Infrared Drying of Rough
Rice for Improved Quality & Processing Efficiency - 2008
Project Leader and Principal Investigators
Zhongli Pan,research engineer, USDA-ARS, Albany, CA
Improving head rice yield is an important goal in rice drying research. Infrared radiation has been shown to shorten drying times without compromising head rice yield and may be superior to conventional air drying. This technique allows for more uniform heating of kernels in the drying process and thus less fissuring.
Research in 2008 focused on temperature and moisture distributions under both infrared and convective drying methods; comparison of fissure rates under these two methods; and developing a better understanding of the relationship between moisture and temperature distributions and, ultimately, milling quality.
In bulk drying experiments, head rice yield and total rice yield were found to be similar or slightly higher from infrared-dried rough M-206. However, infrared drying methods showed slightly lower whiteness – similar to experiments in 2007.
Measuring moisture distribution inside a rice kernel experimentally has been very difficult, primarily because of its small size. However, with recent advances in Magnetic Resonance Imaging (MRI), moisture distribution inside the rice kernel can be observed with high resolution (photo above). This technology is shedding new light on how drying affects the integrity of the rice kernel.
MRI images of convectively dried samples revealed more moisture loss in the husk region than the endosperm of the rice kernel. This finding was confirmed with other analytical tools, as well.
Uneven moisture distribution within a rice kernel – moisture content gradients – plays a significant role in fissuring and thus head rice yield. That’s because the stress in a rice kernel caused by higher moisture differences can overcome the forces holding it together.
The strength of the kernel depends on its moisture content. During rice drying, kernels having higher initial moisture content have greater strength. In multipass drying methods, moisture content gradients are not as pronounced as in the continuous method, and thus have higher head rice yield.
Infrared heating time can also affect head rice yields. After a certain threshold, yields begin to drop. This is caused, again, by high moisture content gradients. By using optimal infrared heating times, moisture content gradients can be lowered, resulting in higher head rice yield.
The optimal heating time depends on the initial moisture content of rice. Experiments determined that the time should be the time needed for rice to reach 140 degrees Fahrenheit with infrared heating.
The information about the impact of moisture content gradients on fissure formation and head rice yield can be used to optimize and design new drying processes for improved milling quality and reduced drying costs.