EFFECT OF COOLING METHODS AND MILLING PROCEDURES ON THE APPRAISAL OF RICE MILLING QUALITY

EFFECT OF COOLING METHODS AND MILLING PROCEDURES
ON THE APPRAISAL OF RICE MILLING QUALITY
Z. Pan, J. F. Thompson, K. S. P. Amaratunga, T. Anderson, X. Zheng
ABSTRACT. The objective of this research was to appraise the quality of medium-grain rice as affected by cooling and two
different Federal Grain Inspection Service (FGIS) milling procedures. Milled rice quality was measured in terms of total rice
yield (TRY), head rice yield (HRY), and whiteness. The cooling study used an internal and an external heat exchanger
developed for the McGill No. 3 mill with room-temperature water and ice cooled water as cooling media. Californian M202
rough rice samples of three different qualities were milled using the McGill No. 3 mill with and without cooling following
the standard FGIS Western rice milling procedure. The cooling methods increased the TRY and HRY, but decreased whiteness.
Every 10°C reduction in the milled rice temperature due to cooling corresponded to an increase of 0.9 percentage points in
TRY and 1.7 percentage points in HRY. The rice samples of M202 from California and Bengal from the Southern region milled
with the Western milling procedure had lower TRY (1.0 to 1.4 percentage points) and HRY (2.3 percentage points) compared
with the Southern milling procedure. Similar quality results obtained using the Southern milling procedure might be produced
using the Western milling procedure with heat exchanger cooling.
Keywords. Cooling, Head rice yield, Heat exchanger, Milling, Quality, Rice sample, Temperature, Total rice yield, Whiteness.
he economic value of rough rice is based on its mill-
The McGill No. 3 mill holds a batch of rice in the milling
ing quality. In the U.S., milling quality is typically
chamber, a space between a lobed shaft (also called the cutter
determined by milling a small rough rice sample
bar) and a surrounding metal screen and a cover. The milling
Tusing the official procedures of the USDA Federal action occurs in the milling chamber due to the relative
Grain Inspection Service (FGIS) (USDA-FGIS, 1994). In the
motion between the rotary cutter bar and rice kernels and
procedures, one of the most important steps is milling, and
among the rice kernels while the batch of rice is under
the McGill No. 3 mill is specified as the official mill. The
pressure. The pressure in the milling chamber is generated by
milling is a batch, single-pass process, which is different
a weight and lever arm assembly pressing a saddle against the
from the current commercial continuous, multi-pass milling
top cover of the chamber (fig. 1a). The milling weights
process. Because of the batch milling process, a large amount
specified in the FGIS procedures vary with the rice varieties
of heat is generated and accumulated in the mill (mainly in
and regions of rice produced. For medium-grain rice, the
the cutter bar) and rice during milling, which could cause a
FGIS milling procedures require that the rice sample is
reduction of the appraised total rice yield (TRY) and head
exposed to a 30 s milling cycle using a 4.54 kg (10 lb) or
rice yield (HRY) (Pan and Thompson, 2002). Meanwhile, the
3.18 kg (7 lb) milling weight for rice produced in the Western
standard FGIS milling procedures specify different milling
or Southern region, respectively (USDA-FGIS, 1994). After
weights (pressures) to be used for rice produced in the West-
the milling cycle, the weights are reduced to 0.98 kg (2 lb) for
ern and Southern regions of the U.S. However, the effects of
Western rice or 0 kg for Southern rice and the mill is operated
different milling weights on the appraised rice quality lack
for an additional 30 s, which is normally referred to as a
scientific documentation.
brushing cycle or polishing cycle. Due to the differences in
milling weights, the rice sample milling procedures are
called the Western or Southern rice sample milling procedure
in this study. After the polishing cycle, the milled rice is
Article was submitted for review in March 2005; approved for
unloaded and visually compared with a standard “well
publication by the Food & Process Engineering Institute Division of
milled” sample. The polishing cycle may be repeated to
ASABE in August 2005.
achieve “well milled” rice.
The authors are Zhongli Pan, ASABE Member Engineer, Research
Engineer, USDA-ARS Western Regional Research Center, Albany,
There is only limited information about the development
California, and Assistant Adjunct Professor, Department of Biological and
of the FGIS procedures and related research (Smith and
Agricultural Engineering, University of California, Davis, California;
McCrae, 1951; Smith, 1955a, 1955b, 1955c, 1955d). Smith
James F. Thompson, ASABE Member Engineer, Extension Specialist, K.
(1955c) studied the effect of milling weight on rice quality
Sanath P. Amaratunga, ASABE Member Engineer, Post-Doctoral
Researcher, and Tom Anderson, Engineering Shop Manager, Department
and found that milling of medium- and short-grain rice
of Biological and Agricultural Engineering, University of California,
required more time than milling of long-grain rice using the
Davis, California; and Xianzhe Zheng, ASABE Member Engineer,
McGill No. 3 mill. The most recently reported rice sample
Professor, Northeast Agricultural University, Harbin, China.
milling research used either the McGill No. 2 mill or IRRI test
Corresponding author: Zhongli Pan, Processed Foods Research Unit,
tube mill (Takai and Barredo, 1981; Banaszek et al., 1989;
USDA-ARS Western Regional Research Center, 800 Buchanan St.,
Albany, CA 94710; phone: 510-559-5861; fax: 510-559-5864; e-mail:
Andrews et al., 1992; Sun and Siebenmorgen, 1993; Archer
zpan@pw.usda.gov.
and Siebenmorgen, 1995; Reid et al., 1998; Bautista et al.,
Transactions of the ASAE
Vol. 48(5): 1865?1871
E 2005 American Society of Agricultural Engineers ISSN 0001?2351
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Milling
weight
External heat
exchanger
Internal heat
exchanger
(a)
(b)
(c)
Figure 1. (a) McGill No. 3 mill set-up with the internal and external heat exchangers, (b) design of external heat exchanger, and (c) design of internal
heat exchanger incorporated in the cutter bar.
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2001). The McGill No. 2 mill uses about 100 to 150 g of rough
cooling using the standard milling weights of the Western
rice, while McGill No. 3 mill uses 1000 g of rough rice, as de-
milling procedure of the FGIS. The three Californian rice
fined in the USDA-FGIS milling procedures (USDA-FGIS,
samples and one additional medium Southern variety Bengal
1994). Andrews et al. (1992) reported that the HRY increased
rough rice sample, obtained from the Agricultural Experi-
with reduced milling time or reduced milling weight (pres-
ment Station of Louisiana State University, were used to
sure) with the McGill No. 2 mill. The HRY was also im-
study the effect of the Western and Southern milling
proved by lowering the brown rice temperature before
procedures on the appraised rice quality. The samples were
milling (Archer and Siebenmorgen, 1995). However, the
milled using both standard Western and Southern milling
HRY was inversely related to the degree of milling as mea-
procedures (USDA-FGIS, 1994).
sured by a Satake milling meter. Archer and Siebenmorgen
Because high temperatures of the cutter bar and milled
(1995) also found that lower brown rice temperatures did not
rice reduce the TRY and HRY of milled rice with excessive
significantly improve the HRY if the HRY yield was mathe-
rice kernel breakage and moisture loss, the current milling
matically adjusted to achieve an equal degree of milling. Mo-
practice at the CDFA Laboratory is to mill rice samples when
hapatra and Bal (2004) did a similar study using a
the cutter bar temperature reaches a temperature of 46°C to
laboratory-scale, abrasive mill and found that the whole ker-
54°C (115°F to 130°F). If the cutter bar is below the
nel yield decreased linearly with an increase in milled rice
prescribed temperature, one or two rice samples are milled
temperature. However, the researchers did not report the
before an official rice sample is milled. If the cutter bar
milling degree of the white rice.
temperature is above the prescribed temperature, a small fan
The rice sample milling procedures have been updated
is used to cool the cutter bar to the required temperature
several times (USDA-FGIS, 1979, 1982, 1994), but the
range. The temperature of the cutter bar at the start of milling
technical information related to the changes in the milling
was called the initial cutter bar temperature. After a rice
procedure is not available. Pan and Thompson (2002) studied
sample was milled and unloaded from the rice mill, the
the relationships between mill heat generation, rice tempera-
temperature of the cutter bar was measured again, and this
ture, and quality (TRY, HRY, and whiteness) using a McGill
temperature was called the ending cutter bar temperature.
No. 3 mill. They found that the highest temperatures of the
Both the initial and ending temperatures at the working
cutter bar and milled rice reached 74°C and 84°C, respec-
surface of the cutter bar were measured using an infrared
tively, after six rice samples were successively milled. The
thermometer. When an infrared thermometer is used to
high cutter bar and milled rice temperatures caused signifi-
measure the temperature of cutter bar surface, the high
cant reduction in the appraised TRY and HRY of milled rice,
reflectivity of the metal surface can result in inaccurate
especially for low quality rice. The high milling weight of the
temperature measurement. Therefore, before measuring the
Western milling procedure may also cause higher milling
temperature, the cutter bar surface was covered with a piece
temperature than the Southern milling procedure. The
of thin paper tape to ensure an accurate temperature
combination of high milling temperature and high milling
measurement. The temperature of milled rice was measured
weight (pressure) could produce lower appraised TRY and
using a thermometer through a thermocouple immediately
HRY, but higher whiteness with the Western milling
after the milled rice was unloaded from the mill.
procedure compared to the Southern procedure. Therefore,
rice producers are interested in developing methods to
HEAT EXCHANGERS
prevent the quality changes caused by the high milling
To reduce the milling temperature (cutter bar and milled
temperature and determining the difference in appraised
rice temperatures) of the McGill No. 3 mill, an external heat
quality obtained using the Southern and Western milling
exchanger and an internal heat exchanger were developed at
procedures.
the University of California, Davis. The external heat
The objectives of this study were to: (1) design heat
exchanger was placed on the top of the milling chamber and
exchangers for cooling a McGill No. 3 mill, (2) determine the
replaced the regular saddle. It was made of brass and had
effect of cooling methods on appraised milling quality of
channels to allow cooling water to flow through it and
medium-grain rice, and (3) compare the appraised rice
remove heat during milling (fig. 1b). Since the heat
quality of medium-grain rough rice from the Western and
exchanger added additional weight to the milling chamber,
Southern regions using the Southern and Western milling
the milling weight was adjusted to keep the same milling
procedures.
pressure as generated by the milling weight specified by the
standard FGIS milling procedure. The internal heat exchang-
er was created by drilling holes through an existing cutter bar
MATERIALS AND METHODS
to allow cooling water to circulate inside it (fig. 1c). A
stainless steel manifold was added to the cutter bar to supply
MATERIALS AND MILLING PROCEDURES
cooling water to it. The two heat exchangers were operated
Three Western variety M202 rough rice lots with different
simultaneously and individually. The cooling water had the
qualities (low, medium, high) from California were used for
flow rate of 0.98 kg min?1 for each exchanger and was
studying the cooling effect on the appraised rice quality. The
pumped to circulate through the heat exchangers during
rice samples were obtained from Farmers Rice Cooperative
milling.
(Sacramento, Cal.). The moisture contents were 12.6%,
13.1 %, and 12.9% for rough rice of low, medium, and high
qualities, respectively. The lots were split into 1000 g
MEASUREMENT OF MILLED RICE QUALITY
samples and milled with McGill No. 3 mill (fig. 1a) at the
The major quantitative rice quality indicators specified in
California Department of Food and Agriculture (CDFA)
the standard rice sample milling procedures are TRY and
Laboratory (West Sacramento, Cal.) with and without
HRY. Whiteness of the milled rice was also examined to
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Table 1. Experimental design for studying effects of milling conditions.
Western Milling Procedure with Different Cooling Conditions[a]
Southern Milling
Western Milling
Rice
Procedure
Procedure (Control)
IHX-Water
IHX-EHX-Water
IHX-Ice
IHX-EHX-Ice
EHX-Ice
Low quality
X
X
X
X
X
X
X
M202
Medium quality
X
X
X
X
X
X
X
High quality
X
X
X
X
X
X
Bengal
X
X
[a] IHX = internal heat exchanger, EHX = external heat exchanger, Water = water at room temperature (20°C - 21°C), and Ice = ice water (1°C - 3°C).
ensure that it reached a level called “well milled” as defined
condition, respectively. All reported values are the averages
by the FGIS standard. The TRY and HRY were determined
of the measured data. All quality data were analyzed using
by following the USDA-FGIS procedures. The TRY and
SAS software (SAS Institute, Inc., Raleigh, N.C.). Analysis
HRY were defined as percentages of milled rice and milled
of variance (ANOVA) and least significant difference (LSD)
whole kernels based on the initial rough rice weight, respec-
(a = 0.05) were used to differentiate the means of rice quality
tively. The whiteness of milled rice was evaluated based on
parameters.
the whiteness index (WI), as determined with a whiteness
tester (C-300, Kett Electronic Laboratory, Tokyo, Japan). A
higher index number indicates whiter milled rice.
RESULTS AND DISCUSSIONS
EFFECT OF COOLING ON MILLING TEMPERATURE
EXPERIMENTAL DESIGN OF MILLING TEST
The uses of different cooling methods reduced both initial
The effects of cooling methods and milling procedures on
and final cutter bar temperatures, which resulted in lower
quality appraisal were studied following the experimental
milled rice temperatures than the control (table 2). After
design shown in table 1. Both room-temperature water (20°C
examining the temperature changes of the cutter bar during
to 21°C) and ice-cooled water (1°C to 3°C) were used for
milling and the milled rice temperatures for different quality
cooling through the heat exchangers. For the milling tests
rough rice samples under a specific cooling method, it seems
without cooling, the initial cutter bar temperature was 49°C,
that the quality of rough rice did not significantly affect the
which was in the prescribed temperature range of the current
temperatures. Therefore, the following discussion is based on
practice at the CDFA Laboratory. When the internal heat
the average temperature values obtained from the samples
exchanger with room-temperature water and ice water
with different rough rice qualities under a specific cooling
cooling was tested, the initial cutter bar temperature was
treatment, unless otherwise specified. The external heat ex-
cooled to 24°C and 12°C, respectively. If the ending cutter
changer with ice water reduced both milled rice temperature
bar temperature was higher than the required temperature
and ending cutter bar temperature by 2°C on average
after milling a sample, the cooling water was pumped
compared with the control (no cooling). The internal heat ex-
through the internal heat exchanger to achieve the desired
changer with room-temperature water or ice water lowered
cutter bar temperature before milling another sample. The
the ending cutter bar temperature by 24°C or 34°C, respec-
initial and ending cutter bar temperatures were measured
tively, and the milled rice temperature by 11°C or 26°C, re-
using the methods described in the Materials and Milling
spectively, compared with the control. Therefore, the internal
Procedures section. The milling tests of the Bengal and M202
heat exchanger was more effective than the external heat ex?
rice were repeated six and three times at each miling
Table 2. Temperatures (°C) of cutter bar and milled rice under different milling treatments.
Treatment[b]
Rough Rice
Temperature
Quality
Measurement[a]
Control
IHX-Water
IHX-EHX-Water
IHX-Ice
IHX-EHX-Ice
EHX-Ice
Ti
49
24
24
12
14
49
Te
58
31
31
21
22
54
Low
?T
9
7
7
9
8
5
Rice
74
65
63
60
58
72
Ti
49
24
24
12
13
48
Te
56
34
34
22
23
55
Medium
?T
7
10
10
10
10
7
Rice
73
61
61
56
56
71
Ti
49
24
24
13
13
Te
57
35
33
26
23
High
?T
7
11
9
13
10
Rice
74
62
59
58
54
Ti
49
24
24
12
13
49
Te
57
33
33
23
22
55
Average
?T
8
9
9
11
9
6
Rice
74
63
61
58
56
72
[a] Ti = initial cutter bar temperature, Te = ending cutter bar temperature, and ?T = difference between Ti and Te.
[b] Control = Western milling procedure without cooling, IHX = internal heat exchanger, EHX = external heat exchanger, Water = water at room temperature
(20°C - 21°C), and Ice = ice water (1°C - 3°C).
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changer in lowering the temperatures due to the direct contact
2002). However, the average WI values of milled low,
of the internal heat exchanger with the rice during milling.
medium, and high quality rice obtained with the cooling were
The difference between the initial and ending cutter bar tem-
42.8, 42.0, and 42.5, respectively, compared to correspond-
peratures for all cooling methods except for EHX-ice was in
ing values of 43.8, 42.4, and 42.3 for the control samples,
the range of 9°C to 11°C, which was only slightly higher than
which may indicate that the low and medium quality rice
8°C of the control test. The reason for the smaller tempera-
samples were not milled to the same degree as the control
ture change (6°C) during milling with the EHX-ice treatment
samples. Using both the internal and external heat exchang-
than with the other cooling treatments is not known. The low-
ers with ice water resulted in the highest average TRY
ered ending cutter bar temperature in the cooling tests was
(68.3%) and HRY (56.4%) compared with the control of
primarily due to the lowered initial cutter bar temperature.
66.6% TRY and 53.3% HRY.
This showed the importance of lowering the initial cutter bar
The ice water cooling caused more TRY and HRY
temperature if a low ending cutter bar temperature and milled
increases than the cooling with room-temperature water. All
rice temperature are desired.
quality parameters discussed below were the average for low,
Using the internal and external heat exchangers at the
medium, and high quality. When only the internal heat
same time resulted in an additional 2°C reduction in milled
exchanger was used, ice water cooling increased TRY and
rice temperature compared with using only the internal heat
HRY by 1.4 and 2.6 percentage points, respectively, while
exchanger, even though the ending cutter bar temperature did
room-temperature water cooling increased them by 1.1 and
not change much.
1.6 percentage points. Similarly, when both the internal and
The ice water cooling reduced milled rice temperature by
external heat exchangers were used, the ice water cooling
5°C more than room-temperature water cooling regardless of
resulted in greater increases in TRY (1.7% percentage points)
the combination of heat exchangers used. When both the
and HRY (3.1 percentage points) than the room-temperature
internal and external heat exchangers were used, room-tem-
water cooling (1.3 and 2.2 percentage points) compared with
perature water and ice water cooling treatments reduced
the control. The external heat exchanger contributed to the
milled rice temperatures by 13°C and 18°C, respectively,
increases in TRY and HRY of 0.2 to 0.3 percentage points and
compared with the control. Since the accuracy of the surface
0.5 to 0.6 percentage points, respectively, in contrast to the
temperature measurement could be up to ±2°C, the effect of
greater increases of 1.1 to 1.4 percentage points in TRY and
cooling medium and methods on the temperature difference
1.6 to 2.6 percentage points in HRY by using the internal heat
between the initial and ending temperatures of cutter bar
exchanger. Such results again showed that the internal heat
might not be significant.
exchanger was more effective than the external heat
exchanger, which is consistent with the milling temperature
EFFECT OF COOLING ON APPRAISED RICE QUALITY
data. The internal heat exchanger cooled with ice water
The cooling treatments significantly increased the TRYs
caused condensation at the surface of the cutter bar after the
and HRYs compared with the control treatment (table 3). The
milled rice was unloaded. Some rice bran absorbed the
milling test for high quality rice using EHX-ice was not
condensed water, formed small wet pieces, and became
conducted due to insufficient rice sample. The average
attached onto the cutter bar surface. The attached pieces
quality parameters obtained with cooling were calculated
needed to be cleaned after milling each sample.
without including the data obtained using EHX-ice. For low,
The internal heat exchanger with ice water quickly cooled
medium, and high quality rice, the average HRY increases
the cutter bar to a prescribed temperature before milling. The
were 1.9, 2.0, and 3.2 percentage points, respectively, and
problem of water condensation on the cutter bar surface may
corresponding TRY increases were 1.3, 1.2, and 1.7 percent-
be minimized by reducing the time of cutter bar exposure to
age points. The increased HRY and TRY could be due to both
room air before milling is conducted. The condensation is not
reduced moisture loss and lowered breakage associated with
a problem if cooling water is above the dew point temperature
the lower milled rice temperature (Pan and Thompson,
Table 3. Quality of milled rice under different milling conditions.
Treatment[b]
Rough Rice
Milled Rice
Quality
Quality[a]
Control
IHX-Water
IHX-EHX-Water
IHX-Ice
IHX-EHX-Ice
EHX-Ice
TRY
67.4 a
68.4 b
68.7 b
68.7 b
68.9 b
68.0 ab
Low
HRY
46.8 a
48.0 ab
48.4 b
48.9 b
49.4 b
46.9 a
WI
43.8 a
43.0 b
42.8 b
42.5 b
42.4 b
43.0 b
TRY
66.7 a
67.7 b
67.8 b
68.0 b
68.1 b
68.1 b
Medium
HRY
55.0 a
56.2 b
56.8 b
57.6 b
57.5 b
57.1 b
WI
42.4 a
41.8 b
41.8 b
41.8 b
42.3 ab
42.4 a
TRY
65.6 a
66.9 b
67.2 b
67.3 bc
67.9 c
High
HRY
58.1 a
60.3 b
61.2 b
61.2 b
62.4 c
WI
42.3 a
42.0 a
42.7 a
42.8 a
42.6 a
TRY
66.6
67.7
67.9
68.0
68.3
Average
HRY
53.3
54.9
55.5
55.9
56.4
WI
42.8
42.3
42.4
42.3
42.4
[a] TRY = total rice yield, HRY = head rice yield, and WI = whiteness index.
[b] Control = Western milling procedure without cooling, IHX = internal heat exchanger, EHX = external heat exchanger, Water = water at room temperature
(20°C - 21°C), and Ice = ice water (1°C - 3°C). Values in each row followed by different letters are significantly different at P < 0.05.
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70
42.9
70
y = ?0.0938x + 73.532
y = 1.8615x ? 64.053
42.8
65
65
R2 = 0.989
R2 = 0.9891
42.7
Whiteness Index
60
60
(%)
y = ?0.1718x + 65.884
Y
42.6
R2 = 0.9866
(%)
55
55
y = 1.6557x ? 55.486
Y
R
42.5
H
R2 = 0.8807
and HR
Y
R
50
50
T
42.4
TRY
HQ
y = 1.6691x ? 65.992
45
45
HRY
MQ
42.3
2
R = 0.8603
WI
LQ
40
40
42.2
65
66
67
68
69
70
50
55
60
65
70
75
TRY (%)
Milled rice temperature (5C)
Figure 3. Relationship between head rice yields and total rice yields of var-
Figure 2. Relationship between averaged milled rice temperature and
ious quality California rice (HQ = high quality rice; MQ = medium quality
quality results.
rice, and LQ = low quality rice).
of ambient air. It is not as convenient to use both heat ex-
Regardless of the cooling methods, the HRY and TRY had
changers for the McGill No. 3 mill compared with only using
a positive linear relationship with similar slopes for different
the internal heat exchanger. Besides the advantage of using
quality rice lots (fig. 3). This indicates that the increases in
the internal heat exchanger in minimizing the reduction of
TRY and HRY were the same for each rice lot, despite
TRY and HRY, it also lowers the initial cutter bar temperature
differences in overall lot qualities. In other words, the TRY
within a few seconds to the desired level, compared with sev-
and HRY increases caused by cooling were similar in lots
eral minutes required for the current air cooling practice.
with HRYs ranging from about 45% to 60%.
The individual quality data showed that the HRY of high
quality rice was improved more than the HRY of low and
COMPARISON OF WESTERN AND SOUTHERN MILLING
medium quality rice. This is different from the results from
PROCEDURES
some of our other tests (Pan and Thompson, 2002). The
The Southern standard milling procedure resulted in
difference could be due to the different histories of drying,
higher TRY and HRY than the Western procedure (table 4).
harvest, and storage, although the exact reasons for the
When the M202 rice was milled, the average TRY and HRY
difference are not known.
obtained with the Western milling procedure were 1.4 and
In general, the effectiveness of the different cooling
2.3 percentage points lower, respectively, than those ob-
methods in reducing rice temperature followed an order from
tained with the Southern procedure. Similarly, when the
high to low: internal and external heat exchangers with ice
Bengal rice was milled, the TRY and HRY obtained with the
water, internal heat exchanger with ice water, internal and
Western milling procedure were 1.0 and 2.4 percentage
external heat exchangers with room-temperature water, and
points lower, respectively, than those obtained with the
internal heat exchanger with room-temperature water. In
Southern procedure. The lower TRY and HRY obtained with
determining the order, the external heat exchanger with ice
the Western milling procedure were associated with the
water treatment was not included since the milling quality
higher milling temperature, which was probably caused by
data of high quality rice with external heat exchanger cooling
the greater milling weights used with the Western produce.
were not available.
The WI values of rough rice from both regions were 0.7
The TRY or HRY increased linearly with decreasing
to 1.7 units lower (darker rice) when they were milled using
milled rice temperature (fig. 2). Every 10°C increase in the
the Southern milling procedure compared with the Western
milled rice temperature corresponded to a decrease of
procedure. However, the current FGIS milling procedure
0.9 percentage points in TRY or 1.7 percentage points in
does not specify the milling degree in terms of WI. If the rice
HRY in the range of temperatures produced by the various
samples were milled to the same whiteness, then the
cooling methods. The milled rice quality results also verified
difference in HRYs from the different milling procedures
our initial hypothesis that the milling temperature was a
might be reduced. However, there were not enough data in
critical factor affecting the appraised rice quality using the
this study to make a mathematical adjustment based on the
Western milling procedure.
whiteness or degree of milling as reported by Archer and
There was no consistent relationship between the milled
Siebenmorgen (1995). The higher WI values of the Bengal
rice temperature and whiteness index (WI). The heat
rice could be due to its variety and lower moisture content
exchangers used with low and medium quality rice caused a
compared to the M202 rice used in the tests. It can also be
small but statistically significant reduction in WI. This was
seen that the M202 quality from the Southern procedure was
not observed in the high quality rice samples. The WI values
very similar to the results from the Western procedure with
of the cooling treatments averaged across all rice qualities
two of the cooling treatments, i.e., the internal and external
were 0.4 to 0.5 units lower than the control. However, all the
heat exchangers with room-temperature water, or the internal
rice samples were “well milled” based on the FGIS standard.
heat exchanger with ice water.
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Table 4. Quality of rice milled with the Western and Southern procedures.
Quality of Milled Rice[a]
Cutter Bar Temperature (°C)
Rough Rice
Milling
Milled Rice
Rice
Quality
Procedures
TRY
HRY
WI
Temperature (°C)
Ending
Initial
Western
67.4 a
46.8 a
43.8 a
74
58
49
Low
Southern
68.7 b
48.7 b
42.6 b
72
55
49
Western
66.7 a
55.0 a
42.4 a
73
56
49
Medium
Southern
67.9 b
56.9 b
41.5 b
69
54
49
M202
Western
65.6 a
58.1 a
42.3 a
74
57
49
High
Southern
67.3 b
61.1 b
42.1 a
69
55
49
Western
66.6
53.3
42.8
74
57
49
Average
Southern
68.0
55.6
42.1
70
55
49
Western
68.7 a
59.2 a
45.7 a
74
58
49
Bengal
Southern
69.7 b
61.6 b
44.0 b
72
54
49
[a] Values from the Western and Southern milling procedures in each category followed by different letters are significantly different at P < 0.05.
The Bengal rice had 10.2% moisture. TRY = total rice yield, HRY = head rice yield, and WI = whiteness index.
CONCLUSIONS
Archer, T. A., and T. J. Siebenmorgen. 1995. Milling quality as
affected by brown rice temperature. Cereal Chem. 72(3):
This study showed that using the internal and external heat
304-307.
exchangers effectively reduced rice and mill temperatures,
Bautista, R. C., T. J. Siebenmorgen, S. C. Millsap, and B. K. Goh.
increased the TRY and HRY, and decreased whiteness of the
2001. Evaluation of the IRRI test tube mill for use in milling
milled samples. The internal heat exchanger lowered mill
small samples of rice. ASAE Paper No. 016100. St. Joseph,
and rice temperatures more than the external heat exchanger,
Mich.: ASAE.
and ice water lowered the temperatures more than room-tem-
Banaszek, M. M., T. J. Siebenmorgen, and R. N. Sharp. 1989.
perature water. The maximum improvements in average
Effects of moisture content at milling on head rice yield and
TRY and HRY of the three different quality rice samples were
degree of milling. Arkansas Farm Research Series 38: 15.
1.7 and 3.1 percentage points when both the internal and
Mohapatra, D., and S. Bal. 2004. Wear of rice in an abrasive milling
operation, part II: Prediction of bulk temperature rise.
external heat exchangers with ice water were used following
Biosystems Eng. 89(1): 101-108.
the standard Western milling procedure. A 10°C decrease in
Pan, Z., and J. F. Thompson. 2002. Improvement of accuracy and
milled rice temperature corresponded to an increase of
consistency of rice sample milling. Research Progress Report of
0.9 percentage points in TRY and 1.7 percentage points in
California Rice Research Board.
HRY. The Western milling procedure caused significantly
Reid, J. D., T. J. Siebenmorgen, and A. Mauromoustakos. 1998.
lower TRY and HRY than the Southern milling procedure,
Factors affecting the head rice yield versus degree of milling
probably because of the high milling weight and high milling
slope. Cereal Chem. 75(5): 738-741.
temperature in the Western milling procedure. Quality results
Smith, W. D. 1955a. The use of the Carter dockage tester to remove
similar to the Southern milling procedure can be produced by
weed seeds and other foreign material from rough rice. Rice J.
using the Western milling procedure with either room-tem-
58(9): 26-27.
perature water cooling of both the internal and external heat
Smith, W. D. 1955b. The use of the McGill sheller for removing
hulls from rough rice. Rice J. 58(10): 20.
exchangers, or with ice water cooling of the internal heat
Smith, W. D. 1955c. The use of the McGill miller for milling
exchanger.
samples of rice. Rice J. 58(11): 20.
Smith, W. D. 1955d. The determination of the estimate of head rice
ACKNOWLEDGEMENTS
and total yield with the use of the sizing device. Rice J. 58(12):
The authors wish to thank Homer Formenteta, Dale Rice,
9.
and Sandra Newell of the California Department of Food and
Smith, W. D., and W. McCrea Jr. 1951. Where breakage occurs in
Agriculture; Michael Johnson and Chuck Britton of the
the milling of rice. Rice J. 54(2): 14-15.
USDA Federal Grain Inspection Service for supporting the
Sun, H., and T. J. Siebenmorgen. 1993. Milling characteristics of
various rough rice thickness fractions. Cereal Chem. 70(6):
experiments; Farmer’s Rice Cooperative for supplying the
727-733.
rice samples; and the California Rice Research Board for
Takai, H., and I. R. Barredo. 1981. Milling characteristics of a
providing partial financial support for the project.
friction laboratory rice mill. J. Agric. Eng. Res. 26(5): 441-448.
USDA-FGIS. 1979. Rice Inspection Handbook for the Sampling,
Grading, and Certification of Rice. HB 918-11. Washington,
REFERENCES
D.C.: USDA Agricultural Marketing Service.
USDA-FGIS. 1982. Rice Inspection Handbook. Washington, D.C.:
Andrews, S. B., T. J. Siebenmorgen, and A. Mauromostakos. 1992.
USDA Agricultural Marketing Service.
Evaluation of the McGill No. 2 miller. Cereal Chem. 69(1):
USDA-FGIS. 1994. Rice Inspection Handbook. Washington, D.C.:
35-43.
USDA Agricultural Marketing Service.
Vol. 48(5): 1865?1871
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