physiochemical properties and cooking quality of …
TRANSCRIPT
SD0500026
PHYSIOCHEMICAL PROPERTIES AND
COOKING QUALITY OF LONG AND SHORT
RICE (ORYZA SATIVA) GRAINS
By:
Lu6na ^aha Mohamed(Ef6ashir
B.Sc. (Agric.) Honours
University of Al-Zaiem AI-Azhari
2001
Supervisor:
(Dr. d
JL dissertation submitted to the "University of'Khartoum in
-partialfulfilment for the requirement of the degree of Waster
of Science in food science and techno fogy
Faculty of agriculture
University of Khartoum
January 2005
(Dedication
the soul of my father,
My mother who gives a Cot.
brothers and sisters,
My fiance
My friends
Jlc^nowCecCgements
Firstly I am most gratefulto jillah who assisted me health and gave me
•patience to compfete this worl^
I woufif Cif{e to express my deepest appreciation and sincere gratitude to
my supervisor (Dr. (Babikgr 'El-'Wasidi for his cCose and carefuC supervision,
endfess patience, continuous guidance and encouragement throughout the course
of this study.
Specialthan/is are due to professorflbd'Llmoneim Ibrahim 'Mustafa for
his interest, guidance and helpful comments.
jAlso 1 am indebted to 'Moidana-SheiUfi/' QLC-Sfiaaram, for his hefp during
coCCection of samples (Sudanese rice).
'My tfianl{s are aCso extended to the staff members of the department of
soiC science, facnCty of Agriculture, University ofTyhartoum.
9dy great gratitude to my friends and colleagues for their wonderful
encouragement andmoralsupport, find any one who assisted me by any means.
(Especially, those supplied chemicals or information.
Finally, my thant{s and appreciation to my family and fiance without
•whom this study would not have been completed.
List of Contents
Title Page
Dedication i
Acknowledgements ii
List of contents iii
List of tables vi
List of figures vii
List of plates viii
Abstract ix
Arabic abstract xi
CHAPTER ONE: INTRODUCTION 1
CHAPTER TWO: LITERATURE REVIEW 3
2.1 The Origin of rice 3
2.2 Rice in the Sudan 4
2.3 Importance of rice 6
2.4 Rice utilization 8
2.5 structure of rice grain 8
2.6 Rice milling 9
2.7 Rice Parboiling 13
2.8 Physical properties 15
2.8.1 Grain dimensions, weight and uniformity 15
2.8.2 Color 17
2.8.3 Density 18
2.8.4 Broken rice 18
2.8.5 Texture 21
2.9 Chemical properties of rice 21
2.9.1 Chemical composition 21
2.9.2 Nutritional value office 23
2.9.3 Gelatinization temperature (GT) 24
2.9.4 Gel consistency (GC) 24
2.9.5 Amylose and amylopectin 24
2.9.6 Starch 25
2.9.7 Starch gelatinization 26
2.10 Cooking quality 28
CHAPTER THREE: MATERIALS AND METHODS 32
3.1 Materials 32
3.1.1 Rice samples 32
3.1.2 Sample preparation 32
3.1.3 Cooking experiment 32
3.2 Methods 33
3.2.1 Physical methods 33
3.2.1.1 Grain dimensions 33
3.2.1.2 Grain shape 33
3.2.1.3 Density 33
3.2.1.4 Viscosity 34
3.2.1.5 1000 kernel weight 34
3.2.1.6 Broken ratio 35
3.2.1.7 Lovibond color system 35
3.2.2 Chemical methods 35
3.2.2.1 Moisture content 35
3.2.2.2 Ash content 35
3.2.2.3 Oil content 35
3.2.2.4 Crude fiber 35
3.2.2.5 Protein content 36
IV
3.2.2.6 Starch determination 36
3.2.2.7 Estimation of the amylose content 37
3.2.3 Cooking quality 38
3.2.3.1 Alkali - spreading value 38
3.2.3.2 Gel consistency (GC) 38
3.2.3.3 Texture of cooked rice 38
3.2.4 Statistical analysis 39
CHAPTER FOUR: RESULTS AND DISCUSSION 40
4.1 Physical properties 40
4.1.1 Dimensions and shape 40
4.1.2 Density, Viscosity, 1000 kernel weight, broken ratio 42
4.1.3 Color 45
4.2 Chemical properties 45
4.2.1 Proximate composition 45
4.2.2 Starch - amylose - amylopectin 49
4.3 Cooking quality 52
4.3.1 Alkali spreading value - gel consistency 52
4.3.2 Texture of cooked rice 54
4.4 CONCLUSIONS 56
4.5 RECOMMENDATIONS 57
References 58
Appendix 71
List of Tables
Table Page
Table (1): Rice production and area harvested in Sudan 7
Table (2): Grain size and shape of traditional U. S. rice types 19
Table (3): Chemical and physical characteristics of U. S. long,
medium, and short grain rice types 27
Table (4): Physical properties (dimensions and shape) 41
Table (5): Physical properties (density, viscosity, 1000 kernel
weight and broken ratio) 44
Table (6) Color of rice 46
Table (7): Proximate composition of long and short rice grains 48
Table (8): Starch, amylosc, amylopectin before and after
cooking 50
Table (9): Selected physicochemical properties of long and
short rice grains 53
Table (10): Sensory evaluation test for texture of cooked rice 55
VI
List of Figures
Figure Page
Figure 1. Structure of the rice grain. 10
Figure 2. Starch standard curve. 71
Figure 3. Standard calibration curve of amylose content as
determined by eoloriwetric procedure 72
vn
List of Plates
Plates Pnge
Plate 1. American rice (parboiled) before cooking 73
Plate 2. American rice (parboiled) after cooking 73
Plate 3. Pakistan rice (Basmati) before cooking 74
Plate 4. Pakistan rice (Basmati) after cooking 74
Plate 5. Thailand rice before cooking. 75
Plate 6. Thailand rice after cooking 75
Plate 7. Egyptian rice before cooking 76
Plate 8. Egyptian rice after cooking 76
Plate 9. Sudanese rice before cooking 77
Plate 10. Sudanese rice after cooking 77
V I I I
Abstract
Five rice grain samples namely long (American (Parboiled
rice), A; Pakistan, P and Thailand, T) and short (Egyptian, E and
Sudanese, S ) types were i nvestigated for their physicochemical and
cooking quality characteristics. Investigations showed that rice grain
of the two types had a length of 6.73 mm (A), 7.49 mm (P), 7 .05
mm (T), 5.46 mm (E) and 5.64 mm (S); width of 2.08 mm (A), 1.73
mm (P), 2.06 mm (T), 2.66 mm (E) and 2.73 mm (S); thickness of
1.59 mm (A), 1.50 mm (P),1.67 mm (T), 1.93 mm (12) and 1.83 mm
(S) and length / width (L / w) ratio 3.24 (A), 4.35 (P), 3.43 (T), 2.06
(E) and 2.07 (S).The L/W ratios obtained were used for determination
of grain shapes. The shapes determined were slender for the long type
samples and bold for the short type samples. Density values were 1.43
g/ml (A), 1.48 g/ml (P), 1.45 g/ml (T), 1.46 g/ml (E) and 1.46 g/ml
(S). Paste viscosity increased from out of scale (A), 12 cm (P), 1 1.5
cm (T), 5.50 cm (S) to 4.50 cm (E).l000 kernel weight values were
16.67 g (A), 14.53 g (P), 18.89 g (T), 18.47 g (E) and 18.32 (S).
Broken ratios obtained were 3.62 (A), 0.31 (P), 1.58 (T), 6.39 (!•) and
6.54 (S).
Also investigations showed that rice grains contained 8.6% -
10.9% moisture; 0.3% - 0.6%, ash; 0.22- 0.48% fiber; 6.2% - 8.0%
protein; and 0.5% - 1.0% oil.
Cooking reduced the starch percentages from 59.82% -
64.27% to the range of 43.97% -55.47% for both types.
For all types of rice amylose seems to be lower (24.00% -
31.50%) than amylopectin (31.66% -39.57%). Cooking reduced both
amylose (13.83% - 18.67%) and amylopcctin (26.30% - 34.32%)
content, however, it increased the amylopectin content ( 39.64%) o f
the Sudanese, S sample.
IX
Alkali spreading values were 3 (A), I - 2 (P), I 2 (T), 6 7
(E) and 6 - 7 for (S) and their gclatinization temperature (GT) was
classified as High - Intermediate (70 - 74°C) for A, High GT (75 -- 79o
C) for P and T, while E and S were classified as having Low GT (55o
- 69 C). Gel consistency (GC) v allies w ere 8 6.00 in m, 4 1.67 m m,
43.67 mm, 28.33 mm and 22.00 mm for A, P, T, E and S respectively.
According to their GC values the rice grain samples, investigated in
the present study could be categorized as soft (A), Medium (P, T) and
Hard (E, S). The sensory evaluation results of the texture of cooked
rice showed that the long grains were located between not sticky to
rather sticky and the short grains were very sticky. Tenderness ol long
type grains were between I lard to firm whereas short type grains were
soft. For moislness the long type lies between very dry to slightly
moist whereas the short type is located between moist to very moist. It
was observed that cooked parboiled rice is harder and less sticky than
cooked raw rice. Texture of cooked rice seems to be improved with
increasing amylase content.
X
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XII
CHAPTER ONE
INTRODUCTION
With improved health services, the world's population has
increased greatly. This motivates every nation to search lor ways and
means to increase production of food with good quality to support its
expanding population. Sudan is not exceptional in this regard.
Rice, the most productive cereal crop has been thought of as a remedy
to supplement wheat, Sorghum and millet as food crops. (Awok,
1995).
Rice (Oryza saliva L.) is one of the leading food crops of the
world and is the staple food of over approximately one halfo ft he
world's population. (Singh ct al, 2003).
Rice is the second cereal in the world, it is grown in the tropics where
rain and sunshine is abundant. This wide adaptability of the rice plant
is the explanation of its importance as a food crop (Kent, 1980).
More than 500 million tons of paddy rice are produced world wide per
year (Ohtsubo el al, 1998) Approximately 90% of the world rice is
produced in Asia but only 4 - 5% enters the global market (Juliano,
1998).Total world consumption of rice is projected to increase from
356 million metric tones in 1994 to 403 million metric tones by 2005
(Hettiaraehehy and Qi, 1998).
Rice is used mainly for human nutrition. In addition it is
particularly valuable for use in medical and functional foods (Medealf,
1998). Besides being the main source of calories and protein, rice is an
important cereal because it has the highest - digestibility, biological
value and protein efficiency ratio (PER) among all the cereals (Kaul,
1973).
The peoples of different countries have varying preferences for
types of rice: round - grain rice is preferred in Japan, Korea and
Puerto Rico, possibly because the cooked grain are more adherent
where as long grain rice is preferred in the U. S. A. People in most
countries prefer white rice, but in India and Pakistan the preference is
for red, purple or blue strains.
Knowledge of the physical and mechanical properties of the
rice is being used in the planting, harvesting, drying, storing, milling,
and processing of rice, (Kunze and Wratten, 1985).
Most of the short - grain rices, as well as some of the medium and
long - grain varieties, have a rather sticky texture when cooked. The
sticky type is preferred by some people, especially those who cat rice
with chopsticks because the grains cling together.
The long - grain varieties have a dry (laky texture when boiled.
This type is preferred by most consumers (Leonard and Martin, 1963).
Quality of rice produced in Sudan is far below world's standard.
Currently the country consumes more than what i t produces. These
conditions in spire great need for more research on rice to improve
yield and quality.
The objectives of this research are, therefore, to study:
1. The physicochemical properties of two types of rice (Long and
short).
2. The cooking quality of the two types of rice under investigation.
CHAPTER TWO
LITERATURE REVIEW
2.1 The Origin of rice
Botanists base their evidence of the origin of rice largely on Ihe
habitats of the wild species. It is presumed that the cultivated species
have developed from certain types of the wild rice (Grist, 1975).
Sharma (1983) reported that the genus Oryza comprises twenty
- five species distributed through the tropical and subtropical regions
of Asia, Africa, central and South America and Australia.
According to Vavilov (1930), the longer a group has been
established in an area, the larger will be the number of species to be
found there. He concludes that the wealth of forms and varieties of
rice found in the south - west Himalayes, which are closely allied to
many Chinese varieties, points to this region as the centre of origin of
rice. It may well be that this and other places in India, South - Bast
Asia, the Philippines and Africa are centres of origin of cultivated
forms of rice.
Ting (1949) concludes that in view of the number of wild rices
found in southern China, rice cultivation is believed to have started in
this region and to have spread north world. Ting (1960) states thai rice
glumes found in the Yangtse River in red burnt clay, thought to
belong to the late Neolithic, have been classified as O. saliva F.
spontanea ssp. Keng and show strong resemblances to the cultivars
now grown in eastern China.
Chandraratna (1964) stated that the country of origin of rice is
not known, but the weight of evidence points to the conclusion that
the centre of origin of Oryza saliva L. is South - East Asia,
particularly India and Indo - China, where the richest diversity of
cultivated forms has been recorded.
3
2.2 Rice in the Sudan
Rice is a unique major food crop of the world by virtue of the
extent and variety of uses and its adaptability to a broad range of
climatic, and cultural conditions. 11 i s u sually g rown u nder s hallow
flood or "wet paddy" conditions, but is cultured w here flood water
may be several meters deep and to the opposite extreme as an upland
cereal. Although rice appears to have a high water requirement, it is
not much different from that of other field crops. Unlike most cereal
crops, however, rice benefits from standing water. It has capability for
anaerobic respiration and has aerenchyma tissue in the aerial organs
through which oxygen diffuses to the roots (Mikkelsen and DeDatla,
1980).
Because of its unique ability to grow and produce high caloric
food values per unit area on all types of land and water regimes,
combined with its adaptation to a wide variety of climates and
agricultural conditions, it is the world's most important cereal crop.
Thousands of cultivars are grown throughout the world, representing a
wild range of plant and grain characteristics (Mikkelsen and DeDalta,
1980).
The introduction of cultivated rice species into (he Sudan from
Congo dated back to 1905 (Hakim, 1963), Oryza pimctuta Kotschy,
was found to be growing wild in rain depressions in the Sudan (Bain,
1948). Since then, and after many trails rice was grown in Gezira,
Bahr ELGhazal (Aweil), Upper Nile (Malakal), White Nile
(ELDuweim) and Jonglei province (Bor). In addition to the growing
areas mentioned, Vachhani (1966) reported that rice can be grown as a
cash crop under irrigation in kenana and Kashim CLGirba. In Malakal,
however, it was found that large scale production of rice can not be
effected without supplementary irrigation by pumps (Hakim, 1963).
Despite this prospective performance of paddy in the Sudan, no
major commercial rice production took off to dale.
In the sixties, Sudan attempted to incorporate rice cultivation
into Gezira Scheme, the country's most mechanized scheme. This was
initiated to increase its yield and improve the quality, as a result of
utilizing facilities in the Gezira Scheme in rice production.
Unfortunately rice brought with it to Gezira scheme more aquatic
weeds. These weeds negatively affected cotton yield, the country's
main cash crop. Consequently after a few years of relatively belter
production, rice cultivation was immediately discontinued in Gc/ira
(Awok, 1995).
It is regrettable that research on rice in Sudan has not been
continuous, eventually it has not achieved much. Despite abundance
of wild rice growth in river basins all over the country and particularly
in swamps of Upper Nile state, the average yield is still very low.
It is just one tone per hectare compared to 6.2 t/ha in Kgypt, 2.1
t/ha in Nigeria, 6.4 t/ha in Spain, 5.7 t/ha in China, 2.7 t/ha in India,
2.8 t/ha in Philippines and 1.8 t/ha in Chile (FAO, 1990).
Quality of rice produced in Sudan is far below world's
standard. Currently the country consumes more than what it produces.
These conditions in spire great need for more research on rice
to improve yield and quality (Awok, 1995).
Weeds are one of the major factors thai reduce both yield and
quality of rice. There is insufficient information regarding the time
and method for optimum weed control in i rrigated d ry seeded rice.
This a major limitation to increasing rice production in the Gezira
scheme (Ghobrial, 1978; Babiker, 1983).On the other hand, nitrogen
fertilizer applied in non - polluting rates often doubles or even triples
yield potential of rice (DeDatta, 1974).
According to the Arab Agricultural statistics (1994, 1998) rice
production and area harvested in Sudan are shown in Table (I).
2.3 Importance of rice
Rice is the basic food for more than half the world population
and provides up to 80 % of the food intake in some countries (Kent,
1980). It is the stable food of a large majority of the people in China,
Pakistan, Japan, Korea, Taiwan, Srilanka, Laos Vietnam, Thailand,
Indonesia, Philippine Republic, Malaysia and Madagascar. Nearly
one- third of the cultivated area in India is planted to rice (Ramiah,
1953).
Besides being the main source of calories, rice is an important
cereal because it has the highest digestibility, biological value and
protein efficiency ratio (PER) among all the cereals (Kaul, 1973).
Rice is one of the most important crops of the world, in
additional to wheat and maize. More than 500 million tons of paddy
rice is produced world wide per year (Ohtsubo et a!., 1998).
Approximately 90% of world rice is produced in Asia but only
4-5% enters the global markets (Juliano, 1998).
Total world consumption of rice is projected to increase from
356 million metric tons in 1994 to 403 million metric tons by 2005.
(Hettiaraehehy and Qi, 1998). This high yield is due mainly to the
heavy use of commercial fertilizers, the development of improved
varieties and efficient cultural methods (Terao, 1929 and Malsuo,
1954).
Thailand, Burma and I ndochina normally supply 9 0% o ft he
rice that moves in international trade. Other important export countries
are the United States, Brazil, Italy and Bgypt.
Table (1): Rice production and area harvested in Sudan
Year
Production
(Metric tones)
Area (Hectares)
1987
1000
1260
1988
1000
1260
1989
1000
1260
1990
720
870
1991
300
2290
1992
200
210
1993
200
170
1994
640
630
1996
2000
2940
1997
2000
2940
Source: Arab Agricultural Statistics (1994, 1998)
7
2.6 Rice milling
Milled rice is defined as a rice obtained after milling which
involves removing all or part of the bran and germ from the husked
rice. It could further be classified into three degrees, under milled rice,
well-milled rice and extra -milled rice. (Iso, 1988)
An efficient mill cleans, scours and polishes the grain with a
minimum of breakage (Collier, 1947). Rice that has been milled by a
scouring process is liable to develop oxidative rancidity; the risk is
minimized by the polishing.
Jn Asia, much of the rice for home or local consumption is
milled by hand with a wooden pounder and hollow block .With such
equipment, parts of the bran layer and germ are left on the milled rice,
which means that much of the rice consumed in Asia is under milled.
Rice is machine-milled in the United States, in most other occidental
countries, and to considerable extent-in Asia too (Leonard and Martin,
1963).
Of the primary factors determining the milling, quality of rice
is the head rice yield (Andrews el al, 1992). The milling quality is also
determined by the degree of milling which i ndicates the amount o f
bran remaining on rice kernels after milling. The United States
Department of Agriculture rice standards (USDA, 1982) currently
specify the use of the Me Gill No .3 mill as part of the overall
procedure in determining these parameters. However, a smaller mill,
the Me Gill No .2, is becoming more popular than the No. 3 in the rice
industry (Andrews et al, 1992).
Awn
Lammo
Polea
Hull -<
v.
Slerilelemmae
Rochillo —
I f " "•_')'!'
' ' I i * 1
- * C|\l.ir y<
.( • • t i t 'JSl
Figure (1): Structure of the rice grain
Source: Juliano and Bechtel (1985)
0
It is well documented that moisture content (MC) at the time of
milling has a significant effect on head rice yield and the degree of
milling (Wratten, 1960; Wasserman, 1960, 1961; Banaszek et al,
1989; Pominski, et al., 1961). As MC decreased, head rice yield
increased and degree of milling decreased as observed by Webb and
Calderwood (1977). They claimed that to obtain a degree of milling al
lower MCs equivalent to those observed at higher MCs, head rice
yield is reduced. Banaszek et al, (1989) determined head rice yield
and degree of milling for rice ranging from 10 to 16% in MC. Few
investigations as to the effects of milling time on head rice yield have
been noted. Velupillai and Pandey (1987) associated the degree of
milling with the milling time used on the No .2 mill, this was done by
milling samples for time period of 0-60 sec. in 5 sec. increments.
They determined that 65-73% of the bran was removed in the lust 20
sec. of milling. With the New bonnet variety, as much as 84% of the
breakage also occurred in the first 20 sec of milling.
Head rice yield and degree of milling can be influenced by the
pressure applied to the rice during milling. The pressure applied in the
milling chamber is controlled by the amount and location of the
weight placed on the weight lever. To obtain equivalent degrees of
milling Webb and Calderwood (1977) increased pressure settings on
the miller. However, no mention was made of how or by what amount
the pressure was increased.
It is speculated that still another factor may influence head rice
yield and degree of milling. It has been observed that the amount of
brown rice placed in the mill tends to alter head rice yield and degree
of milling.
The USDA (1982) states that milling yield shall not be
determined when the M C o f the rough rice exceeds 1 8%. An initial
rough - rice sample of 1 kg is required. Total weight on the weight
holder is adjusted according to the type of rice (short - medium or
long - grain).The milling duration is usually set at 30 sec for all types
of rice. (USDA, 1982).
II
The difficulty in studying the properties of different layers of
rice kernel lies in removing the outer layer uniformly with minimal
change in its properties and keeping the central core integral. Various
abrasive and friction type mills have been used for this purpose in the
laboratory, but a suitable one could not be found because dry milling
increases the temperature of rice (Yanase and Tani, 1969) and
damages the starch granules (Gen He and Suzuki, 1988). Solvent
extraction milling of rice has been practiced commercially (Lynn,
1966). However Gen Me and Suzuki (1988) found that only the bran
layer was softened, and hardness of starchy endosperm was essentially
unchanged in rice oil and hcxane, so undamaged starch granules were
not obtained from the outer layer of starchy endosperm by following
the solvent extraction milling procedure.
The concentration of the vitamins and the minerals in surface
layers and also, in the easily detachable germ leads to considerable
loss of the nutrients during the milling of the grain. Rice is being
milled often to the extent of 15%.So as to ensure better keeping
quality and to obtain an attractive product with good cooking quality
(Ghose et al, 1960).
As a result of this, a substantial part of the essential minerals
and vitamins are lost to the consumers. To avoid, at any rate, some
part of this loss, under milling has been repeatedly suggested
especially for people whose main article of diet is rice. The Bagnio
Nutrition Conference on rice (1948) and the Nutrition Advisory
committee of the India Council of Medical Research have
recommended the milling of rice to such a degree that it will still leave
at least 1.8 y per g of thiamine in the rice. This has been considered a
safe level for the prevention of beriberi in people consuming rice
diets.
12
2.7 Rice Parboiling
Parboiling is essentially precooking of rice within the husk
(Bhattacharya and A li, 1 985). 11 i s an important industry, for about
half the rice produce in South Asia is consumed alter parboiling
(Unnikrishnan and Bhattacharya, 1987). It is possible to prepare
parboiled rice of diverse quality by adopting different systems of
parboiling and also by varying the degree of heat treatment in each
(Bhattacharya, 1985).
Parboiling rice normally consists of soaking rough rice in hot
water for several hors until the kernels are saturated, draining the
water, steaming the saturated rice for several minutes at or above 100°
C to gelatinize the starch, and drying the parboiled product (Marshall
etal, 1993).
Parboiling, a processing step aimed at getting more milled rice
from paddy, is practiced in many countries including India,
Bangladesh, Pakistan, Burma, Malaysia Thailand, Italy, Spain,
Uruguay, Brazil, France and the United States (Pillaiyar, 1990). The
advantages claimed for parboiled rice have included improved nutrient
availability (particularly lliiamin), decreased susceptibility to insect
attack during storage, decreased washing and cooking loss, more
swelling when cooked to the desired softness, improved digestibility
with high protein efficiency ratio, and stabilization of the oil content
of the bran (Pillaiyar, 1990).
Unnikrishnan and Bhattacharya (1987) reviewed the work of
several authors who suggested that the initial quality of the rice to be
parboiled influences the quality of the products.
As reported, from the work of other authors, by Pillaiyar (1990) that
all varieties of paddy can not be parboiled uniformly by a single
technique. Among the different properties, the apparent (Biswas and
13
Juliano, 1988) and insoluble amylosc contents (Unnikrishnan and
Battacharya, 1987) and GT (Mohandoss and Pillaiyar, 1982) seem to
greatly influence the properties of parboiled rice. By properly
selecting the variety suitable for a processing technique, parboiled rice
of a given texture can be produced. Initial differences in quality
attributes of rice under raw condition are for the most part maintained
even after parboiling (Unnikrishnan and Bhaltacharya, 1987)
Compared to raw rice, parboiled rice is harder and gives a
higher yield of head rice upon milling (Raghavendra Rao and Juliano,
1970). In complete parboiling has caused increased kernel breakage
upon milling (Subrahmanyan et al, 1955; Bhattacharya and Subba
Rao, 1966). Also parboiled rice is less sticky after cooking (Kalo et al,
1983) and requires a longer cooking time than raw rice (Ali and
Bhattacharya 1982). These differences between the properties of
parboiled and raw rice are due to changes in the starch upon
parboiling.
Marshall et al, (1993) reported that parboiled rice harder and
less subject to transverse breakage than raw rice, so parboiling has
become popular among rice millers because it result in increased head
yield. Improved head yield translates into higher profits for the miller
because unbroken kernels command a higher price in the market place
than broken kernels. In addition, parboiled rice commands a higher
price than non parboiled milled rice because some consumers prefer
its cooking, flavor and textural qualities.
Priestley (1976) examined the effectiveness of parboiling in
reducing milling breakage in relation to degree of gelatinization.
Compared to raw rice, milling breakage in parboiled rice was
improved only after the starch was completely gelatinized, However,
Itoh and Kawamaura (1991) observed that as the degree of rice starch
gelatinization increased from 2 to 60% the percentage of kernel
breakage decreased from 7 to 1% in a linear manner.
14
Advantages of parboiling over raw milled rice are a better
recovery of whole grains during milling, making a translucent hard
grain, resistant to breakage, inactivation of enzymes, biological
sanitation, easier removal of the hull during milling, better grain
swelling during cooking ,less starch in the cooking water, change in
taste and texture of rice (Sujatha et al 2004).
2.8 Physical properties
Rice, unlike most other cereals, is consumed as a whole grain.
Therefore physical properties such as size, shape, uniformity and
general appearance are o f u tmost i mportancc. Furthermore, because
most rice is milled, the important physical properties are determined
primarily by milled endosperm.
2.8.1 Grain dimensions, weight and uniformity
Since rice is produced and marketed according to grain size
and shape, the physical dimensions, weight, and uniformity are of
prime importance. Grain - type categories are based upon three
physical qualities: length, width and weight. Varieties of each of the
three grain types must conform to rather narrow limits of size and
shape (Mutters, 1998).
The USDA recognizes seven classes of milled rice based on
length/width ratios and known as long - grain, medium - grain, short
grain, mixed, second head, screenings and brewers. While Atukorale
(2002) classified rice broadly into two types namely long grain and
short grain. Long rice grains are preferred when the grains are wanted
to stay separate during cooking, while short rice grains are used to
give a sticker more viscous appearance.
Kernel dimensions are primary quality factors in many areas of
processing, drying, handling equipment, breeding, marketing and
grading (Kramer, 1951; Adair et al, 1973). There are established size
15
and shape requirements for each grain type and varieties must
conform to these specifications. The various grain types are
objectively classified according to length, width, length/width ratio
thickness and grain weight (Luh, 1980).
Grain length was classified (Khush et al, 1979) as extra long
(> 7.50). Long (6.61-7.5rnm), medium (5.51- 6.60mm), or short
(< 5.50mm), Grain shape based on length - width ratio was classified
as slender (> 3.00), medium (2.01-3.00), or bold (1.01-.00) (Khushtv
al, 1979).
As the caryopsis develops, it attains its maximum length and
width before it attains its maximum thickness (Del Rosario el al,
1968). In current rice cultivars, the thickness of individual kernels al
harvest varies widely. The effect of variation of kernel thickness in
harvested rice is a little - studied aspect office research, although it
has implications for many phases of rice investigation, including
cultural practices, breeding, drying, processing, quality and
composition (Wadsworlh et al, 1979).
Matthews and spadaro's research on rice milling (1976)
involved the range of thickness of individual kernels in harvested rice;
that research showed significant differences in milling quality related
to thickness.
Long - grain types predominate in the tropical regions of Asia,
except for coarse grain types for home use, where as the short - grain
types prevail in the northern subtropical region (Leonard and Martin,
1963). Rice varieties in the Bengal province of India have a wide
range of variation in size and shape of kernel. Unhulled grain range in
length from 5.15 - 11.27mm and in thickness from 1.61- 2.59mm
(Nagai, 1959). Uniformity of grain size, shape and weight is
determined by calculating the coefficient of variation for each
measurement on randomly selected kernels from a representative
sample. Grain weight (size) is expressed in g/1000 kernels. The range
average values for grain size and shape of rough, brown and milled
forms of traditional U.S. commercial long, medium and short grain
types are given in Table (2) (Luh, 1980).
Test weight measure of quality is a useful relative indicator of
total milled rice yields. Test weight provides a measure of the amount
of unfilled, shriveled and immature grain based on the size standards
established for the three - grain types (Mutters, 1998).
2.8.2 Color
Color is used one criteria of quality in all rice in the U. S. The
assessment, however, is performed on well - milled, whole grain rice.
The color of rice is often referred to as "general appearance". Color is
influenced by such things as smut which imparts a gray color or red
rice seed that gives the milled rice a rosy color (Mutter, 1998).
Color and anthocyanin pigmentation in the apiculus of the rice
hull are factors influencing different aspects of rice quality. Rice
varieties produced in the United States are classed as either light
(straw) or dark (gold) hulled. While hull color is not of major
importance in the production of regular white milled rice, it is of
considerable importance in the manufacture of parboiled rice.
Varieties with light colored hulls are generally preferred by parboilers
because they tend to produce a lighter colored parboiled product than
dark hulled varieties processed under similar parboiling conditions
(Gariboldi, 1974).
17
2.8.3 DensityThe density of rice was reported to range from 1.327 to 1.375
g/cm3 for rough rice (medium grain), from 1.365 to 1.381 g/cW for
rough rice (long grain) and from 1.442 to 1.379 g/cm1 lor brown rice
(long grain).Bulk density increased with moisture content for the
rough rice (medium and long grain) but showed little variation for the
brown rice (Kunze and Wratten, 1985).
Varieties also differ in their fraction of high density grain due
to differences both in the degree of filling of the hull by caryopsis and
in actual caryopsis density (Venkateswarlu el cil, 1986).
2.8.4 Broken riceThe productions of rice have been increased by adopting
modern technology and e fficient management techniques. However,
quality and quantity losses still occur at the pre - and post - harvest
phases were many factors influence the ultimate quality of the grain
(Velupillai and Pandey, 1990).According to them, internal cracking is
an important factor affecting rice grain quality. As reported by Kunze
and Hall (1965) a rice grain with two or three cross - sectional cracks
has lost its commercial value. Sharma and Kunze (1982) defined
"Fissure" as the large internal fracture usually found to be
perpendicular to the long axis of the grain. It has long been known that
fissured kernels of rice usually break in the milling process.
One of the major problems of the rice processing industry is fissured
or cracked brown rice brought about by environmental and varietal
factors. The resultant excessive cracked grains (brokens) reduce the
market value of milled rice. The fissures occur during moisture
adsorption in relatively dry rough rice (kunze, 1985; Srinivas and
Bhashyam, 1985). The rice grain is hygroscopic and responds
dynamically and physically to moisture and temperature changes in its
environment; however, moisture gradients arc more responsible than
temperature gradients for producing stress fissurcss (Juliano ct al,
1993).
18
Table (2): Grain size and shape of traditional U.S. rice
types
Grain Grain Average Average Average Average Average
Type form Length width Length/width Thickness 1000
(mm) (mm) ratio (mm) Grain vvl.
(gms)
Long
Medium
Short
Long
Medium
Short
Long
Medium
Short
Milled
Brown
Rough
(Paddy)
6.7 to 7.0 1.9 to 2.0 3.4:1 to 3.6:1 1.5 to 1.7 15 to 18
5.5 to 5.8 2.4 to 2.7 2.1:1 to 2.3:1 1.7 to 1.8 17 to 21
5.2 to 5.4 2.7 to 3.4 1.7:1 to 2.0:1 1.9 to 2.0 20 to 23
7.0 to 7.5 2.0 to 2.1 3.4:1 to 3.6:1 1.6 to 1.8 16 to 20
5.9 to 6.1 2.5 to 2.8 2.2:1 to 2.4:1 1.8 to 2.0 18 to 22
5.4 to 5.5 2.8 to 3.0 1.8:1 to 2.0:1 2.0 to 2.1 22 to 24
8.9 to 9.6 2.3 to 2.5 3.8:1 to 3.9:1 1.8 to 1.9 21 to 24
7.9 to 8.2 3.0 to 3.2 2.5:1 to 2.6; 1 1.9 to 2.1 23 to 25
7.4 to 7.5 3.1 to 3.6 2.4:1 to 2.4:1 2.1 to 2.3 26 to 30
- Data based on measurements o< fully developed mature kernels
of typical varieties with in each grain type.
Source: Luh (1980).
Broken rice is generally at only 30 to 50% of whole grain. The
accurate measurement of the amounts and classes of broken grain is
very important (Mutters, 1998).
Rice consumers like the percentage of broken and spoiled
kernels in their rice whole rice to be as a small as prices and their
incomes will permit (Abu Agla, 1988). The average broken kernels
content of rice varies from 5 to 10% in luirope and 5 to 15% in Costa
Rica, Japan and Thailand, to more than 30% in some Asia countries
which import rice (Abbott et a/, 1972).
Broken kernels have a considerably lower commercial value
than whole, unbroken kernels; therefore, rice processors strive to
produce milled rice with the least amount of broken rice (Velupilla
andPandey, 1990).
The causes of rice grain breakage have been classified as those
due to the properties of the grains and those due to the conditions
under which the grain is milled (Spadaro et al, 1980).
The properties of grain are strongly influenced by variety,
moisture content, and the conditions to which the grains arc subjected.
Foremost among the causes for grain breakage are the fissures present
in the rice when it enters the mill. Fissuring of rice has thus received
much attention by researchers but the trade in general has not
considered the presence of fissured whole kernels important. Hence
information relating fissured grains in specific lots of rough rice to
mill yields would be useful to the processing industry.
Nguyen and Kunze (1984) studied the fissures in rough rice
resulting from drying and post drying treatments and reported that the
breakage strength of rough rice was inversely proportional to the
percentage of fissured grain at the end of any storage period after
drying. On a study of rice breakage during milling, it was conclude
that grain breakage resulted primarily from grain defects, such as
fissures (Indudhara Swamy and Bhattacharya, 1971).
From the same study, it was noted that the number of broken
grains after milling rarely exceeded the number of defective grain
before milling.
20
2.8.5 Texture
Szczesnick (1968) defined texture as "the sensory
manifestation of the structure of food and the manner in which that
structure reacts to applied force".
Gonzalez et al, (2004) claimed that texture is an important
attribute of food acceptance by consumer and is therefore considered
as a critical control step in quality assessment.
Rice is consumed principally as a whole grain and therefore;
the texture of the whole grain is a matter of primary importance.
Different cultivars of waxy and non - waxy rice arc usually classified
according to their grain dimensions, amylose content, amylograph
consistency, gelatinization properties of the extracted starches and the
texture (From both Instorm hardness and sensory measurements) of
cooked rice (Juliano, 1982; Juliano, 1985).
Rice texture is affected by factors such as rice variety, amylose
content (AM), gelatinization temperature (GT) and processing Factor
(Meullenet et al, 1998).
Conventionally, sensory and processing qualities of rice have
been assessed by a contribution of preference sensory and
physicochemical properties evaluations. Sensory evaluations are
generally preference rating of llavor and texture, with the emphasis in
texture attributes. Through statistical methods, relationships between
sensory and physicochemical properties are determined, allowing
assessment o f sensory q uality for a target population or application
(Champagne, 1997).
2.9 Chemical properties of rice
2.9.1 Chemical composition
Composition of the grain makes it a palatable food of high-energy
value, which leads the nutritionist to have a major interest in the
composition of the kernel.
21
Moisture content affects rice quality in several different ways.
Of great significance is its effect on the keeping quality of all forms of
rice. Sound dry rice can be maintained for year if properly stored but
only a few days are required for wet rice to spoil. Rough rice moisture
content of 13% is commonly accepted as a safe level for storage for
less than 6 months. Where as 12% or less moisture is recommended
for long term storage (Wasserman and Caldcr wood, 1972, Johnston
and Miller, 1973; Boiling et al, 1977). Moisture contents of rough rice
in access of 14% are designated as sample grade under the U.S.
standards for rice.
The fat content of rice is low and most of it is removed in the
process of milling and is contained in the bran (Grist, 1975). Lipids
are also known to influence viscoelastic properties by forming
inclusion complexes with the helical structure of amylose. Maningat
and Juliano (1980) found that defatting rice starch reduced both
gelatinization temperature and gel viscosity of starch. Morrison and
Azudin (1987), however, could not find n predictive relationship
between starch lipid content and viscoelastic properties.
The protein content of milled rice is low in comparison with
other cereals, although the whole rice grain content N ><5.95 % rice
ranged from 7.0 to 10.8% of which 70 - 80% is in the glulelin. The
invitro protein digestibility was relatively good (87.6 - 91.8%) as
shown by Yousif (2000). Protein, as the other major constituent of
rice, has not been thought to strongly influence cooking and eating
qualities. When differences in gross protein content were examined in
relation to texture of cooked rice, only a weak relationship was found
the higher protein rices being some what less tender than low protein
rices (Onate et al, 1964; Juliano et al, 1965). Because commonly eaten
rices generally contain about 7% protein and do not fluctuate widely
form this level, protein content is not considered an important
indicator of quality (I (amakcr and Griffin, 1990).
22
Rough rice has higher fiber and ash content but lower protein
and available carbohydrates (Nitrogen - free extract, by d i(Terence)
than brown rice. The differences are readily explained by the high
fiber and ash content and the low content of protein and available
carbohydrate in the rice hull (Juliano and Bechtel, 1985).
The important B vitamins in rice are thiamine, riboflavin and
niacin. Modern milling removes most of these vitamins because they
are found largely in the bran and germ. Milling removed 72.25% of
the niacin, 86.27% of the biotin and 50.96%> of the panlolhcnic acid.
The vitamin losses from milling parboiled rice were only 27.55, 48.60
and 24.55%, respectively (Kik, 1951).
The mineral composition of the rice grain depends
considerably on the availability of soil nutrients during crop growth
and on the diverse sampling, preparation, and analytical methods used
by various investigators. Minerals are generally present in higher
levels in brown than in milled rice. A considerable portion of the rice
caryopsis ash is accounted for by phosphorus. Potassium, Magnesium
and silicon are present also in large amounts in brown and milled rice.
By contrast, silica is the major element in hull ash (Juliano and
Bechtel, 1985).
2.9.2 Nutritional value of rice
Some losses of nutrient were observed during rice milling. These were
reported in section 2.6.
One cup of brown rice cooked ( 195 gin) has 2 16.4 calories,
1.76g of fat,0.64g of monosaturated fat, 0.63g of poly unsaturated fat,
0.35g of saturated fat, 44.8g carbohydrate, 5.03g of protein, 0.66g of
fiber, 0. 19 mg of pantothenic acid, 0.28ing of vitamin B6,7.80 mg of
folic acid, 19.50mg of calcium, 0.82mg of iron and l.23mg of zinc
(Atukorale, 2002).
23
2.9.3 Gelatinizntion temperature (GT)
Gelatinization Temperature of the rice grain is recognized as
one of the most important determinants of cooking quality (Rao el al,
2004).
It is generally indirectly tested by dispersal in dilute alkali
using the alkali spreading value (ASV) test of Little ct al., (1958). The
time required for cooking milled rice is determined by gclatinization
temperature. Environmental conditions such as temperature during
ripening influence GT: In many rice growing countries there is a
distinct preference for rice with intermediate gclalinization
temperature (IRRI, 2004).
2.9.4 Gel consistency (GC)
Gel consistency measures the tendency of the cooked rice to
harden after cooling. Within the same amylose group, varieties with a
softer gel consistency are preferred and the cooked rice has a higher
degree of tenderness. Harder gel consistency is associated with harder
cooked rice and this feature is particularly evident in high amylose
rice. Hard cooked rice also tends to be less sticky. Gel consistency is
determined by heating a small quantity of rice in a dilute alkali (IRRI,
2004).
2.9.5 Amylose and amylopectin
The amylose content of starches usually ranges from 15 to
35%.Cooking and eating qualities of rice have long been associated
with amylose content (Hamaker and Griffin, 1990). Rices low in
amylose are generally known to be sticky and moist, where as those
high in amylose are non sticky. Flaky and dry (Juliano et al, 1965).
However, deviations from this correlation exist, such as low -
amylose rices that are non sticky vice versa. Also, rices containing the
same amylose content may differ substantially in hardness (Firmness)
and stickiness (Perez and Juliano, 1979).
24
Other components of rice have been studied in respect to their
relationship with quality. The structure of the amylopectin molecule,
in particular, appears to influence viscoelastic properties of rice.
Juliano et al, (1987) studied three high - amylose rices that contained
similar amounts of amylose but differed in gel consistency, a
measurement used to differentiate high - amylose rices. They found
that the rices that produced hard gels (the firmer, less sticky cooked
rice) had more long - chain linear portions in the amylopectin
molecule than the softer gel rices. The long chain amylopectin also
apparently increases iodine - binding capacity. Amylopectin structure
also differed between apparent low - and high - amylose rices
(Takeda et al, 1987).
Based on amylose content, milled rice is classified in "amylose
group" as follows:-
- Waxy (1 - 2% amylose).
- Very low amylose content (10 - 20% amylosc).
- Intermediate amylose content (20 - 25% amylose).
- High amylose content (25 - 33% amylosc).
Amylose content of milled rice is determined by using the
colorimetric iodine assay index method (1RR1, 2004).
Table (3) shows the range of chemical and physical characteristics
(quality) among U.S. long, medium, and short grain rice types.
2.9.6 Starch
Starch comprises approximately 90% of the total dry weight of
polished rice, and rice eating cooking quality is mainly affected by the
starch properties (Bao, 2004).
The rice endosperm starch may be common (non glutinous) or
waxy (glutinous). The mature, dry common rice grain is more or less
translucent, where as that of waxy rice is opaque or waxy hard,
"waxy" or "glutinous" refers to glue-like or waxy structure, not to the
protein present - The starch in waxy rice consists entirely of
amylopectin, branched - chain starch, while ordinary (straight - chain)
starch is about one - fourth amylose and three - fourths amylopectin
(Leonard and Martin, 1963).
25
2.9.7 Starch gclntiiiizntion
The texture of cooked rice is largely determined by the
gelatinization properties of its starch granules (Juliano, 1985). The
factors that influence gelatinization include granules size and shape,
amylose content, degree ol crystal I inity in the amylopectin fraction,
chain length in amylopectin, and, possibly, placement and content of
starch granule-associated protein and lipid (Juliano ct a/, 1965, 1987;
Manginat and Juliano, 1980; Hamaker and Griffin, 1990; Tester and
Morrison, 1990). These factors differ in starches from different rice
varieties. However, it is still not clear why different rice starch
granules are have differently under gelalinization condition (I latnaker
and Griffin, 1993).
The hot- paste viscosity of rice flour is implicitly related to
gelatinization behavior of the starch granule. Important factors that
Influence paste viscosity are: the degree to which the granule swells
(indicated by swelling potential), the dispersibility of the swollen
granule, and the amount of exudales in the intergranular spaces
(Hamaker and Griffin, 1993). Gelation properties are dependent both
on solubilized amylose and on swollen granules (Morris, 1990).
Granule- swelling potential, on particular, has been related to
differences in rigidity of gelatinizing starch granules (Sandhya Runi
and Bhattacharya, 1989) and differences in degree of crystanillity
within the amylopectin fraction (Tester and Morrison, 1990). Paste
viscosities differ among rice varieties and certain viscosity
parameters, such as Brabender visco/ Amylograph relative breakdown
viscosity, correlate to cooked rice texture (Bhattacharya ct ul, 1982).
Amylose content a laboratory indicator of cooked rice texture, is
positively correlated to amylograph breakdown viscosity, final
viscosity at 95°c, viscosity on cooling to 50°C, and set back (Juliano,
1985).
26
Table (3): Chemical and physical characteristics of 11. S. lonj»,
medium, and short grain rice types.
Characteristic
Amy lose (%)
Alkali spreading value
Gelatinization temp (°c)
Gelatinization class
Water uptake (ml/lOOg)
Protein (%)
Parboiling stability solid
Loss (%)
Amylographic paste viscosity
peak
Cooked 10 min at 95 °c
Cooled at 50 °c
Brewing Cook ability, Sec
Long grain
23-
3 _
71 -
-26
5
-74
Intermediate
121
6 -
18-
765
400
770
120
-136
7.5
-21
-940
- 500
-880
Medium grain
15-20
6 - 7
65-68
Low
300-340
6 - 7
31 - 3 6
890-820
370-420
680-760
5-15
Short grain
18 - 20
6 - 7
65 - 67
Low
310-360
6 - 6.5
30-33
820 870
370-400
680 - 690
5 - 10
Webb. (1985). In rice chemistry and technology.
27
The most useful chemical predictor of cooked rice texture is the
amylose - amylopcclin ratio (Juliano, 1985). However, because the
amylose- texture relationship does not always hold true, and rices with
the same apparent amylose content may differ in texture, other
constituents of the rice kernel probably play a role in governing
texture. Hamaker and Griffin (1990), I lamakcr et al, (1991) suggested
that specific proteins associated with the starch granule may influence
viscoelastic properties of the cooked grain and Hour.
2.10 Cooking quality
Historically, the cooking and processing characteristics of rice
have always been factors of primary importance in rice eating areas of
the world. Cooking and processing quality, along with milling quality,
are the fundamental components of quality that determine and
establish the economic value of the rice grain (Lull, 1980).
Concepts of rice cooking and processing quality may be
described in several ways. In the United States, one of the most
successful has been to characterize cooking and processing quality on
the basic of chemical and physical terms which serve as i ndices of
specific rice quality (Beachell and llalick, 1957; Adair el al, 1973).
Moreover Sujatha et al (2004) reported that the cooking quality of rice
is determined on the basis of the variety and its physico - chemical
properties, but mainly the amylose content.
Eating quality of rice is a great concern to consumer, millers
and rice breeders. It is influenced by varietal characteristics,
conditions of cultivation and post harvest storage and processing
operations. The degree of milling and cooking methods influence the
cooking quality of rice to a significant extent (Satish et al, 2004).
Good eating quality relates to high stickiness, sweet flavor,
gloss of cooked rice and palatability. Eating quality of a particular lot
of rice is evaluated objectively by a taste panel. Taste panel scores are
then used to calibrate "taste analyzers" which determine taste scores
based on physiochemical properties. Protein, amylose and moisture
are the primary determinates of taste in while rice (Mutters, 1998).
28
The methods for preparing the rice for testing also vary widely.
The following variations have all been used in i nslrumenl o r panel
evaluation of cooking procedures: large or small sample size, excess
or limited water, optimum or fixed rice to water ratios, direct or
indirect boiling, steaming, or oven heating, fixed or optimum cooking
times and various cooling procedures.
Batcher et al, (1963) who compared a standard oven cooking
method to wide variety of native cooking methods and rices from
different countries concluded that:" whether the rice was prepared in
the oven, steamed, cooked in small, medium or large amount of water
or in water with oil added, the palatability characteristics of the
cooked product from a given sample and country were similar". This
implies that varietal differences will be evident regardless of the
cooking method used as standard. During studies of cooking kinetics,
Suzuki et al, (1976) found that prcsoaking reduced the cooking time
required to soften the center of the rice kernel (optimum). Their
graphs indicate that a cooking time of about 25min at 100°c is needed
to reach optimum without presoaking. Cooking to optimum has been
recommended by IRR (1979). Others have reported that testing for
doneness by pressing rice kernels between glass plates did not
correlate well with panel estimation of doneness (llogan and
Roseman, 1961).
Kurasawa et al, (1962), who used both excess and limited
water for cooking, pointed out that solids leached during cooking are
removed in excess water but are redeposited when I imitcd water is
used.
Cooking index (CI) was formulated as a function of optimum
cooking time (Staish et al, 2004). Most of the short - grain rices, as
well as some of the medium and long varieties, have a rather sticky
29
texture when cooked. The sticky type is preferred by some people,
especially those who eat rice with chopsticks, because the grains cling
together. The long - grain varieties grown in the United Slates, and
many of these in South - Cast Asia, have a dry flaky texture when
boiled. This type is preferred by most consumers (Leonard and
Martin, 1963).
Cooking qualities depend on cooking time, stickiness,
cohesiveness and tenderness. Some correlation has been found
between the cooking properties and the protein content (Juliano et al,
1965). The levels of isoleucine, lysine and riboflavin decreased dining
making of rice chips (Tongnual and Fields, 1979). Starch, protein and
lipids are the main rice grain components which affect cooking and
eating quality (Zhou et al, 2002).
The wasteful effect of washing rice in large amounts of water,
prior to cooking, and also the loss of nutrients as a result of discarding
the gruel obtained by cooking the rice in excess water have been
demonstrated (Ghose et al, 1960).If these practices are followed, only
about 25% of the vitamins present in the original grain has been
shown to be retained by the cooked rice (Swaminathan, 1942).
Parboiled rice has been demonstrated to behave better than raw rice in
this respect because washing of this rice prior to cooking removes less
vitamins than in the case of raw rice (Ghose et al, 1960).
In most rice eating countries, the rice as purchased from the
market is not completely free from bran residues, d irt, dust, etc. 1 n
practically every home, washing rice two or three times is practiced as
a regular procedure to remove these impurities. Even the very first
washing will leach out a considerable amount of vitamins and
minerals. A specific recommendation to reduce the number of
washing will not, therefore, materially reduce the extent of loss of
30
nutrients by washing. As far as possible, dirt and bran should be
removed by winnowing and if washing has to be restored too, the
grain should not be rubbed under water as is usually done but (he
grain poured into the cooking water and the floating impurities
removed by skimming out as reported by Ghose el al, (1960).
According to them the practice of cooking rice in excess water
which results in loss of nutrients in the gruel has arisen out of
necessity. If the gruel is left behind, the cooked rice is pasty.
This is especially true in the case of freshly harvested rice,
which cooks to a very pasty mass which is difficult to digest. The
gruel is usually discarded although in some poor homes it is used
along with soup. Such gruel loss is more when the rice is cooked in a
vessel directly over fire. The ebullition of the boiling water makes the
grain hit against each other and starch is leached out into the cooking
water and a thick sticky gruel will result. This difficulty is not
experienced during the cooking of parboiled rice, because the
parboiled grain is hard and leaching of starch into the cooking water is
usually very little.
31
CHAPTER THREE
MATERIALS AND METHODS
3.1 Materials
3.1.1 Rice samples
Five samples of rice grains (two short and three long) were
used in this study. Four of these types locally known as American rice
(Rosana), parboiled rice; Pakistanian rice (Basmati); Egyptian rice and
Thailand rice were obtained from Khartoum North market. The fifth
type, locally known as Sudanese rice was obtained from White Nile
(ElDuweim). All samples (three kilogram each) were stored in a deep
freezer for further analysis.
3.1.2 Sample preparation
The rice grain samples were cleaned to remove immature
kernels and foreign materials. A portion of the rice grains were then
milled using a small scale milling machine. The milled rice flour
obtained was kept for further analysis.
3.1.3 Cooking experiment
Broken rice was removed before cooking. Rice was cooked
with water in a ratio of 1:2.5 in all cases as described by Bhaltacharya
et al. (1978), since all rice varieties absorb about that proportion of
water when optimally cooked (Bhaltacharya and Sowbhagya, 1971).
To ensure uniform absorption of water by all grain, cooking was done
in shallow, round, stainless - steel dishes (121 mm diam.) containing
no more than two or three layers of the grain (14 - I6g in each dish).
Up to four of these dishes, provided with loosely lilting aluminum
covers, were arranged in a stainless steel stand one above the other,
the dishes and the stand being well machined to keep i 11 evel. The
assembly was steamed at 0 psig for 45min a leveled autoclave. The
32
cooked rice in each dish was then quickly covered with a piece of
filter paper (weighed down by the upturned aluminum cover and then
stirred with a spatula to break the Lumps, once after 5 min and again
after 15 min. A rice of a sample from two or more dishes were then
pooled and transferred into a big Pctri dish with a piece of filter paper
placed below the lid. The Petri dish was allowed to stand for lh at
room temperature about 30°C).
3.2 Methods
3.2.1 Physical methods
3.2.1.1 Grain dimensions
Grain Dimensions were determined using general virnia
caliber. Ten grains from each sample were collected at random and
the dimensions were measured to obtain the average length, width and
thickness of the milled rice.
3.2.1.2 Grain shape
Based on the length to width ratio (L/w), the shape of the
milled rice was determined (IR.RI, 2004).L/w ratio is calculated as
follows:
Average length of rice mmL/W =
Average width of rice mm
3.2.1.3 Density
This was determined by the Pycnometer method according to
AOAC(1990).
Clean and dry pycnometer (50ml capacity) was HI led with distilled
water, The water level was adjusted to proper point on pycnometer
and stoppered, then weighed. The procedure was repealed again using
2g of rice and completed with distilled water, then weighed.
33
The density of rice sample was calculated as follows:-
Weight of riceDensity =
Volume of rice
Where:-
Volume of rice = Volume of replaced water
(M, + M 2 ) - M 3Volume of replaced water
Density of water
Where:-
M|= Weight of rice.
M2= Weight of pycnometer filled with water.
M3= Weight of pycnometer + Weight of rice + water added.
3.2.1.4 Viscosity
Viscosities of sample (Rice Hour pastes) were measured
according to Hamaker and Grifiin (1993) using the Bostwick
consistometer. A beaker containing an 8% flour slurry was placed in a
boiling water bath and stirred (approximately 20 revolutions / mm) for
15min with. aTeflon - coated stir rod to keep Hour particles in
suspension. Care was taken to introduce only a minimal amount of
shear to the mixture.
The hot paste was cooled to 70°C. Relative viscosity
measurements on the Bostwick consistometer were reported as the
distance (cm) the 70°C— paste traveled down the metal slide in 30 sec.
3.2.1.5 1000 kernel weight
The 1000 — Kernels from each sample were counted randomly
in triplicate and weighed separately to determine 1000 Kernels weight
(Singh et al, 2003).
34
3.2.1.6 Broken ratio
Broken ratio is determined by the method oflRRI (2004) using
the grain grader. The broken grains were separated from the whole
ones and the percentage of the broken was calculated using the
following equations:-
Wt of broken grains x 100% Broken = :
Wt of sample3.2.1.7 Lovibond color system
Ten grams of rice Hour were shaken in a glass - stoppered
flask with 100ml of 70% alcohol by volume. The flask then
centrifuged and the color of clear liquar determined in the lovibond
type colorimeter on the lovibond scale with 70% alcohol as the
standard reference fluid (Kerr, 1950).
3.2.2 Chemical methods
3.2.2.1 Moisture content
Moisture content of samples was determined according to
A.O.AC (1984).
3.2.2.2 Ash content
Ash content was determined by the ignition of samples at
550°C according to A.O.A.C (1984).
3.2.2.3 Oil content
Oil content of the sample was determined according to
A.O.A.C (1984).The oil was determined by extracting a 2g sample in
soxhlet apparatus for 8h using petroleum ether as solvent.
3.2.2.4 Crude fiber
Crude fiber was determined according to A. O. A. C (1984)
using the fibertec system. About two grams of defatted sample were
weighed and used for crude fiber determination.
35
3.2.2.5 Protein content
Protein content was determined using the standard kjeldahl
method (A. O. A. C, 1975). Using kjcldahl catalyst tablets containing
mixture of copper sulfate and sodium sulfate.
3.2.2.6 Starch determination
Starch of fresh and cooked rice was determined by the method
of dispersal in CaCl2, followed by iodine spectrophotometry (Kerr,
1950).The sample was ground to pass 40 mesh screens.
Two grams were taken in a suspension of 10ml water in a glass
beaker, stirred with a glass rod; 60ml of 1.2% CaCI2 and 2ml of
aqueous solution of acetic acid (0.8%) were added to the sample
suspension with continuous stirring, water was added during boiling to
compensate for loss by evaporation. The solution was cooled to room
temperature and then 4ml of 4% stannic chloride were added with
stirring. The Liquar was then transferred to 100ml volumetric flask
and brought up to volume by the addition of CaCl2 solution. After
filtration, 5ml portion was placed in one liter volumetric flask
containing about 700ml water. Then 20ml iodine - potassium iodide
aqueous solution was prepared by adding 0.5g of iodine to 0.75g of
potassium iodide and adjusted the volume by distilled water to 100ml
in a volumetric flask. The volume was made to one liter by distilled
water.
The blue color intensity was measured at 610nm using spectra-
photometer. Reasonable spectrophotometric reading were observed
for samples 50% starch (CLDaw, 1994). If starch content was more
than 50%, then dilution may be necessary as reported by ELDaw
(1994). A dilution of 2.5 was used in the current work. The standard
curve (see appendix 1) prepared by ELDaw (1994) was used for the
determination of starch concentration. The starch equivalent of the
different samples were used to calculate the starch percentage.
36
Calculation:-
C x d f x100Starch % = —; ; 7-r—,
Samples weight (g)
Where:-
C: concentration corresponded to absorbance.
df: Dilution factor.
3.2.2.7 Estimation of the amylose content
A rapid colorimetric method described by William el al (1970)
was used for estimating the amylose content of starches.
Reagents:-A. Stock iodine solution: Potassium iodide (20g) was weighed into
100 ml beaker together with (2g) resublimcd iodine. The
reagents were dissolved in minimum water and carefully diluted
to 100ml volumetric flask.
B. Iodine reagent: 10ml of stock A was pipetted into volumetric
flask and diluted to 100ml with distilled water.
Procedure:-Starch samples (20mg) were weighed into 100 ml beaker.
Exactly 10mI of 0.5N KOM solution (28.05g per liter) was added and
the starch was dispersed with stirring rod magnetic stirring bar for 5
minutes or until fully dispersed. The dispersed samples were
transferred to a 100ml volumetric flask and diluted to the mark with
distilled water with careful rinsing of the beaker.
An aliquot of the test starch solution (10ml) was pipetted into
50ml volumetric flask and 5ml 0.1N MCI (8.17ml AR cone. MCI per
liter) was added followed by 0.5ml of iodine reagent L3.
The volume was diluted to 50ml and the absorbance of blue
color was measured at 625nm after 5 minutes. For calibration of the
colorimetric test, increments of amylose from 2 to I2mg were plotted
against absorbance at 625nm wave length (Taha, 2000) as shown in
the calibration curve (appendix 2).
37
3.2.3 Cooking quality
3.2.3.1 Alkali - spreading value
Alkali - spreading value was determined by the method of
little et al (1958) using duplicate samples of six grain soaked lor 23b
in 10ml of 1.7% KOI I at 30°C.Gelatenization temperature (GT) was
classified (Juliano, 1990) as low, intermediate, or high. It is indexed
by the alkali spreading value using a 1 to 7 scale: low, 6 - 7;
intermediate, 4 - 5 ; high intermediate, 3; high, 1 - 2.
3.2.3.2 Gel consistency (GC)
Gel consistency was determined by the procedure of
Cagampang et al, (1973).Gel consistency measures the length of the
gel of 90 or lOOmg rice flour in 2.0ml 0.2N KOI I in 100 x 13mm test
tubes after lh in a horizontal position. Gel consistency (length) data
are classified as soft (100mm), medium (41 - 60mm) and hard
(25-40mm) (Juliano, 1990).
3.2.3.3 Texture of cooked rice
For estimating the texture of cooked rice, rice was cooked with
2.5 times its weight of water (Deshpande and Bhattacharya, 1982).
For the sensory test, 20g of milled rice and 50ml of distilled water
were placed in a 250ml beaker, covered with a Pciri dish, and open
steamed in an autoclave for 45 min., 50min and GOmin for raw mild,
and severely parboiled rice, respectively.
Sensory test were made using fingers alone rather than by
mouth (Sowbhagya et al, 1987). All samples were code numbered and
several samples arranged randomly were presented two times to a
panel of six judges who scored them sequentially.
Each sample was tested on two different occasions, and the
mean of all the scored for tenderness, inoistness and stickiness, in
which the highest score corresponded to the highest quality. The score
38
card for sensory evaluation of texture of cooked rice reported by
Unnikrishnan and Bhattachrya (1987) is shown below:
Score
1
3
5
7
9
Stickiness
Not sticky, well separated grains
Not sticky, partially separated grains
Rather sticky
Very sticky
Like paste
Tenderness
Hard
Firm
Moderate
Soft
Mashy
Moist ness
Very dry
Rather dry
Slightly moist
Moist
Very moist
3.2.4 Statistical analysis
Data were assessed by analysis of variance (ANOVA) using SPSS
program version 10.0 and means were separated by Duncan's multiple
range test with a probability (P < 0.05) (Duncan, 1955).
39
CHAPTER FOUR
RESULTS AND DISCUSSION
4.1 Physical properties
4.1.1 Dimensions and shape
The dimensions (Length, widlh, and Thickness and L/w ratio)
and shapes of long type (American A; Pakistan P and Thailand T) and
short type (Egyptian E and Sudanese S) rice grains were measured and
their results were shown in Table 4.
The length of the various samples, under study, was found to
be 6.73mm A, 7.49mm P, 7.05mm T, 6.46mm E and 6.64mm S. A
significant difference (P > 0.05) was observed within the long type
samples. The long and short types were significantly different. This
was supported by the findings of Khush et al, (1979) who classified
grain length as extra long (> 7.50mm), long (6.61-7.50mm) and short
(< 5.50mm). Webb (1980) found the average length of long type in the
range from 6.7 to 7.0 and short type from 5.2 to 5.4mm.The results are
in conformity with the range of 6.6-7.7 for long type and 5.5mm for
short type reported by Leonard and Martin (1963).
The width measured was 2.08mm A, 1.73mm P, 2.06mm T,
2.66mm E and 2.73mm S.A significant difference was observed
within the long type samples. The long and short types were
significantly different. These values (Except that of P) were located
within the range of 1.9 to 2.0 (Webb, 1980) and 2.0-2. lmm (Mutter,
1998) reported for the long type rice. The results are similar to those
obtained by Webb (1980) who reported a range of 2.7 to 3.1mm for
the short type rice. The values obtained, in this study, were
comparable to the findings of Mutters (1998) who reported a range of
2.8-3.0 for the short grain rice.
40
Table (4): Physical properties (dimensions and shape):
Sample
A
P
T
E
S
Length
(L) mm
6.73C
7.49a
7.05b
5.46d
5.64'1
Width
(w) mm
2.08"
1.73C
2.0 6"
2.66'
2.7 3 '
Thickness
mm
1.59C
1.50''
1.67L
1.93'
1.83"
L/vv ratio
3.24C
4.35'
3.43"
2.06''
2.07''
Shape
Slendei
Slendei
Slendei
Bold
Bold
• Each value is a mean of 10 determinations.
• Means in the same column with the same superscript letters are
not significantly different.
Where:-
A: American rice
P: Pakistanian rice
T: Thailand rice
E: Egyptian rice
S: Sudanese rice
41
Thickness in the live samples studied was 1 .59 mm A, I .50
mm P, 1.67 mm T, 1.93 mm E and 1.83 mm S. The results showed
significant differences within and between different types of rice.
These results were in conformity with the range of 1.5 to 1.7 mm for
long type (Webb, 1980). The later worker reported a range of 1.9 to
2.0mm for the short type rice. The results of this study were lower
than the range of 2.0 - 2.6 mm for long type and 3.2 mm for short
type rice as reported by Leonard and Martin (1963).
Length/ width ratio (L/W) values were 3.24 A, 4.35 P, 3.43 T,
2.06 E and 2.07 S. A significant difference (P> 0.05) was observed in
the long type samples. The long and short types were significantly
different. The long and short type o f r ice, i n t he p resent s tudy, a re
classified as slender and bold respectively according to Khush el al,
(1979) who reported that grain shape based on length / weight ratio
was classified as slender (>3.00) or bold (1.01 - 2.00).
4.1.2 Density, viscosity, 1000 kernel weight, broken ratio.
The physical properties (Density, Viscosity, 1000 kernel
weight and broken ratio) of the long type (A, P and T) and the short
type (E and S) of rice grains were investigated and their results were
shown in table 5.
The densities for the five samples investigated were 1.43 g/ml,
1.48 g/ml, 1.45 g/ml, 1.46 g/ml and 1.46 g/ml respectively. Significant
differences (P> 0.05) were observed between the three samples of the
long grain rice type whereas; no differences were seen between the
two samples of the short grain type. A significant difference (P> 0.05)
was observed between the (A) long grain type and the two short grain
type studied. These values were slightly lower if compared to the
range of 1.49 -1.51 g/ml for non waxy rice starch reported by Juliano
(1985). The values obtained are higher than the range of 1.365 -
1.381g/ml for rough rice reported by Kunze and Wratten (1985) and
this could be due to differences both in the degree of filling of the hull
by caryopsis and in actual caryopsis density (Venkalcswarlu, 1986).
42
The Bostwick consistomcter, which measures distance a paste
flows down a sloping metal tray, showed increased paste viscosity
with long grain types for out of scale (A), 12 cm (P) and I 1.5 cm (T).
These results are compatible with the findings of Hamaker and Griffin
(1993) who reported a range of 10-12 cm for long type varieties. Since
sample (A) is parboiled rice, this can offer an explanation for the fact
that no value was read for this sample on consistometcr and hence the
very low viscosity. Significant differences (P>0.05) were observed
within each of the long and short types and between the long and short
types. A decreased paste viscosity were measured in the short type
with values of 4.5 (E) and 5.5 (S). These results disagree with the
findings of the same investigators who reported a paste of 22-24 cm.
Difference in paste viscosity observed in this study could be attributed
to difference among rice varieties (Bhattacharya el al, 1982). Also
Hamaker and Griffin (1993) suggested that specific proteins
associated with the starch granule might influence viscoelastic
properties of the cooked grain and Hour.
Weight of 1000 Kernel in the five samples studied were 16.67g
A, 14.53g P, 18.89g T, 18.47g l.< and 18.32g S. A significant
difference (P>0.05) was observed within the long type samples. The
long and short types were significantly different. These values were
located within the range of 15-18g reported by Webb (1980) and.the
range of 16—18g reported by Mutters (1998) for the long type except
the T sample which is slightly higher, but lower if compared to the
range of 20 to 23g and the range of 22-24g reported by the same
investigators for the short type. This may be attributed to ripening
conditions as reported by Pillaiyar (1988).
43
Table (5): Physical properties (density, viscosity, 1000
kernel weight and broken ratio)
Sample
A
P
T
E
S
Density
g/ml
1.43C
1.48"
1.45lK
1.46"b
1.46"b
Viscosity
cm
-
12.00"
1 1.50'1
4.50d
5.5Oc
1000 kernel weight
(8)
I6.67C
I4.531'
18.89:1
I8.47'1
I8.3211
Broken ratio
%
3.62b
0.3 la
I.58C
6.39"
6.54:1
- No viscosity read
• Each value is a mean of triplicate determinations
• Means in the same column with the same superscript letter are not
significantly different.
Where:-
A: American rice
P: Pakistanian rice
T: Thailand rice
E: Egyptian rice
S: Sudanese rice
44
Broken ratios were 3.62% A, 0.3% I\ 1.58% T, 6.39% II and
6.54% S. as can be seen from results in table 5. A significant
difference was observed in the long type samples. The long and short
types were significantly different. These values lie with the lower
values of the range of 3.79-22.53%, 4.8-25.72% and 3.80-22.12% for
the Labelle, Tebonnet and New bonnet varieties respectively reported
by Velupillai and Pandey (1990), however the P and T samples were
shown to have relatively low broken ratios which may be attributed to
the drying process.
Rice consumers like the percentage of broken in their whole
rice to be small as prices and their incomes will permit (Abu Agla,
1988). The resultant excessive cracked grains (broken) reduce the
market value of milled rice (Juliano, 1993).
4.1.3 Color
The lovibond instrument (using three (liters) was used for the
determination of the colors of the five samples studied and (he results
were shown in table 6.
From the result of table 6 a significant difference was observed
within and between the different types studied.
Color is used as one criteria of quality in all rice in the U. S.
The color of rice is referred to as "general appearance".
(Mutters, 1998).Color is influenced by such things as smut which
imports a gray color or red rice seed that gives the milled rice a rosy
color (Mutters, 1998) or improper polishing
4.2 Chemical properties
4.2.1 Proximate composition
The proximate composition analysis of the live rice grains
investigated A, P and T, (long type) and E and S (short type) were
shown in table 7.
45
Table (6) Color of rice
Sample
A
P
T
E
S
Red filter
0.1"
0.2;i
0 . 1 b
0.2;1
O.I1'
Yellow filler
O.4;|
0.31'
0.2c
0.2c
0.2c
Blue filtei
0.2c
0.41'
0.5;l
0.5a
0.2'
• Each value is a mean of triplicate determinations.
• Means in the same column with the same superscript letters are not
significantly different.
Where:-
A: American rice
P: Pakistanian rice
T: Thailand rice
E: Egyptian rice
S: Sudanese rice
46
Chemical analysis revealed thai the moisture content (M(') was
10.1%, 10.5%, 10.9%, 10.4% and 8.6% in the 5 samples studied
respectively. The result showed significant di(Terence (P>0.()5) within
and between different types of rice. The present values fall within the
range of 9 - 1 3 % given by Kent ( 1980) except t he S udanese sample
which gives values lower than this range. The lower MC of the
Sudanese sample could be attributed to differences in the temperature
used in the drying process .
Ash values in the 5 samples studied were 0.6%, 0.4%, 0.3%,
0.5% and 0.5%. A significant difference was observed in the long type
sample. The long and short types were significantly different. These
values were inline with the findings of Juliano (1985) who reported
0.3 - 0.8% a sh a nd a lso a greed w ith t he w oik o f K ent ( 1980) w ho
found 0.4-0.7%, however the T sample was shown to contain
relatively low ash.
Crude fiber values of the 5 samples were 0.48%, 0.22%,
0.35%, 0.32% and 0.3% respectively. Significant differences were
observed within the long type samples. The long and short types were
significantly different. These results were fairly compatible with the
range of 0.2-0.5% reported by Juliano (1985).
Crude protein values were 7.4%, 8%, 6.2%, 6.9% and 7.2%
respectively. Significant differences were observed within the long
type samples and between short and long types. These results were in
conformity with the range of 5.7-9.9% reported by .luliano ct al
(1993) and the range of 5-9 % reported by Kent (1980).
Oil values of the five samples were 1%, 0.9%, 0.5%, 0.9% and 0.7%
respectively. The results showed significant difference within and
between the different types of rice. These values were located within
the range of 0.4-1 % reported by Kent (1980) but higher if compared
to the range 0.3-0.5% reported by Juliano (1985). Sometimes samples
of the same variety can be expected to vary, depending on differences
in sample histories (Fellers, 1983).
47
Table (7): Proximate composition of long and short rice
grains:
Sample
A
P
T
E
S
Moisture %
10.1c
10.5b
10.9a
10.4bc
8.6t!
Asli %
0.6a
0.4c
0.3d
0.5b
0.5b
Fiber %
0.48;i
0.22c
0.3511
0.32"
0.30b
Protein %
?4ah
8.0;i
6.2C
6.9b
7.2b
Oil %
1.0"
0.9;i
0.5c
0.9'1
• 0.7"
• Each value is a mean of triplicate determinations.
• Means in the same column with the same superscript loiters are not
significantly different.
Where:-
A: American rice
P: Pakistanian rice
T: Thailand rice.
E: Egyptian rice
S: Sudanese, rice
48
Parboiled rice retains more protein, fat, ash, crude fiber and
amylose is thus more nutritious than raw milled rice (Sujatha et al,
2003). The present study supported this conclusion because this
applies in A sample.
4.2.2 Starch, amylasc and amylopcctiii
The percentages of starch, amylose and amylopectin were
determined before and after cooking and the results were presented in
table 8.
Starch contents before cooking were found to be 64.22% (A),
64.27% (P), 63.57% (E and 59.82% (S). No significant differences
were observed within the three samples of the long rice type, but
significant differences were seen within the two samples of short type
grain and no significant difference was observed between long type
and short type with the exception of S sample which was shown to be
significantly different from the rest of the group. These values were
lower than that obtained by Juliano and Bechtel (1985) who reported
77.6% for milled rice and this could be due to the relatively high
protein content of the samples under investigation (see section 4.2.1).
Bhattacharya et al, (1978) reported that a high protein variety would
contain a relatively low starch.
Cooking significantly decreased starch from the above results
to 51.40% (A), 50.13% (P), 43.97% (T), 48.15%, (II) and 55.47% (S).
Significant differences (P > 0.05) were observed within the
long type and each sample of the short type and between the long and
short types. Amylose contents before cooking were 26.83% (A),
31.00% (P), 31.50% (T), 24.00% (II) and 24.33% (S). No significant
differencces were observed within each of two types with the
49
Table (8): Starch, amylose, amylopectin before and after
cooking
Sample
A
P
T
E
S
Before cooking
Starch
64.22a
64.27a
63.16a
63.57a
59.82b
Amylose
26.83b
31.00a
31.50'
24.00c
24.33C
Amylopectin
37.39
33.27
31.66
39.57
35.49
After cooking
Starch
51.40b
5O.131'
43.97ci
48.15 '
55.4711
Amylose
18.67"
18.0011
17.67"
13.83'
15.831'
Amylopeclin
32.73
32.13
26.30
34.32
39.64
• Each value is a means of triplicate determinations.
• Means in the same column with the same superscript letters are
not significantly different.
Where:-
A: American rice.
P: Pakistanian rice.
T: Thailand rice
E: Egyptian, rice.
S: Sudanese, rice.
50
exception of A sample which was shown to be significantly different
from the rest of the group, but a significant difference (P > 0.05) was
observed between the long and short types. The present values fall
within the range of 20-25% for short type, classified as intermediate
and a range of 25-33% for long type classified as high as proposed by
Juliano (1990). These results are also similar to the findings of
Hamaker and Griffin (1990) who reported a range of 29.5 - 31% for
long types with the exception of A sample which has a relatively
lower value. The values obtained for the short type were higher if
compared to the range of 19.8-22% reported by Namaker and Griffin
(1990).
Cooking significantly decreased amylosc content, mentioned
earlier, to 18.67% (A), 18.00% (P), 17.67%. (T), 13.83% (I:) and
15.83% (S).No significant differences were observed within long
types but a significant difference was seen in the short type samples.
A significant difference was shown between the two types
investigated.
Amylopectin contents, obtained by difference, were 37.39%
(A), 33.27% (P), 31.66% (T), 39.57% (13) and 35.49% (S) before
cooking and 32.73% (A), 32.13% (P), 26.30% (T), 34.32% (E) and
39.64% (S) following cooking. It is worth mentioning that the
amylopectin content of S sample increased by cooking and this is due
to the fact that the reduction of the starch content, which was
originally low compared to other rices, after cooking is low. The
increase in amylopectin content after cooking could be responsible for
high viscosity, of the Sudanese rice, reported in this study ( section
4.1.2). The structure of the amylopectin molecule in particular,
appears to influence viscosclastic properties of rice (llamaker and
Griffin, 1990).
51
4.3 Cooking quality
4.3.1 Alkali spreading value - gel consistency
Selected physicochcmical properties of long and short rice
grains were determined and the results were shown in table 9.
Alkali spreading values obtained, using 1 - 7 scale of Juliano
(1990), described in section 3.2.3.1, were 3 (A), 1-2 (1>) and (T) and
6-7 (E) and (S). Rice samples, in the present study, were classified as
high - intermediate GT 70-74°C (A), High GT 75- 79°C (P) and (T)
and low GT 55 - 69°C (E) and (S) according to the ranges reported by
Juliano (1990) as shown in section 3.2.3.1. These values disagree with
the range of 3 -5 for 1 ong g rain t ype r eported b y W ebb ( 1980) a nd
Mutters (1998) except sample A. The present values arc located
within the range of 6-7 for short type reported by Webb (1980) and
Mutters (1998).
The importance of gelatinization temperature is for
determining, the time required for cooking milled rice. The
differences in GT could be due to the environmental conditions such
as temperature during ripening (IRRI, 2004).
Gel consistency values were 86.00mm A, 41.67mm l\
43.67mm T, 28.33mm E and 22.00mm S. A significant difference was
observed within and between the two types studied.
Rice samples, in the present study, were categorized as soft (A),
Medium (P) and (T) and Hard (E) and (S) according to the ranges,
reported by Juliano (1990) as shown in section 3.2.3.2.
52
Table (9): Selected physicochemical properties of long
and short rice grains:
Sample
A
P
T
E
S
Alkali
spreading
value
3
1 -2
1 - 2
6 - 7
6 - 7
Gelatinization
temperature
(GT) class*
High-intermediate
High GT
High GT
Low GT
Low GT
Gelntinization
temperature
°c
70 - 74
75 - 79
75 - 79
55 - 69
55 -69
Gel
consistency
(mm)
86.00a
41.67U
43.671'
28.33C
22.00'1
Category
Soil
Medium
Medium
1 lard
Hard
According to Juliano (1990).
• Each value in Alkali spreading is a mean of 2 determinations.
• Each value in Gel consistency is a mean of triplicate
determinations.
• Means in the same column with the same superscript letters
are not significantly different.
Where:-
A: American rice.
P: Pakistanian rice
T: Thailand rice
E: Egyptian rice
S: Sudanese rice
53
4.3.2 Texture of cooked rice
Sensory evaluation was done as described in section 3.2.3.3.
The sensory evaluation results of the texture of cooked rice were
shown in table 10. There is a distinct increase in stickiness from long
to short type. Long type located between not sticky to rather sticky
and short type was very sticky. Tenderness of long type is between
Hard to firm whereas short type is soft. For moistness the long type
lies between very dry to slightly moist whereas the short type is
located between moist to very moist. The mean values of the sensory
texture scores indices (Stickiness, Tenderness, and moistness) are
significantly different (P>0.05) within the long type and between short
and long types. This difference may be due to the fact that rice texture
is affected by factors such as rice variety, amylose content,
gelatinization temperature (GT) and processing factor (Meullenet at
al, 1998). It was observed that sample A (parboiled sample) had score
of not sticky, well separated grains, hard tenderness and very dry. This
result confirms the popular observation that cooked Parboiled rice is
harder and less sticky than cooked raw rice (Unnikrishnan and
Bhattacharya, 1987).
Texture is an important attribute of food acceptance by
consumers and as such, a critical step in quality assessment (Gonzalez
et al, 2004).
The texture of cooked rice showed a distinct relationship to the
amylose content (Unnikrishnan and Bhattacharya, 1987). The present
study supports this conclusion.
Photographs of the five samples, before and after cooking, are
shown in appendices 3-12.
54
Table (10): Sensory evaluation test for texture of cooked
rice
Sample
A
I*
T
E
S
Stickiness
1.00"
4.17"
2.33C
7.6711
7.83;1
Tenderness
1.33C
3.83h
3.00b
7.0011
7.33;1
Moistness
1.67"
4.331'
3.00c
6.00"
7.83"
• Each value is a mean of 12 cletenninations.
• Means in the same column with the same superscript letters are
not significantly different.
Where:-
A: American rice
P: Pakistanian rice
T: Thailand rice
E: Egyptian rice
S: Sudanese rice
55
4.4 CONCLUSIONS
The present work was intended to investigate some physical,
chemical and cooking qualities of (wo types of rice grains i.e. long
type and short type. The long type grain includes American (A),
Parboiled rice; Pakistan (P) and Thailand (T) while the short type
includes Egyptian (E) and Sudanese (S).
Significant differences were observed in the physical properties
studied (dimensions, density, viscosity. 1000 kernel weight, broken
ratio and color) within the long and between short and long types,
while no significant differences were observed within the short types.
The long and the short types were significantly different in their
proximate composition.
Cooking of rice grains (short and long) significantly reduced
starch, amylase and amylopcctin contents. According to (heir amylose
content A, P and T were classified as high while E and S were
classified as intermediate. A significant difference was observed
between short and long types in cooking qualities including alkali
spreading gel, consistency and texture of cooked rice. Texture of
cooked rice seems to be improved with increasing amylose content.
Sensory evaluation show that sample A (parboiled) had a score
of well separated grains (not sticky), hard tenderness and very dry
indicating the importance of the process of parboiling in improving
the cooking quality of rice grains.
56
4.5 RFXOMMENDATIONS
The results obtained form the study suggests the necessity to
proceed carrying further research on rice grain and therelbre:-
1. Further investigations are required to see the possibility of
parboiling the Sudanese rice as it proved its good cooking
qualities in this study.
2. Research may be directed to study the characteristics which
were not covered by this study, such as water uptake ratio,
volume increase and color changes.
57
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Reading Cone.0.046 0 0120.144 00240 28? 0.0360.408 0048
0.8 1.2 1.4
Reading
Figure 2. Starch standard curve
71
Appendix 2
4 6 0 10Weight of Qinyloae (nig)
Figure (3). Standard calibration of amylose content as determined by colorimelricprocedure
72
Appendix 3.
Plate 1. American rice (Parboiled) before cooking
Appendix 4.
Plate 2. American rice (Parboiled) after cooking)
73
Appendix 5.
Plate 3. Pakistan rice (Basmati) before cooking)
Appendix 6.
Plate 4. Pakistan rice (Basmati) after cooking)
74
Appendix 7.
Plate 5' Thailand ricet»tp>iftcooking
Appendix 8.
Plate 6. Thailand rice after cooking
75
Appendix 9.
h
I ' *
Plate 7. Egyptian rice before cooking
Appendix 10.
Plate 8. Egyptian rice after cooking
76
Appendix 11.
Plate 9. Sudanese ricelo^^ cooking
Appendix 12.
L *
Plate 10. Sudanese rice after cooking
77