Rice Thickness Grading

by Susan Reidy
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One of the goals of rice processing is to avoid overmilling of rice by carefully controlling the degree of milling and thus maximizing milled rice yield and head rice yield.

Monitoring lot-to-lot variability and adjusting milling parameters is one strategy for reducing variability in degree of milling among lots. Size-grading techniques, such as thickness grading, could also reduce milling variability.

Researchers from the University of Arkansas Rice Processing Program in Fayetteville, Arkansas, U.S., examined whether thickness grading could concentrate and partition chalky and fissured kernels to secondary processing streams in order to improve milling yields of the primary processing stream. Research was conducted by Brandon C. Grigg, manager, Rice Quality Laboratory, and Terry Siebenmorgen, professor and director, University of Arkansas Rice Processing Program.

They found that removing the fissured and chalky kernels did significantly improve milling yield. However, researchers said the economic and logistic impacts on commercial milling operations have yet to be considered.

Importance of rice thickness

Thickness grading of rice has been proposed as a means of improving kernel uniformity by removing the thinner kernels to be used for applications such as flour.

Previous research has shown that thin kernels often break during milling. When thin kernels were removed prior to milling, there was a trend of improved milled rice yield and head rice yield.
When fractioned as rough rice prior to milling, thin kernels yielded a lower surface lipid content and head rice yield, compared to thicker kernels milled for the same duration, previous research has shown. Thinner kernels would thus possibly require a shorter milling duration, possibly resulting in reduced breakage and increased milling yield, if milled as a separate process stream.

Chalkiness may also be associated with thin, incompletely-filled kernels. Chalkiness has been linked to the process of starch accumulation in the rice endosperm.

Head rice yield is also reduced by kernel fissuring, which may result from rapid moisture adsorption by kernels of low moisture content in the field or due to conditions during the drying process. Prior research has shown that thicker, bolder kernels were more susceptible to fissuring than thinner kernels.

Research method

Researchers used four lots of long-grain rice cultivars (pureline Wells and hybrid XL753), of both superior and inferior milling quality. Lots were cleaned, conditioned and graded using a laboratory-scale precision sizer equipped with rotary screens.

The resulting thickness fractions included thin (less than 2 mm), medium (2-2.05 mm) and thick (more than 2.05 mm).

Following thickness grading, milling and physical properties were determined for each thickness fraction. An analysis was conducted for each fraction to determine the milling duration needed to reach the desired degree of milling.

Unfractioned and fractioned rough rice were dehulled. The resulting brown rice samples were milled for different durations to develop relationships between degree of milling and milling duration.

Following milling, researchers measured head-rice surface lipid content and determined milled rice yield and head rice yield. Head rice was separated from broken kernels using a sizing device.

Researchers then examined physical properties including bulk density of rough rice. Brown rice properties were determined by dehulling four rough rice samples.

Using a scanning system, brown rice chalkiness as a percentage of kernel area was determined. Fissured kernel percentage was visually determined using a grain scope. A kernel was counted as fissured if one or more fissures were detected.

Statistical software was used to analyze the data.

Results

Unfractioned milled rice yields of superior and inferior lots within each cultivar were not different. However, differences in the head rice yields were substantial, with the superior Wells cultivar being 27 percentage points greater than the inferior sample of the same cultivar. The XL753 superior head rice yield was 15 percentage points greater than the inferior sample.

The reduced head rice yield was due to the chalkiness and the fissured kernel percentage (see Figure 1, page 82). There was significantly greater chalkiness observed in the inferior lots, as well as a greater fissured kernel percentage.

Thickness grading of rough rice resulted in mass percentages for the >2 mm fractions ranging from 57% to 77%, about 10 percentage points less across the range of lots than the 66% to 89% observed by researchers previously. See Figure 2a and 2b (this page).

For all lots, the >2 mm kernels comprised the majority of the mass. For both cultivars, there was a trend for a greater mass of thicker kernels in the lots with inferior milling quality. Since chalkiness has been shown to be more prevalent in thinner kernels, decreased head rice yield for the unfractioned, inferior lots may be associated with increased fissuring of the thicker, bolder kernels.

A further thickness grading of the >2 mm kernels was added to determine if the greater portion of fissured kernels was concentrated in the very thickest kernels. This would enable the thickness fraction to be partitioned and transferred to an alternate processing stream, such as parboiling.

The >2 mm fraction was thickness graded with a 2.05 mm screen, resulting in the previously described thin, medium and thick kernel fractions. This potentially created a medium kernel stream with reduced fissured kernels, reduced chalkiness, and improved milling characteristics.

The mass percentage of the thin kernels ranged from 25% to 42%. The superior lots had the greatest mass percentages of thin kernels, about 42%. The mass percentage of medium kernels generally exceeded that of thin kernels and was greater than that of thick kernels. A trend existed for greater mass percentage of thick kernels in inferior lots.

Milled rice yield increased with increased kernel thickness for all lots, agreeing with previous research. For superior lots of both cultivars, there no differences in milled rice yield between medium and thick kernels. However, for the inferior lots, the milled rice yield of thick kernels was greater than that of medium kernels. See Figures 3a and b (page 85).

For the superior lots, milled rice yields of both thick and medium kernels exceeded those of unfractioned rice. Therefore, removing the thin kernels increased milled rice yield for inferior and superior rice lots. For inferior lots, the thick kernels had greater milled rice yield and medium kernels had equivalent milled rice yield when compared to unfractioned rice.

The head rice yields of thin kernels were significantly less than those of medium kernels. The general trend for decreased head rice yields of thin kernels occurred even though the degree of milling was the same as medium and thick kernels.

Counter to the trends seen in milled rice yields, head rice yields of thick kernels were significantly less than medium kernels, with the exception of the superior Wells lots. Those lots showed a 1.5 percentage point decrease in head rice yields of thick kernels.

The head rice yields of medium thickness kernels followed the same pattern of milled rice yields.
For superior lots, head rice yields of medium kernels were significantly greater than unfractioned rice, while for inferior lots, head rice yields of medium kernels were statistically equivalent to those of unfractioned rice.

In superior lots, head rice yields of thick kernels trended greater than unfractioned rice while inferior lots were less than unfractioned rice.
Trends for milling yields across the three thickness fractions may be explained by the associated physical properties.
Bulk densities of thin kernels were significantly less than those of medium and thick kernels. This is likely a result of more completely filled kernels and greater relative mass of endosperm associated with the medium and thick kernels.

For chalkiness, the overall trend was a reduction in chalkiness with increased kernel thickness.
The fissured kernel percentage varied by cultivar, increasing with increased thickness fraction for both Wells lots, while no differences were observed in the XL753 variety.

Conclusions

Thickness grading of rough rice resulted in fractions with distinct properties. For all lots, there was a trend of increasing milled rice yield with increasing kernel thickness. Head rice yields did not necessarily follow the same trend.

While head rice yields of medium kernels were greater than those of thin kernels for all lots, head rice yields of thick kernels tended to be less than medium kernels due to increased fissured kernels in the thick fractions.

Milled rice yield was significantly linked to bulk density. However, head rice yields were linked to physical properties primarily kernel defects such as chalkiness and fissured kernel percentage.

Thickness grading of long grain rough rice could concentrate chalky and fissured kernels and partition them into alternative processing streams, thereby improving the milling yields.

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