Characteristics of One-Pass Vertical Friction-Type Rice

Japan. J. Trop. Agr. 33 (2) : 88-99, 1989

Characteristics of One-Pass Vertical Friction-Type Rice Whitener (I)

•\ Effect of configuration of whitening screen•\

Danilo O. VARGAS*, Takaakl SATAKE**,

Sumihiko MIYAHARA*** and Shlgeru YOSHIZAKI**

*Doctoral Program in Agricultural Sciences, Graduate School at University of Tsukuba

**Institute of Agricultural and Forest Engineering , University of Tsukuba, Tsukuba City, Ibaraki Prefecture 305***Japan Bio-Oriented Technology Research Advancement Institute , Omiya City, Saitama Prefecture 330

Abstract

One-pass vertical friction-type rice whitener was evaluated by introducing a whitening index defined

as the ratio of the vertical resistance exerted by the whitener and the rice flow rate . As a result, there existed a curvilinear relationship between the flow rate and the vertical resistance.

The configuration of the whitening screen did not affect much the characteristics of this whitener

although the hexagonal screen exhibited a relatively higher performance compared with the rest .Also, the angle of slit has no significant effect over the characteristics of this whitener .

Key words Head rice yield, Miling recovery, Total milling yield, Vertical friction-type rice whitener , Whitening index, Whitening screen

Introduction

Most of the researches on rice whitening con-ducted so far dealt mainly with the japonica or short-grained rice varieties. Matsuda8) studied the whitening characteristics and the opera-tional conditions involved in using a horizontal rice whitener. Kawamura5) conducted a basic study on the elastic behavior of whitening rice

grain and the whitening pressure developed in a horizontal rice whitener.

Also, Namikawa2) performed an analytical and experimental study on the basic action of the abrasive-type whitening mechanism. Moreover, studies on the whitening character-istics and the properties of brown rice, particu-larly the japonica rice varieties, were reported by Kawamura et al.6), and others.

There are comparatively few studies con-ducted in Japan concerning whitening of indica or long-grained rice varieties. The re-sults of the studies on the relationship between the whitening characteristics and the shape of milled rice by using two types of whiteners were already reported7). Morishima et al. 11)

studied an impact-type of whitening. Also,

Yoshizaki et al. 17) evaluated the properties of

rice bran produced during the whitening pro-

cess. In addition, they studied the losses of rice

encountered during the said whitening

process18).

Sarker and Miyahara14) jointly studied the

breakage characteristics and bran particle dis-

tribution of selected rice varieties during the

whitening process using a horizontal friction-

type whitener.

Researches1•`4) conducted outside Japan inso-

far include applied and fundamental studies,

i. e., studies on the improvement of the En-

gelberg rice mill, breakage of rice grains and its

effect on the whitening characteristics, and

others.

The introduction of the one-pass vertical

friction-type rice whitener was focused in

answering the problem of high breakage in

whitening of indica varieties. Due to the rad-

ially uniform internal pressure and gravitati-

onal flow inside the whitening chamber, it is

expected that the percentage of broken rice

could be decreased by avoiding the excessive

internal pressure developed in the whitening

chamber. However, few data on the operationReceived 26 August, 1988

89VARGAS et al.: Characteristics of One-Pass Vertical Friction-Type Rice Whitener (I)

Table 1 Physical properties of brown rice used

Notes: *Mean triaxial diameter= (Length+Width+Thickness)/3**Koshihikari

***Nipponbare

Fig. 1 Schematic diagram and specification of

the rice whitener Note: Dimensions are

in millimeter

of this type of whitener are available.

This study was carried out to obtain basic

data relating to the rice whitening process

which will become design criteria of this new

type of whitener.

Materials and Methods

1. Materials Used For experiment regarding the shape of whit-

ening screen, CH-45, an indica variety, grown in Agricultural and Forestry Research Center at University of Tsukuba in 1984, was utilized. For experiment regarding the perforation or slit of whitening screen, additional two japoni-ca rice varieties, Koshihikari and Nipponbare,

were utilized for comparative purposes. Both

japonica varieties were grown at Ibaraki Pre-fecture in 1984 and 1985, respectively.

Table 1 shows the physical properties of the brown rice used in this experiment.

2. Equipment Used1) Rice whitenerOne-pass vertial friction-type rice whitener

manufactured by Yamamoto Manufacturing Company (VP-150) was used throughout the experiment, and will be hereafter referred to as VP-150. The schematic diagram of this white-ner is shown in Fig. 1. This is a 1.5 kW rice whitener with a 3-phase 200V and 50/60 Hz standard power source. Running at a main shaft revolution of approximately 700 rpm, this whitener has a hopper holding capacity of about 30 kg with a throughput capacity of about 120 kg/h (brown rice).

The whitening operation starts when brown rice is placed in the intake hopper. The flow of brown rice from the intake hopper is controlled by an adjustable gate valve which allows the discharge of brown rice to the whitening cham-ber. The passage of brown rice from the intake hopper to the whitening chamber is facilitated by a pitch screw conveyor connected to the cast-iron roll or the whitening roll.

The whitening chamber is the enclosed space formed by the whitening screen and the white-ning roll.The whitening screen is made up of 3 separate triangular perforated hardened steel

plates which are clamped at both ends. Due to the presence of the vertical iron supports on these plates, the screen that is formed is a nonagon with unequal sides. The upper por-tion of the screen set up is linked to a spring system which makes it possible to be mobile.

90 Japan. J. Trop. Agr. 33(2) 1989

For brevity, this type of screen will be hereaf-ter referred to as the existing or nonagonal screen.

The whitening roll is made up of a cylindri-cal cast-iron roll which is tightly fitted to the main shaft. The whitening roll has a helical

protrusion with an opening at the side to permit air to pass through the whitening cham-ber for cooling purposes.

Brown rice upon reaching the whitening chamber moves between the cylindrical cast-iron roll and the movable perforated screen vertically. Bran removal is accomplished by shearing action between the screen wall and the grains, and the friction created by the sur-face contact among the grains. The amount of bran removed from brown rice during whiten-ing depends on the force developed inside the whitening chamber. The force is controlled through the springs and metallic bars which are attached to the movable whitening screen.

There are 3 springs and 3 metallic bars which are situated equiangularly within the periphery of the upper portion of the whiten-ing chamber. The linkage system of this spring-metallic bar is such that one end of the spring is linked to one end of the metallic bar, and this is joined to the movable whitening screen by means of pins. The remaining end of spring is connected to a geared round metal

plate movable at desired adjustment. The remaining end of the metallic bar, however, is fixed at the upper part of the cylindrical cast-iron wall which covers the feed screw. Thus, the nature of configuration of the pins of spring, metallic bar, and movable whitening screen can be viewed as a 3-member truss system. During operation, the whitening screen provides resistance to brown rice being whitened, and moves in somewhat helical downward direction. But this downward dis-

placement is controlled by the spring elonga-tion. Depending on the spring adjustment level, the amount and quality of whitened rice desired could be obtained.

Whitened rice passing through the whiten-ing chamber are discharged at the outlet located below directly to the collecting bin. Under the experimental set up adopted for this study, except for the use of different types of whitening screen, no modification for any

mechanical elements of this whitener was car-

ried out.

2) Whitening screens used

Three shapes of whitening screen, namely;

hexagonal, cylindrical, and nonagonal were

used throughout the experiment.

The hexagonal screen was varied according

to the perforation or slit categorized depending

on their slit angle orientation, namely; -45•‹,

+30•‹, +45•‹, and +60•‹. Fig. 2 shows the

schematic illustration of the screens used

during the experiments.

3. Methods1) Preparation of test samples

Dehusking of the paddy was performed by utilizing an Otake Mini-Top Dehusker. One-

pass dehusking operation was done for all paddy. Unhusked paddy produced during this operation were separated from the brown rice by means of a 4-mm (U 4.0) cylindrical length

grader. Immatures were removed by means of a thickness grader and sieves.

All the thoroughly-cleaned, one-pass, whole brown rice were weighed and separated into a 1 to 3-kilogram samples. The physical prop-erties of brown rice shown in Table 1 were determined by using standard measuring de-vices.

2) Determination of the vertical resistance exerted by the whitener and the down-ward force exerted by the rice kernels

By the nature of configuration of the VP-150 as described earlier, let the free-body diagram shown in Fig. 3 represent a segment of spring-bar-screen attachments. A represents the pin

joint between the spring and the geared round metal plate, B the pin joint between the metal-lic bar and cast-iron cylindrical wall, and C the

pin joint between the metallic bar, the spring, and the movable whitening screen.

After the initial adjustment of the distance between A and B by setting the whitener at a desired level, the movable whitening screen will be displaced from the original ho to the final displacement h during whitening opera-tion. Simultaneously, the initial length of spring x0 will be elongated to length x. Howev-er, the length of the metallic bar l will be constant since its motion is restricted curvili-nearly.

91VARCAS et al.: Characteristics of One-Pass Vertical Friction-Type Rice Whitener (I)

Fig.2 Schematic illustration of the screens

a0, a and ƒÀ0, ƒÀ represent the initial and final

inclusive angles of the pin joints A and B,

respectively, during whitening operation.

The forces acting will be R1, R2, FAC, FBC, and

V. R1 and R2 are forces due to the reactions at

A and B. Since the linkage acts as a truss and

there are no other outside forces acting, R1 and

R2 will be acting respective to their individual

direction.

FAC is the force acting on the spring while FBC

92 Japan. J. Trop. Agr. 33(2) 1989

Fig. 3 The mechanical balance of forces of the

spring-bar-screen attachment under operating condition

is the force acting on the metallic bar. V is the

vertical force which acts on the rice grain

inside the whitening chamber equal to the ver-

tical resisting force exerted by the spring. This

force is also equal to the downward force ex-

erted by the rice kernels inside the whitening

chamber.

The vertical force V exerted by a spring can

be easily calculated by applying the cosine and

sine law and the geometric relation of the truss,

that is

V=•¬ (1)

where

x=•¬(2)

and E is a spring constant. The displacement h was measured by attach-

ing a metal plate at the side of the whitening chamber connected to a dial gage (Kyowa Elec-tronic Instruments Co., Ltd.: Type DT-10D). The dial gage was linked to a strain amplifier

(Kyowa Electronic Instruments Co., Ltd.: Type DPM-6H) directly connected to a multi-pen re-corder (Yokogawa Electric Works, Ltd.: Type 3061). The change in length occurring during the whitening operations was recorded by the multi-pen recorder.

3) Whitening experimental test conditions and procedures

The experimental conditions were based on the type of whitening screen, type of rice vari-ety, and the level of adjustment of the rice whitener. The level of adjustment of rice whitener corresponds to a specific setting of the spring within the whitener. In Fig. 3, this corresponds also to L measurements, and dep-ending on the desired level of adjustment, L

Fig. 4 (a) Cross-section of the rice particle stack

showing the real and imaginary broken surface (b) Rice particle showing physi-

cal dimensions

values varied. The initial value of L at pre-whitening operations was 106 mm with ho and l equal to 57.6 and 81.2 mm, respectively. From these values, xo was determined.

At operating conditions, there were 3 level of adjustments selected for this study with L values of 107.6 mm, 115.6 mm, and 119.6 mm, respectively. These levels of adjustment were assumed to correspond with the low, medium, and high level of whitening based on pre-test values. During operation and at each test con-dition, the recorded h value and other whiten-ing data were obtained.

Also, from the results of preliminary tests, the flow rate of brown rice was controlled at one-half of the hopper gate opening (based on machine characteristics) which showed stable conditions (less breakage and overmilling) during operations and was maintained con-stantly throughout the experiments. The rice whitener when operated at full hopper gate opening resulted in rather high breakage and overmilling which made it very difficult to clarify the effect of screens and the angles of slit.

The whitening experimental procedure ad-opted is as follows: The brown rice test run sample was placed in the hopper of the rice whitener and the measurement of brown rice


temperature was done by placing the thermo-couple at the bulk of brown rice. The test condition was set up after which the test run was conducted. At specified interval, 10 or 11

seconds, a whitened rice sample was obtained

approximately 1 or 2 minutes after the initial discharge of whitened rice. The sampling time was recorded on the basis of the value regis-tered on the timer when the sampling was

simultaneously obtained. The whitened rice sample was coded by the appropriate test run

code. After whitening, the temperature of the whitened rice was immediately measured by again placing the thermocouple at the bulk of

whitened rice, and the weight after whitening determined. Thereafter, the rice whitener was

prepared for the successive test runs in repeat-ing the same procedures. For the entire exper-

iments, a total of 54 test runs was conducted.4) Whitening index of the rice whitenerFor simplicity of calculation, assume the rice

particles being whitened inside the whitening chamber as homogeneous spheres each having a volume of v, density of p5, and diameter of dp.

Further, assume that the particle stack inside the whitening chamber has a homogeneous

void ratio E.Considering an imaginary broken surface at

any cross-section inside the whitening cham-ber represented by dash line in Fig. 4a, the

mean cut area of the cut sphere Sp can be calculated by the following equation13):

Sp=(1/dp)•¬f(z)dz= ƒÎdp2/6 (3)

Since the void ratio E is statistically equiva-lent to the void ratio at the section of the

broken surface, the number of particles n f per unit area of the imaginary area is given by eq.

(4).nf=(1-ƒÃ)/(ƒÎdp2/6) (4)

In the case of particles in contact with the

screen wall, the situation of the rice particles

could be viewed as those shown in Fig. 4a

except that the dash line represent the screen

wall. At any condition, the rice particles po-

sitioned outside the imaginary broken surface

will not exist. The choice of the side being

taken to be arbitrary. Therefore, the number of

rice particles per unit area of the screen wall nW

would probably be reduced by half its original

number, that is

nw=3(1-ƒÃ)/(ƒÎdp2) (5)

Considering the vertical resistance fw acted by the single particle in contact with the screen

wall, the following equation is derived.

fw=V/(2ƒÎRLsnw)

=Vdp2/[6RLs(1-ƒÃ)] (6)

where R is the hydraulic radius of the screen,

and L5 is the length of the screen.

The rice particle flow rate Gs is given by eq.

(7),

Gs=ƒÎR2 us(1-ƒÃ)ƒÏ5 (7)

where us is the mean particle velocity inside

the whitening chamber.

Combining eq. (6) with eq. (7), we get

fw=(ƒÎdp2VusƒÏsR)/(6LsGs) (8)

Since the particle residence time T inside the

whitening chamber is expressed as

T=Ls/us (9)

combining eq. (8) with eq. (9) will give a

physical quantity referred to as the whitening

index Iw, defined by:

Iw=V/Gs=kfwT (10)

where k is a constant equal to 6/(ƒÎRƒÏsdp2).

Eq. (10) indicates that the ratio of the vertical

resistance and the particle flow rate will be

proportional to the shearing force exerted by a

single particle in contact with the screen wall

and the particle residence time inside the

whitening chamber.

5) Analysis of milled rice samples

Using a 3-mm (U 3.0) rotary cylindrical

length grader, the head rice and brokens of

the milled rice sample were separated. The

weights were measured, respectively.

The whiteness degree was measured using a

Kett C-3 whiteness meter. Five hundred head

rice grain from each milled rice sample were

collected and their weights determined.

To verify whether the whitened is fully

milled or not, the milling degree analysis was

performed using a freshly prepared New M. G.

Solution in accordance with the JFA. The

other whitening parameters were calculated by

using the following equations:

Total Milling Yield ƒÅt (%)

=[(Tot . Wt. of Milled Rice after Whiten-

ing)/(Total Weight of Brown Rice

Used)]•~100 (11)

Milling Recovery ƒÅm (%)

=[(Weight of 500-grains of Head Rice)/

(Weight of 500-grains of Brown Rice)]

94 Japan. J. Trop. Agr. 33(2) 1989

•~100 (12)

Head rice Yield ƒÅh (%)

=([Total Weight of Head Rice Recover-

ed)/(Total Weight of Milled Rice

Sample)]•~ 100 (13)

Eqs. (11), (12) and (13) are related to each

other by the following equation.ƒÅ

m=ƒÅt•~ƒÅh

The definition of brokens was based on the

length of the rice particle and is expressed to in

1/8 t h units of the length of the whole un-

broken milled rice grain16). The percentage of

brokens was determined as the difference be-

tween the percentage of whole unbroken

milled rice grain and the head rice yield. How-

ever, no attempt was made to segregate the

small, medium and large brokens. The head

rice was defined as 75% or greater than the

whole unbroken milled rice grain. The temper-

ature rise was obtained by the temperature

difference between the brown rice and milled

rice measured by C-A thermocouple (Yokog-

awa Electric Works, Ltd.: Model 2541).

The flow rate of the milled rice was deter-

mined by dividing the total weight of the

milled rice collected during the steady state

flow by the total collecting time.

It must be noted, however, that the data

obtained from this study are data from one-

pass whitening operation.

Results and Discussion

1. Vertical Resistance V, and Flow Rate Gs, and

Whitening Index Iw

Results from the experiments indicated dis-

placement h values ranging from 61.85 mm to

63.64 mm for all levels of adjustment and for

all types of whitening screen. The vertical

resisting force exerted by the rice whitener or

the downward force exerted by the rice kernels

determined from these values ranged from

114.15 N to 308.91 N.

The flow rate of whitened rice obtained from

the different samples ranged from 51.27 kg/h

to 69.00 kg/h for CH-45, 65.57 kg/h to 93.69

kg/h for Nipponbare, and 73.55 kg/h to 89.53

kg/h for Koshihikari.

Fig. 5 shows a typical example of the rela-

tionship between the particle flow rate based

on the whitened rice GS and the vertical resist-

ance V exerted by the whitener. Regardless of

Fig. 5 Relationship between the particle flow

rate and the vertical resistance

the shape of the whitening screen, the angle of slit, and the rice variety used, GS and V is correlated with each other in the following equations.

V=Vmax-aG2s (15)Gs=a'(Vmax- V)1/2

=a'V 1/2max(1-V/Vmax)1/2 (16)where a and a' are constants, and where a'=1/ a1/2. Vmax is the maximum vertical resistance at which the rice particles cannot flow through the whitening chamber. The value of a' for CH-45 ranged from 2.23 to 2.43 while for Kos-hihikari and Nipponbare, the value of a' ranged from 2.75 to 3.35. The above relation-ship is similar to the empirical formula of par-ticle flow rate charging into a pressurized bin9).

Expanding eq. (16) by using Taylor's series, and taking the first term because V/Vmax<<1, we get the following expression.

Gs-a'Vm/1/2max[(1/2)(V/Vmax)] (17)Assuming the bran removed from the whit-

ening chamber negligible and combining eq. (17) with eq. (10), we get an approximated ex-pression as

Gs=(2a'Vmax)/(a'Iw+2V1/2max) (18)Eq. (18) indicates that GS decreases with an


Fig. 6 Relationship of the whiteness degree and

milling degree

increase of IW. The values of whitening index

obtained from the experiments ranged from

1.65 N•Eh/kg to 5.78 N•Eh/kg for CH-45, 1.26 N•E

h/kg to 4.54 N•Eh/kg for Nipponbare, and 1.34

N•Eh/kg to 3.95 N•Eh/kg for Koshihikari. The

calculated values of whitening index as were

determined from the experiments suggest that

whitening index is an indicator of the charac-

teristics of rice whitener that could be consid-

ered during the evaluation of its whitening

characteristics.

Although the unit of whitening index could

be interpreted as the unit of impulse, the phys-

ical meaning attached to the whitening index

makes it different from impulse. Whitening

index, as suggested from eq. (10), represents

the amount of force per unit mass of rice ker-

nels exerted by them at certain time interval

during the instantaneous whitening of the rice

kernels.

Since whitening index is a physical quantity

derived during the whitening process indica-

tive of the characteristics of whitening, this

value could probably be considered as one of

the whitening parameters. Thus, the following

discussions show the behavior of whitening

index related to other whitening parameters.

2. Whiteness Degree and Milling Degree

Results of whiteness degree obtained from

the experiments under all levels of adjustment

and all types of whitening screen ranged from

32.6 to 44.5 for CH-45, 34.5 to 43.5 for Nip-

ponbare, and 28.7 to 41.5 for Koshihikari. At

these whiteness degree ranges, the milling

degree obtained under all levels of adjustment

and all types of whitening screen varied from

80-100% for all types of rice variety. Fig. 6

shows an example of the relationship between

whiteness degree and milling degree.

In order to provide a basis for comparison for

all types of whitening screen utilizing the

values of whitening index, it is necessary to

introduce and define the term critical whiten-

ing index Iwcrit. The critical whitening index is

the minimum value of whitening index with a

milling degree of 100% and whiteness degree

of 40. The whiteness degree of 40 was selected

because the samples tended to obtain 100%

milling degree when they had approximately a

whiteness degree of 40. The value of whiteness

degree equal to 40 reaching 100% milling

degree is consistent with other research

findings4,10) concerning the relationship be-

tween whiteness and milling degree.

The effect of the whitening index on white-

ness degree wd by using different angles of slits

is shown as an example in Fig. 7.

The general relationship between them is

expressed as

wd=fI1/2w+c (19)

where c is the whiteness degree of the brown

rice according to the rice variety to be white-

ned, and f is a constant depending on the con-

figuration of the screen used and the rice vari-

ety whitened. From the result of experiment,

the value of c chosen was 20. The value of f

ranged from 7.69 to 11.10.

Considering Iwcrit, the hexagonal screen ex-

hibited the lowest with 3.43 N•Eh/kg, followed

by the cylindrical with 4.71 N•Eh/kg, and lastly

by the nonagonal with 5.59 N•Eh/kg.

For the screen with different angle of slit,

similar tendency was exhibited. However, the

difference was very small. Irrespective of the

rice variety whitened, the angle of +45•‹

showed the lowest value of IWcr;t .

3. Total Milling Yield

Under this study, the results of the total

milling yield for all levels of adjustment and all

types of whitening screen varied from 73.96%

to 91.56% for CH-45, 80.50% to 93.50% for

Nipponbare, and 82.80% to 95.67% for Koshi-

hikari. To obtain the relationship between

total milling yield ƒÅt and Iw the corresponding

values were plotted as shown in Fig. 8. Fig. 8 is

an example of this relationship, and this rela-

96 Japan. J. Trop. Agr. 33(2) 1989

Fig. 7 Relationship of the whitening index and whiteness degree

tionship could be generally expressed asƒÅt

-100e-ƒÓIw (20)

where ƒÓ is a constant depending on the config-

uration of the whitening screen and the rice

variety used. The value of cb was between

0.017 and 0.026 for CH-45, and between 0.011

and 0.021 for both Koshihikari and Nipponb-

are. AtIwcrit, the ƒÅt value ranged from 75.5% to

88.6%.

4. Head Rice Yield

The values of head rice yield for all levels of

adjustment and all types of whitening screen

which varied from 60.54% to 93.64% for CH-

45, 96.46% to 99.69% for Nipponbare, and

97.48% to 99.85% for Koshihikari. The per-

centage of brokens could be obtained from the

subtraction of the value of the head rice yield

from 100%. It must be noted, however, that

since no size classification was done, the

brokens referred to in this study consisted of a

mixture of small, medium and big brokens.

An example of the relationship between the

head rice yield 1h and 1W is shown in Fig. 9. The

general relationship between them is ex-

pressed as

ƒÅh=100e-ƒÓIw (21)

where ƒÓ is a constant depending on the rice

variety whitened and the screen configuration.

For CH-45, the value of ƒÓ ranged from 0.019 to

0.032, while for both Koshihikari and Nip-

ponbare, the value of ƒÓ ranged from 0.002 to

0.003. ƒÅh depends much on the type of rice

variety whitened and the shape of whitening

screen used.

Fig. 8 Relationship of the whitening index and

total milling yield


head rice yield

At the value of Iwcrit, the highest value of ƒÅh

was obtained by the hexagonal screen, fol-

lowed by the cylindrical, and lastly by the

nonagonal. The range of values obtained was

between 76 and 80%.

Increasing the value of IW did not affect much

the value of ƒÅh for Koshihikari and Nipponbare,

but for CH-45, there was a sharp drop of the

value of ƒÅh as the value of Iw increased. Using

screen with different angle of slit had very

small variations for all varieties as the value of

Iw increased.

5. Milling Recovery

The milling recovery indicates the extent of

bran removal. The calculated values of milling

recovery obtained from the experiments for all

levels of adjustment and all types of whitening

screen varied from 83.73% to 91.82% for CH-

45, 78.20% to 88.87% for Nipponbare, and

83.75% to 92.54% for Koshihikari. The gener-

al relationship between the milling recovery ƒÅm

and Iw is expressed asƒÅ

m=100 e-ƒÁIw (22)



milling recovery

where y is a constant depending on the rice

variety whitened and the screen configuration,

and has a value ranging from 0.012 to 0.025.

As shown typically in Fig. 10, ƒÅm has very

slight variation at Iwcrit value for all types of

screen. The range of values obtained was be-

tween 86 and 88%, which is 4 to 6% lower than

reported values14).

6. Temperature Rise

The temperature rise recorded from this

study under the different levels of adjustment

and all types of whitening screen varied from

13.1•Ž to 29.2•Ž for CH-45, 13.5•Ž to 29.3•Ž for

Nipponbare, and 11.5•Ž to 28.3•Ž for Koshihi-

kari.

Fig. 11 shows the relationship between the

temperature rise ‡™ƒÑ and Iw. The general rela-

tionship between them is expressed as‡™ƒÑ

=j•EIw1/2 (23)

where j is a constant depending on the rice

variety whitened and the screen configuration.

The value of j for CH-45 was between 10.8 and

12.2, while for Koshihikari and Nipponbare, the

value was between 13.2 and 14.6.

At the value of Iwcrit, the temperature rise for

all types of screen has fallen within a range of

22-25•Ž. These values coincided with the data

obtained by Miyahara et al.10), but, approxi-

mately 10•Ž higher than the values reported in

•g Satake Technical News Report•h15). This differ-

ence could probably be attributed to a differ-

ence in operating conditions. Under this study,

the temperature values were obtained from

one-pass whitening operation while those of

reported from •gSatake Technical News Report•hFig. 11 Relationship of the whitening index and temperature rise

eration. In multi-pass whitening operation, the

process of whitening rice kernels is usually

conducted gradually and repeatedly which

would probably attain lower values of temper-

ature rise.

Conclusion

The whitening characteristics of one-pass

vertical friction-type rice whitener was ex-

plained through the introduction of a whiten-

ing index and relating this to some of the

whitening parameters.

1. Using different shapes of whitening

screen has not significantly affected the overall

characteristics of the rice whitener although

the hexagonal screen seemed to have exhibited

higher performance over the others. This is

particularly evident on the CH-45 rather than

the Koshihikari and Nipponbare especially if

the head rice yield would be considered.

2. The utilization of different types of per-

foration on the whitening chamber has not

affected the characteristics of this type of

whitener regardless of the rice variety used for

whitening.

3. There was an indication that the flow

rate which was correlated with the vertical

resistance was similar in tendency as that of

particle flow rate charged in a pressurized bin,

and that it decreases with an increase of whit-

ening index.

4. The whitening index as a function of the

different whitening parameters was affected

by the configuration of the whitening screen

and the type of rice variety used for whitening.

The study involved mainly that of vertical

friction-type rice whitener. It is suggested that

future studies should include a thorough inves-were obtained from a multi-pass whiktening op-

98 Japan. J. Trop. Agr. 33(2) 1989

tigation of whitening index as applied to hori-zontal friction-type rice whiteners. Also, it is suggested that an evaluation of the whitening index using rice whiteners with a higher power

requirements (over 1.5 kW) should be con-ducted.

List of Symbols

A pin joint between the spring and the geared round metal plate [-]

a constant defined by eq. (15)a' constant defined by eq. (16)B pin joint between the metallic bar and

cast-iron cylindrical wall [-]C pin joint between the metallic bar, the

spring, and the movable whitening screen [-]

C whiteness degree of brown rice [-]dp diameter of rice particle [mm2]E spring constant [4.21196 N/mm]FAC force acting on the spring [N]FBC force acting on the metallic bar [N]

f constant defined by eq. (19)fW vertical resistance acted by a single

particle in contact with the screen wall [N]

G5 particle flow rate or the flow rate of

milled rice [kg/h]

h final displacement [mm]

h o original or initial displacement [mm]Iw

whitening index [N• h/kg]

Iwcrit critical whitening index [N• h/kg]

j constant defined by eq. (23)

k constant equal to 6/ƒÎRƒÏdp2 [kg-1]

L length between A and B [mm]

Ls length of the screen [mm]

l length of metallic bar [mm]

of number of particles per unit area of

the imaginary area

[no. of particles/mm2]

nw number of particles per unit area of

the screen wall [no. of particles/mm2]

R hydraulic radius of the screen [mm]

R1 force due to the reaction at A [N]

R2 force due to the reaction at B [N]

Sp mean cut area of the cut sphere [mm2]

T particle residence time [s]

us mean particle velocity inside the

whitening chamber [mm/s]

V vertical force which acts on the rice

grain inside the whitening chamber

equal to the vertical resisting force ex-

erted by the spring. This force is also equal to the downward force exerted by the rice kernels [N]

Vmax maximum vertical resistance [N]

v volume of rice particle [mm3]

wd whiteness degree [-]

x elongated length of spring [mm]

xo initial length of spring [mm]

z distance from the center of the sphere

to the cut area [mm]

Greek Symbols

a final inclusive angle at pin joint A

[deg.]

a0 initial inclusive angle at pin joint A

[deg.]ƒÀ

final inclusive angle at pin joint B

[deg.]ƒÀ

initial inclusive angle at pin joint B

[deg.]ƒÁ

constant defined by eq. (22)•¢ƒÑ

temperature rise [•Ž]ƒÃ

void ratio of rice kernels [-]

ƒÅh head rice yield [%]ƒÅ

m milling recovery [%]ƒÅ

t total milling yield [%]ƒÏ

5 density of rice particle [kg/mm3]ƒÓ

constant defined by eq. (20)ƒÓ

constant defined by eq. (21)

References

1. ARBOLEDA, J.R. 1975 Improvement of the Kis-kisan Rice Mill. Agric. Eng. Dept. Saturday

Seminar IRRI2. AUTREY, H. A. et al. 1955 Effect of Milling Con-

ditions on Breakage of Rice Grains. Journal of Agric. and Food Chem. 3(7): 593-599

3. BHATTACHARAYA, K. R. 1969 Breakage of Rice during Milling and Effect of Parboiling. Cereal Chemistry 46(5): 478-485

4. CHUNG, C. J. et al. 1983 Various Designs of the Perforated Screen Affecting the Performance of a Rice Whitening Machine. Final Report of

Post-production Rice Systems (Korea) Phase III Project: 187-212

5. KAWAMURA, N. 1951 Study on the Milling op-erations. Journal of the Jpn. Soc. Agric. Mach. 12(3,4): 43-51

6. KAWAMURA, S. et al. 1979 Studies on the Rice Milling (Parts 2, 3). The Summary of the 38th Annual Meeting of the Japanese Society of

Agricultural Machinery: p. 141

99VARCAS et al.: Characteristics of One-Pass Vertical Friction-Type Rice Whitener (I)

7. Laboratory of Postharvest Technology, Facul-

ty of Agriculture, Univ. of Kyoto 1979 Studies

on the Drying, Threshing and Milling Charac-

teristics of Indica Type Rice. Report of the

Lab. of Postharvest Tech., Univ. of Kyoto, No.

5402

8. MATSUDA, R. 1961 Experimental Studies on the

Milling Characteristics of the Rice Mill, Ph. D.

Dissertation, University of Kyoto.

9. Members of the Grains Conveyance Equip-

ment Committee 1976 Storage and Convey-

ance Equipment of Grains. 78-82. Nikkan

Kougyou Shinbunsha Tokyo. (Japanese ver-

sion)

10. MIYAHARA, S. 1984 Related Fundamental

Researches on the Milling Characteristics of

Rice, Ph. D. Dissertation, University of Tsu-

kuba: 104-109 (Japanese version)

11. MORISHIMA, H. et al. 1980 Several Tests on

Milling Characteristics of Indica and Japonica

Type Rice Grain. The Summary of the 38th

Annual Meeting of the Japanese Society of

Agricultural Machinery: p. 26

12. NAMIKAWA, K. 1959 Studies on the Rice Pearl-

ing Mill with Grinding System. Journal of the

Jpn. Soc. Agric. Mach. 21(2): 65-6913. RUMPF, H. 1962 Agglomeration. Edited by

Knepper, W. A., Interscience Publications: 379-418

14. SARKER, N. N. and S. MIYAHARA 1984 Breakage Characteristics and Bran Particle Distribution

of Selected Rice Varieties during the Whiten-ing Process Using Friction Roll Type Machine.Report of Special Research Project on Tropical Agricultural Resources 3: 157-168

15. Satake Technical News. Satake Engineering Co., Ltd., Tokyo (No. 15, 20, 21)

16. VAN RUITEN, H. Th. L. 1979 Physical Proper-ties of Paddy and Milled Rice. Grain Post-harvest Processing Technology, Publication of

Pustaka IPB: 3-1217. YOSHIZAKI, S. et al. 1982 Some Properties of

Rice Bran Produced during the Whitening Process. Report of Special Research Project on Tropical Agricultural Resources 1: 101-108

18. YOSHIZAKI, S., S. MIYAHARA and N. N. SARKER 1983 Losses of Rice during Whitening Process. Report of Special Research Project on Tropical

Agricultural Resources 2: 121-132

摘要

ワンパスたて型摩擦式精米機の特性

第1報精白スクリーン形状の影響

ダニロO.バルガス*,佐竹隆顕**,宮原佳彦***,吉崎繁**

*筑波大学大学院農学研究科305茨城県つくば市

**筑波大学農林工学系

***生物系特定産業技術研究推進機構330埼玉県大宮市

長粒型米の精白において砕米発生が比較的少ないとい

われるワンパスたて型摩擦式精米機について,精白主軸

方向の抵抗力と穀粒流量の比として定義される精白指標

を導入することにより,精白特性の検討を行った.その

結果,両者の関係は非線形であることが明らかとなっ

た.

精白スクリーンの形状は精米機の特性に大きな影響を

与えないものの,断面形状が6角形であるスクリーンは

他のスクリ-ンに比べて良好な精白特性を示した.また

精米機の特性に対するスクリーンスリット角の大きな影

響も認められなかった.

キーワード真精白歩留,精白指標,精白スクリーン,

精白歩留,たて型摩擦式精米機,非砕米歩

合

1988年8月26日受理熱帯農業33(2)88―99,1989

Documents

Characteristics of One-Pass Vertical Friction-Type Rice