Improving Rice Quality & Processing Efficiency - 2010



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Project Leader and Principal  Investigators

Zhongli Pan, research engineer, USDA-ARS, Dept of biological & Ag Engineering, UC Davis




This project continues to develop the knowledge necessary to improve drying efficiency and rough rice quality. Work in 2010 examined more closely the physical, chemical, and hygroscopic differences between M-202 and M-206 and their impact on fissuring.

During harvest season, relative humidity in the Sacramento Valley can change from 90% in the morning to 10% in the afternoon. Rice kernels absorb moisture when relative humidity is high and then “desorb” it when relative humidity decreases. Fissuring of rice kernels can occur as a result of these changes in humidity. Fissuring reduces economic value of the rice.

The newer variety M-206 has shown consistently higher head-rice yield than the older industry standard, M-202. Fissuring is one of the reasons why. To determine the underlying cause of their difference in fissuring, scanning electron microscopy (SEM) measured carbohydrates, proteins, and lipids in brown rice of the two varieties. SEM images showed that the distribution of these components is very similar. This observation was reinforced by analytical tests that showed the chemical composition of the two varieties to be virtually identical. Thus, the fissuring differences could be more likely discovered in how the rice responds to field moisture.


Differences in moisture gains and losses between M-202 (top) and M-206 (bottom) may be related to the size of a gap between rice kernel and husk.

In previous studies, mathematical models predicted changes in moisture content of white rice, brown rice, and rough rice. These models also can be applied to different grain types —short, medium, long—to better understand changes in moisture content. The 2010 research focused on how husk and bran thickness might affect the moisture profile of medium grain rough rice under high relative humidity. Increasing or decreasing bran thickness by 50% resulted in an insignificant impact on moisture profiles. However, changing husk thickness by 50% resulted in larger variations in moisture profiles.

Moisture movement during drying and desorption was not uniform from all regions of the rice kernel. Using magnetic resonance imaging, researchers found that during drying, moisture removal rate is higher in regions near the kernel embryo (germ) than other regions. Moisture absorption research on different parts of the rice kernel found that the portion containing the rice germ absorbed moisture faster than the other portions.

Imaging with SEM and X-ray technology has shown the existence of a gap between the brown rice kernel and the husk. The size of this gap can affect moisture losses or gains significantly. To gain a deeper understanding of moisture transfer between environment and rice kernel, researchers measured the size of this gap at different relative humidity conditions with imaging technology. They found that with increasing relative humidity, brown rice kernels expand at a higher rate than rough rice kernels, which decreases the size of the gap.

Research in 2010 made important progress in understanding rice structure, the pathways of moisture movement and mathematical modeling of drying processes at different temperatures and relative humidities. This knowledge could assist in predicting fissure resistance of different varieties and aid in the design of drying operations.


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