Single-Pass Drying of Rough Rice Using Glass Transition Principles

The objective of this research was to study the drying characteristics, milling quality, and functional properties of rough rice subjected to single-pass drying while controlling kernel material states. Drying experiments were conducted at 60, 70, and 80C and relative humidities ranging from 13 to 83%. High drying air relative humidities (greater than 63%) maintained both the kernel surface and core in a rubbery state during drying, whereas low drying air relative humidities caused rapid transitioning of the surface layers from a rubbery to a glassy state. Long-grain pureline cultivar, Wells, medium-grain pureline cultivar, Jupiter, and long-grain hybrid cultivar, CL XL729, were dried from harvest moisture content to 12.5% moisture content in a single-pass. Immediately after drying, samples were tempered in the drying container or in sealed plastic bags at the drying air temperature for 0, 30, and 60 min, after which they were spread in thin layers and cooled to ambient conditions. For all drying air temperatures and tempering conditions, milling quality was not significantly different from the controls when the relative humidity of the drying air was maintained above 63% (both the kernel core and surface maintained in a rubbery state during drying) and rice was tempered immediately after drying in sealed plastic bags and at the drying air temperature for at least 60 min. Minimal reduction to milling quality was observed at the low drying-air relative humidities when samples were tempered immediately after drying in sealed plastic bags and at the drying air temperature for at least 60 min: tempering in containers having large headspaces or for shorter durations failed to reduce the intra-kernel stresses created during drying due to differences in material state between the surface and the core, thereby causing kernel fissuring and breakage during subsequent milling. The high-temperature conditions did not affect color, degree of milling, and thermal properties. However, pasting viscosity profile was significantly affected