World Grain - February 2011 - 73

RICE QUARTERLY: MILLED RICE FISSURING

CULTIVAR, MOISTURE EFFECTS 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. RELATIVE HUMIDITY, TEMPERATURE IMPACTS Researchers found that the rate of
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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 percent-

ages 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

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World Grain - February 2011

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