World Grain - February 2015 - 64

RICE QAURTERLY

MILLING PROPERTIES
OF COMMINGLED RICE

ommingling of rice commonly occurs during harvest, drying and storage. Because there are differences in milling
properties of different rice cultivars, there is a need to study
the impact of commingling on the milling properties of rice.
These properties include degree of milling, milling yields,
color and chalkiness. To examine the impact, researchers from
the University of Arkansas, Fayetteville, Arkansas, U.S., commingled two long-grain hybrid cultivars and two long-grain
pureline cultivars in various proportions.
Milled rice yield (MRY), head rice yield (HRY), surface
lipid content (SLC) and head rice chalkiness were measured
for individual samples as well as the commingled samples.
Kernel dimensions, total lipid content (TLC), chalkiness and
bulk density of brown rice samples of the individual-cultivar
lots were also measured.
This article will review the process and results obtained
by researchers, which included Nikhil N. Basutkar, graduate
research assistant; Terry J. Siebenmorgen, university professor and director, Rice Processing Program; and Andronikos
Mauromoustakos, professor, agricultural statistics laboratory.
CULTIVARS
Acreage of hybrid rice cultivars has increased due to greater
cient use of water and soil nutrients compared to pureline cultivars. But differences have been observed in the milling properties of hybrid and pureline cultivars, researchers said. For
example, hybrid cultivars have been generally found to mill
to a desired degree of milling more quickly than purelines do.
Rice kernel characteristics such as kernel topography,
physical dimensions and moisture content have been shown
to affect rice milling performance. Variation in these characteristics among rice cultivars leads to increased complexity in
managing milling and processing operations.
Milling involves the removal of the germ and bran layers, which
contain most of the lipids present in a rice kernel. The remaining
endosperm consists of mostly starch. Milling decreases the lipid
content and increases the relative starch content of white rice.
The degree of milling, or the extent of bran removal during milling, affects milling and head rice yields. Head rice
is more valuable than broken rice, so maximizing HRY is of
economic importance.
Commingling rice with differing properties that affect
the degree of milling may result in over-milling of some
64

by Susan Reidy

Commingling of cultivar lots can
occur during harvest, drying and
storage operations
rice, thereby reducing milling yields.
Chalkiness, which is a major rice kernel defect, usually occurs
when high nighttime air temperatures are experienced during
critical stages of kernel development. Cultivars vary in their susceptibility to these temperatures, and thus can differ in chalkiness.
Because kernels of different physical dimensions, topographies
and chalkiness may be commingled, there could be an impact on
milling properties, researchers said, particularly when cultivars
with dissimilar properties are commingled.
MATERIALS AND METHODS
Researchers used two hybrid (CL XL729 and CL XL745)
and two pureline (CL 151 and Wells) cultivars grown in 2011
and 2012. The 2011 lots were selected to have high HRYs
while the 2012 lots were selected to have low HRYs. This was
done to determine if commingling had a similar effect on rice
of different levels of milling yield.
All lots were cleaned using a dockage tester and conditioned
to 12% moisture content. Samples from the cultivar lots were
commingled in various ratios as shown in Table 1 (page 66).
To prepare for milling, 150-gram rough rice samples were
prepared for each commingling ratio. Individual lots were divided into close approximation of the required quantities using a grain divider, weighed to achieve the proportions in the
table and then mixed in respective proportions.
Each commingled sample was mixed for 2 minutes using a
rotary rice grader. For each of the 11 commingling ratios and
four individual lots, four 150-gram samples were prepared, allowing for samples from each to be milled for either 10-, 20-,
30- or 40-second durations. This procedure enabled quantifying the duration required to reach the desired degree of milling levels, researchers said.
Milled rice yield was calculated as a mass percentage of the
original 150 grams of rough rice remaining as milled rice, comprising intact (head rice) and broken kernels. Head rice yield was exFebruary 2015 / World Grain / www.World-Grain.com

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