Milled rice fissuring

by Susan Reidy
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Researchers have found that relative humidity and temperature have a significant impact on the rate and amount of fissuring seen in milled rice.

Fissured milled rice kernels cause great financial losses to post-milling processors in terms of waste and production plant limitations, according to a study published by Terry J. Siebenmorgen and other members of the Rice Processing Program, University of Arkansas, Fayetteville, Arkansas, U.S.

Previous studies have quantified the amount of fissure formation after exposing rice kernels to certain environments in a narrow range of temperatures. However, milled rice can be exposed to a wide range of air conditions during transport and conveyance, especially during winter. With this in mind, researchers set out to measure fissure occurrence kinetics of milled rice exposed to a wide range of environments, including low temperatures.

Fissures form when internal hygroscopic stress exceeds material strength, the researchers said. Stress is caused by moisture transferred to or from the kernel from the surrounding environment. The moisture adsorbing or desorbing potential of an environment is characterized by the grain equilibrium moisture content associated with the surrounding air temperature and relative humidity.

When kernels adsorb moisture, the surface expands causing compressive stresses at the kernel surface and tensile stresses at the drier inner core. Fissures initiate at the inner core wthe tensile stress exceeds the tensile strength of that portion of the kernel.

Researchers found that rice cultivar and moisture content played only a minor role in the rate and amount of fissuring as compared to the air’s relative humidity and temperature. Fissuring was greatest at low and high relative humidity levels, and greater air temperatures considerably increased the rate and amount of fissured kernels.

Little fissuring was found in a relative humidity range of 30% to 75%, at any air temperature tested, researchers said, providing a safe window for milled rice exposure to environmental conditions.

The findings have significant implications for process applications or systems where milled rice is exposed to given air conditions for various durations, researchers said. Maintaining air temperature and relative humidity levels, or maintaining minimal exposure duration to air conditions outside the safe range, should limit fissure formation and breakage.

For the study, milled rice of three cultivars — Bengal (medium-grain), Wells and CL161 (long-grains) — was exposed to a range of air temperature and relative humidity conditions. The cultivars were harvested at moisture contents ranging from 18% to 22%. Rough rice samples were dried to 14% moisture content. Four samples from each cultivar were dehulled and then milled for 30 seconds with a lab mill.

All tests were conducted inside an environmental chamber capable of controlling temperature from -35 degrees C to 150 degrees C. Samples were placed inside open plastic dishes.

The samples were exposed to 35 environmental chamber air conditions including five temperatures (5 degrees C, 10 degrees C, 15 degrees C, 20 degrees C and 30 degrees C) and seven relative humidity levels (10%, 20%, 30%, 45%, 60%, 75% and 90%). The air settings ranged from dry, low relative humidity conditions causing moisture desorption from kernels, to moist, high relative humidity conditions causing moisture adsorption by kernels.

During exposure, fissures in the kernels were illuminated by fiber optic light sources. Images were captured by a video camera and cassette recorder every four minutes over the course of a day. The images were later viewed and the number of fissured kernels counted. Fissuring kinetics, in terms of percentages of fissured kernels throughout a 24-hour period, were quantified.

Bengal, the medium-grain cultivar, generally showed greater fissuring than the long-grain cultivars, especially after long exposure, the researchers said. Fissure formation was greatest when kernels were exposed to 30 degrees C and 10% relative humidity. Kernel size can affect fissuring, with thicker kernels more susceptible to moisture sorption than thinner kernels.

While fissuring is mostly driven by temperature and relative humidity, it is influenced by kernel moisture content. The effects of moisture content on fissuring became more prominent as the exposure temperature increased, particularly in moisture-adsorbing environments, according to the study.

When exposed to 30 degrees C air temperature and 10% relative humidity, the percentage of fissured kernels was directly related to moisture content. More kernels at 14% moisture content fissured compared to those at 11% moisture content at any exposure duration.

Conversely, when relative humidity was 90% and the air temperature was 10 degrees C or 30 degrees C, the percentage of fissured kernels was inversely related to moisture content. A greater percentage of kernels at 11% moisture content fissured as compared to 14% moisture content.

This can be explained by the intrakernel hygroscopic stresses resulting from rapid moisture transfer rates to or from the kernel. The moisture transfer rates are dictated by the difference in moisture content between the kernel and the rice equilibrium moisture content associated with the exposure air.

For example, high moisture content rice kernels (14%) experienced greater moisture gradients than low moisture content kernels (11%) when exposed to low relative humidity levels (10%), resulting in a greater percentage of fissured kernels, the study said.

Researchers found that the rate of fissuring was greater when exposing milled rice kernels to 10% and 90% relative humidity than at any other level, regardless of exposed temperature. At any temperature, increasing the relative humidity from 10% to 20% to 30% showed progressively reduced fissuring levels.

Relative humidities of 45% and 60% produced little to no fissuring, the study said. The moisture gradient between the air and rice was too small to cause intra-kernel stresses sufficient to produce fissures. But as the relative humidity increased to 75% and beyond, fissuring rates and amounts progressively increased due to increasing moisture adsorption levels.

Exposing rice kernels to 10 degrees C and 10% relative humidity for 4, 16 and 60 minutes, the percentage of fissured kernels increased from 7% to 43% to 83%. At 10 degrees C and 90% relative humidity, the corresponding percentages of fissured kernels increased from 0% to 10% to 43%.

Researchers examined the rate of fissure formation when exposing milled rice to severe moisture adsorbing and desorbing conditions that could exist during transport and conveyance. Exposing milled rice kernels to 10% relative humidity and 30 degrees C showed 50% fissuring almost immediately. At 90% relative humidity and 30 degrees C, 20% of the kernels fissured immediately.

After 16 minutes of exposure to 30 degrees C and 10% and 90% relative humidity, 80% and 50% of kernels fissured, respectively. This corroborated the previous findings that greater percentages of kernels fissured when exposed to 10% relative humidity compared to 90% relative humidity.

At a relative humidity of 75% there were no temperature effects as there were only low levels of fissuring. This also held true at relative humidity levels of 45% and 60%. At lesser and greater relative humidity levels, fissure formation increased as the air temperature increased.

Increasing air temperatures promoted greater moisture transfer to or from kernel surfaces, including greater intrakernel moisture content gradients and fissured kernels. For example, at 10% relative humidity, exposure to 10 degrees C, 20 degrees C, and 30 degrees C for 4 minutes resulted in 7%, 27% and 67% fissured kernels. After 16 minutes at the same relative humidity and temperatures, the percentage of fissured kernels increased to 43%, 67% and 80%.

Fissuring increased to 100% after exposure to 10% relative humidity after 24 hours of exposure, regardless of air temperature. At a high relative humidity (90%), fissuring increased with temperature, but at lesser rate than at a low relative humidity (10%).

For example, after 4 minutes of exposure to 30 degrees C and 90% relative humidity, milled rice had 30% fissured kernels. That compares to 67% fissured kernels at 10% relative humidity and 30 degrees C.

These results agree with previous studies that showed increases in rice breakage are usually associated with increases in exposure air temperature.

Researchers found a “window” of air conditions in which minimal fissuring occurred. It can be viewed as the relative humidity range, at a given temperature and exposure duration, in which fissuring was minimal.

In general, this range spanned 30% to 75% relative humidity for exposure durations within 16 minutes. Longer exposures reduced the safe relative humidity window. The range compressed somewhat as air temperatures increased, researchers found, resulting from the increased rate of moisture transfer to (high relative humidity) or from (low relative humidity) kernel surfaces. Milled rice did not fissure, even after 24 hours of exposure, at 60% relative humidity, regardless of air temperature.