Stress fissuring and process duration during rough rice convective drying affected by continuous and stepwise changes in air temperature

ABSTRACT

During rough rice drying, gradients of moisture content and glass transition temperature cause thermal and mechanical stresses inside the kernel. These stresses eventuate to kernel fissuring during the milling process. In this study, convective drying of Hashemi (long grain) rough rice was applied to investigate the effect of continuous and stepwise changes in air temperature on stress cracking index and process duration. Toward this objective, the concepts of glass transition and analysis of moisture contents distributions within the rice kernel were determined through a numerical modeling of mass transfer. For stepwise temperature change, the drying experiments were conducted at temperatures above the glass transition temperature. Results indicated that the stress cracking index under stepwise temperature change conditions (i.e., within the rubbery state) was reduced compared to the continuous mode probably due to a drip in the moisture content gradients created inside the kernels during the drying process. Moreover, the drying duration significantly was shortened when the kernel was dried within the rubbery state due to faster diffusion moisture within the kernel.

KEYWORDS:

• Continuous drying
• glass transition
• modeling
• rough rice
• stepwise

Abbreviations::

• DMTA, dynamic mechanical thermal analysis
• DSC, differential scanning calorimetry
• HRY, head rice yield
• MC, moisture content
• RH, relative humidity
• RMSE, root mean square error
• SCI, stress cracking index
• MR, moisture ratio

Additional information

Funding

This work was supported by the Bu Ali Sina University [Grant Number 1502-93].