Multiphysics simulation and characterisation of parboiling of long grain rice during hydration

Abstract

We examined the moisture uptake kinetic and morphological behaviour of paddy rice during hydration to find the range of process conditions that allow for parboiling in this stage. The heat and mass transfer phenomena were modelled and simulated using a three-dimensional geometry representing a long grain rice kernel. We observed an endothermic event associated with starch gelatinisation in the hydration step, which can enhance process efficiency. Scanning electron micrographs (SEM) showed that intermolecular spaces were eliminated from the grain microstructure, indicating the rice was parboiled. A tracer pigment added to the hydration medium revealed that the water diffuses through the fissures and forms bonds with starch. The numerical simulation of the hydration process agreed well with the experimental data and elucidated the relationship between the moisture and heat gradients across the grains and starch gelatinisation. This research demonstrates that the parboiling process can be optimised by eliminating the steaming step as long as proper temperature-time combinations are used.