COMPARISON OF DIFFERENT RICE MILLING METHODS.pdf

7/28/2019 COMPARISON OF DIFFERENT RICE MILLING METHODS.pdf

1/17

COMPARISON OF DIFFERENT RICE MILLING METHODS

by

Sadegh Afzalinia, Ph. D. Student1Mohammad Shaker, Research Engineer2

Ebrahim Zare, Research Engineer2

1 Department of Agricultural & Bioresource Engineering,University of Saskatchewan, SK, Canada S7N 5A9

2 Agricultural Research Center of Fars provinceZarghan, Shiraz, Iran

P. O. Box: 73415-111

Written for presentation at the

2002 ASAE/CSAE North-Central Intersectional Meeting

Sponsored by ASAE and CSAE

Parktown Hotel

Saskatoon, Saskatchewan, CANADA

September 27-28, 2002

Summary:This study was carried out to determine the best rice milling method in the Fars province of Iran.

First, the effect of paddy moisture content on milled rice breakage was evaluated. Then four differentmilling systems were compared and finally economic evaluation was done to justify the economicperformance of the selected method. Results of this study showed that the optimum paddy moisture contentfor milling process was 12 to 14% wet basis (wb) and using three abrasive whiteners in series and onefriction whitener as a polisher had the least rice breakage, so this method was the best choice for ricemilling operation in Fars province. Economic evaluation also approved the result of method comparison.

Keywords: rice, rice milling system, rice grain breakage, paddy moisture content

The author(s) is solely responsible for the content of this technical presentation. The technical presentation does not necessarilyreflect the official position of ASAE, and its printing and distribution does not constitute an endorsement of views which may beexpressed.

Technical presentations are not subject to the formal peer review process by ASAE editorial committees; therefore,they are not to be presented as refereed publications.

Quotation from this work should state that it is from a presentation made by (name of author) at the (listed) ASAE meeting.

EXAMPLE From Authors Last Name, Initials. Title of Presentation. Presented at the Date and Title of meeting Paper No. X,ASAE, 2950 Niles Road, St. Joseph, MI 49085-9659 USA.

For information about securing permission to reprint or reproduce a technical presentation, please address inquiries to ASAE.

ASAE, 2950 Niles Road, St. Joseph, MI 49085-9659 USAVoice: 616-429-0300 Fax: 616-429-3852 E-Mail:

Paper No: MBSK 02- 214An ASAE/CSAE Meeting

Presentation


2/17

2

COMPARISON OF DIFFERENT RICE MILLING METHODS

S. Afzalinia1, M. Shaker

2and E. Zare

2

1Department of Agricultural & Bioresource Engineering, University of

Saskatchewan, SK, Canada S7N 5A9

2Agricultural Research Center of Fars province

Zarghan, Shiraz, Iran

P. O. Box: 73415-111

ABSTRACT

This study was carried out to determine the best rice milling method in the Fars

province of Iran. First, the effect of paddy moisture content on milled rice breakage was

evaluated. Then four different milling systems were compared and finally economic

evaluation was done to justify the economic performance of the selected method. Results

of this study showed that the optimum paddy moisture content for milling process was 12

to 14% wet basis (wb) and using three abrasive whiteners in series and one friction

whitener as a polisher had the least rice breakage, so this method was the best choice for

rice milling operation in Fars province. Economic evaluation also approved the result of

method comparison.

INTRODUCTION

Rice kernel is covered by two layers. The most outer layer is called husk (hull)

and the inner one called bran. The whole rice kernel before removing these layers called


3/17

3

paddy (rough rice). Husk and bran are not eatable; therefore, they must be removed from

paddy. Husk has not tightly stuck to the kernel, so it is easily removed from the kernel.

When this layer is removed the kernel is called brown rice. Bran is more difficult to be

removed from brown rice, because it has tightly attached to the kernel. The process over

which the bran is removed from brown rice is called whitening or pearling process.

During this process rice kernels are subjected to an intensive mechanical and thermal

stresses which cause some damage to the rice kernels and break some of them. There are

some parameters such as the type of whitening machine, paddy characteristics, and

environmental factors that affect the rice kernel damage and breakage during the milling

process. In this study, effect of paddy moisture content and milling system components

on rice kernel breakage during milling process was investigated and an economic

evaluation was done to determine the most economic system for rice milling process .

LITERATURE REVIEW

Rice kernel breakage during the milling process is affected by different

parameters such as paddy harvesting conditions, paddy drying, physical properties of

paddy kernels, environmental conditions, and type and quality of milling system

components. There are many papers in this area.

Davis (1944) reported that the optimum harvesting moisture content for paddy of

Caloro variety was 20 to 24%. Pominski et al. (1961) reported that paddy moisture

content had significant effect on milling system yield so that for one percent reduction of

paddy moisture in the range of 10 to 14%, performance of milling system increased by

0.7 to 3%.


4/17

4

Matthews et al. (1970) reported that rice breakage was mostly due to mechanical

stresses rather than thermal stresses. Matthews and Spadaro (1975) evaluated the effect of

harvesting method on rice breakage during the milling process. They found that rice

breakage of samples that had been harvested by combine was 5% more than that of the

manually harvested samples. Matthews and Spadaro (1976) found that rice breakage

during the milling process increased with the decreasing kernel diameter.

Dilday (1987) reported that rice breakage during the milling process decreased

with the increasing paddy moisture content in the range of 12 to 16%. Luh (1991)

reported that to have a high quality milling process with reasonable rice breakage, paddy

must be harvested at the optimum moisture content and at the suitable stage of maturity.

Clement and Seguy (1994) found that long and tiny rice kernels were more susceptible to

breakage during the milling process.

Peuty et al. (1994) reported that paddy drying conditions affected the rice

breakage during the milling process so that rice breakage rapidly increased with the

decreasing moisture content of paddy drying air. Autrey et al. (1995) showed that

difference between paddy temperature and milling environment temperature decreased

the performance of rice milling system. They also found that relative humidity of milling

environment had significant effect on milling system yield.

MATERIALS AND METHODS

Paddy (rough rice) must be milled after harvesting and drying. In milling process

uneatable hulls and bran are removed from paddy and white rice is produced. In general,

rice milling process consists of five main operations:


5/17

5

Cleaning

When paddy comes to the milling system it may contain some foreign materials

such as stones, stalk, dust, soil particles, and weed seeds; therefore, it is necessary to pass

the paddy though a cleaning system. This cleaning system can be a simple sieve or a

progressive system.

Shelling

In this stage, the most outer rough shell of paddy is removed. Rubber roll sheller

(Fig. 1) is the most common machine that is used for paddy shelling, however friction

type whitener is sometimes used as a sheller. Paddy goes between two rubber rollers that

are rotating in opposite direction with different velocities. There is a small clearance

between the rollers so that when paddy passes through, it is subjected to some shear

forces and husk is removed from it.

Figure 1. Rubber roll sheller (Luh 1991)


6/17

6

Whitening

In this process, bran- a tightly attached layer to the brown rice- is removed from

rice kernel; therefore, in this stage the maximum mechanical and thermal stresses are

applied to the kernel and rice is subjected to the highest rate of breakage. Two most

common whitening machines are friction type whitener (Fig. 2) and abrasive type

whitener (Fig. 3). In the friction type whitener, rice grains are whitened by contacting

against each other, while in the abrasive one bran is removed by abrading the kernels

against an abrasive surface.

Figure 2. Friction type whitener (Luh 1991)


7/17

7

Figure 3. Abrasive type whitener (Luh 1991)

Polishing

Whitened rice may still have some loose bran which is removed in this process by

polisher. There is a rubber polisher that polishes the whitened rice using a rubber brush,

however friction type whitener is sometimes used as a polisher.

Grading

Rice milling process always creates some broken kernels; therefore, these broken

kernels must be separated from whole ones by using grading sieves at the end of milling

process.

This study was carried out at three different parts. At the first part, two type of

shelling machines were compared from viewpoint of shelling performance and rice


8/17

8

breakage during the shelling process. The second part was evaluation of the effect of

paddy moisture content on rice breakage during the milling process and the final part was

the comparison of four different combinations of milling machines from standpoint of

rice breakage.

Sheller comparison

Rubber roll Sheller is the most common sheller in Iran, but in some areas of

country such as north, friction type whiteners are also used as sheller because of high

variable costs of rubber sheller. For this reason, a comparison was done between rubber

roll sheller and friction whitener as sheller from viewpoint of shelling performance and

rice breakage in Gilan province. Both machines were used at the same conditions and

samples were taken from their outlet. Broken kernels were separated from whole kernels

and the percentage of rice breakage was calculated by dividing the weight of broken

kernels by the total weight of sample. Shelling performance (ratio of the weight of

shelled kernels to the total weight of sample) of each sheller was also determined. The

rice variety that was used in this research was Safidrood with 6% wb moisture content

and t test was used to analyse the data.

Effect of moisture content

This part of study was carried out in Fars province to evaluate the effect of paddy

moisture content on rice breakage during the milling process. Three levels of paddy

moisture content (8 to 10%, 10 to 12%, and 12 to 14%) were considered. The milling

system was consisted of rubber roll sheller, abrasive whitener, and friction whitener as

polisher and a local rice variety (Kamfiroozi) was used. Paddy with selected moisture

content was entered to the milling system and samples were taken from outlet of each


9/17

9

machine to measure the percentage of rice breakage. Split plot design with two factors

(machine type and paddy moisture content) and five replications was used to analyze the

data.

Milling methods

Four different combinations of whitener and polishers in the milling system were

compared from viewpoint of rice breakage, whitened rice appearance, and economic

aspect in Fars province. Considered combinations were as follow:

a. Three abrasive type whiteners in series and rubber roll polisherb. Three abrasive type whiteners in series and friction type whitener as a polisherc. Two friction type whiteners in series without polisherd. Four abrasive type whiteners in series without polisher

The same sheller, paddy separator, and cleaning and grading systems were used for all

treatments. Samples with 100 g weight were taken from system outlet and broken kernels

were separated from whole ones and finally breakage percent for each treatment was

calculated.

Appearance of the whitened rice is an important parameter in rice market value;

therefore, samples with one kilogram weight were taken to evaluate the appearance of

each treatment output. These samples were marked by nine different experts in the rice

market to compare their marketability. Assigned marks were out of 10. In the meantime,

economic evaluation was carried out to find the most economic treatment. The rice

variety that was used in this study was Kamfiroozi with 12 to 14% moisture content and

complete block design with five replications was used to analyze the data.


10/17

10

RESULTS AND DISCUSSION

Sheller comparison

Results of comparing two types of shellers showed that there was a significant

difference (p=0.99) between them from viewpoint of rice breakage so that rice breakage

resulted from friction whitener as sheller was approximately twice as high as that of the

rubber roll sheller, while its shelling performance was only 8.7% higher than that of the

rubber roll sheller (Table I). However, shelling performance of friction whitener was

higher than that of the rubber roll sheller, using it as a sheller is not recommended

because of its high percentage of rice breakage.

Effect of paddy moisture content

The results of the effect of paddy moisture content on rice breakage showed that

moisture content had a significant effect on rice breakage so that rice breakage decreased

with the increasing paddy moisture content in the tested range (Table II). The range of 12

to 14% was the optimum moisture content for paddy at the time of milling, because the

lowest rice breakage was occurred at this range.

Milling methods

The results of this part of study showed that milling method had significant effect

(P=0.99) on the rice breakage during the milling process so that the method having

friction type whitener had the highest amount of rice breakage and treatment having

abrasive whitener without polisher had the lowest amount, however there was no

difference between methods containing abrasive whitener without polisher and abrasive

whitener with friction whitener as polisher from viewpoint of rice breakage (Table III).


11/17

11

Results of rice appearance comparison showed that output of the treatment

containing three abrasive whitener in series and friction whitener as polisher had the best

appearance and marketability and obtained the highest mark (Table IV). Economic

evaluation also showed that the method containing three abrasive whitener in series and

friction whitener as polisher had the lowest cost to whiten the unit weight of paddy

(Table V), therefore it was the most economic method.

CONCLUSSIONS

The results of this study led to the following conclusions:

1. Using friction type whitener as a sheller during the rice milling process was notreasonable, because of causing high rice breakage.

2. The optimum paddy moisture content for the tested variety at the milling time was12 to 14%.

3. Method containing three abrasive type whitener in series and a friction whiteneras polisher had the lowest rice breakage, lowest milling cost, and its output had

the best appearance and marketability; therefore, it was the best rice milling

system for the tested variety and region.

ACKNOWLEDGMENTS

This project was financially supported by Iranian Scientific and Industrial

Research Council, so authors would like to acknowledge the assistance of this

council. We also appreciate the help extended by Shahid Khosrow Zaree Company.

We thank A. Bordbar, S. Zaree, and A. Ghiaci for their valuable assistance.


12/17

12

REFERENCES

Autrey, H. S., W. W. Grigorief, A. M. Altschul and J. T. Hogan. 1955. Effect of milling

conditions on breakage of rice grains. Journal of Agricultural Food Chemistry 3:593-599.

Clement, G. and Jl. Seguy. 1994. Behaviour of rice during processing. Agriculture and

Development 16:38-46.

Davis, L. L. 1944. Harvesting rice for maximum milling quality in California. Rice

Journal 47(3):3-4, 17-18.

Dilday, RH. 1987. Influence of thresher cylinder speed and grain moisture at harvest on

milling yield of rice. Arkansas Acad. Sci. 41:35-37.

Luh, B. S. 1991. Rice. I. Production, 2nd ed. New York, USA: Van Nostrand Reinhold.

Matthews, J., T. J. Abadie, H. J. Deobald and C. C. Freeman. 1970. Relation between

head rice yields and defective kernels in rough rice. Rice Journal 73(10):6-12.

Matthews, J. and J. J. Spadaro. 1975. Rice breakage during combine harvesting. Rice

Journal 78(7):59-63.

Matthews, J. and J. J. Spadaro. 1976. Breakage of long-grain rice in relation to kernel

thickness. Cereal Chemistry 53(1):13-19.

Peuty, MA., A. Themelin, C. Bonazzi, G. Arnaud, VM. Salokhe and G. Singh. 1994.

Paddy drying quality improvement by process optimization. In Proceeding I International

Agricultural Engineering Conference, 298-304. Bangkok, Thailand, 6-9 Dec.

Pominski, J., T. Wasserman, E. F. Schultz, Jr. and J. J. Spadaro. 1961. Increasing

laboratory head and total yield of rough rice by milling at low moisture levels. Rice

Journal 64(10):11-15.


13/17

13

Table I. Average rice breakage and shelling performance of two shellers

Sheller type Average rice breakage(%)

Average shelling performance(%)

Rubber roll sheller 9.3 83.7

Friction whitener as asheller

17.6 92.4


14/17

14

Table II. Average rice breakage of milling system at differentpaddy moisture contents

Paddy moisture content (% wb) Average rice breakage (%)

8-10 21.64

10-12 21.26

12-14 17.09


15/17

15

Table III. Average rice breakage of different milling methods

Milling system Average rice breakage (%)

Two friction whitener without polisher 25.15

Three abrasive whitener with rubber roll polisher 19.05

Three abrasive whitener with friction whitener as a polisher 16.9

Four abrasive whitener without polisher 16.77


16/17

16

Table IV. Marketability of whitened rice with different milling methods

Milling system Average marks (out of 10)

Three abrasive whitener with friction whitener as a polisher 9.49





17/17

17

Table V. Average rice milling cost of different milling methods

Milling system Average milling cost(CAN$/kg)

Three abrasive whitener with friction whitener as apolisher

0.014




Documents

COMPARISON OF DIFFERENT RICE MILLING METHODS.pdf