Coupled multiphase transport, large deformation and phase transition during rice puffing

Abstract

Puffing of biomaterials involves mass, momentum and energy transport along with large volumetric expansion of the material. Development of fundamentals-based models that can describe heat and moisture transport, rapid evaporation and large deformations can help understand the factors affecting the puffing processes and optimize them. In this context, salt-assisted puffing of parboiled rice is described. A multiphase porous media model involving heat and mass transfer within the rice kernel undergoing large deformations is developed. The transport model involves different phases and multiple modes of transport. During puffing, intensive heating of rice leads to rapid evaporation of water to vapor resulting in large pressure development. Also, the rice starch undergoes Glass Transition from a rigid, glassy state to a soft, rubbery state. Development of large pressures within a soft matrix results in large volumetric expansion of the kernel causing it to puff. The developed model was validated against moisture changes and volumetric expansion of the rice kernel during the puffing process and good agreement was found. Gas porosity development in puffed rice was determined via 3D reconstruction of micro-CT images of rice puffed at different times which compared favorably well with model predictions. The expansion of the kernel began from the tip of the grain and the model could successfully capture this phenomenon. Expansion ratio, a key quality parameter associated with puffed products, was found to be sensitive to intrinsic permeability and bulk modulus of the solid matrix. The modeling framework for salt-assisted puffing was then extended to the process of gun-puffing (a completely different puffing process) without significant reformulations thus showing the applicability of the framework for a variety of puffing processes. The final expansion after gun-puffing was much higher compared with salt-assisted puffing and was found to be sensitive to the gun opening time.