2.1 HISTORY
OF RICE
Starting in 2500BC, rice has been a
source of food for people. Rice production originated in china, and was spread
to countries such as Srilanka and India. It is believed that rice was bought to
West Africa and Greece in 300BC, by Alexander the Great’s armies. In 800 AD.
People in East Africa traded with people form India and Indonesia and were
introduced to rice, it was a common belief in the middle Ages that rice fields
were a breeding ground for malaria. People believed that mosquito easily layed
their eggs in the water of the rice fields.
Rice was introduced to Greece and
neighboring Mediterranean (344-324B.C.E) and then gradually to Europe and
Africa (IRR 1997) there have been debates about the introduction to Asia rice
into Africa oryza sativa was believe to have been introduced to Africa
primarily from malayoolynesia a few centuries B.C.E or form sri Lnaka and
Indonesia. The other possibilities for the introduction of rice into Europe are
form Persia, Central Asia, or directly form China. Later the Portuguese brought
rice to Brazil and the Spanish introduced rice to Central and south America
Oryza Sativa was introduced to Egypt form India and finally to west Africa by
Portuguese spice traders between the 15th and 17th
century (Grist 1986).
Origin and diffusion of rice are
still unsettled (Huke 1990) Asian cultivated rice originated in the region from
south china to the changes ins south and south east Asia, including the river
valleys and deltas of the Brahaneputra of northern India, the irrawaddy of
Burma, the making of vietenam and the Yangtse of china based on the number of
wide rice specie and evidence of rice glumes in the burnt day from the late
Neolithic period (Ting, 1949) concluded that rice might have in south china and
spread northwards (Capeland 1924) and some Japanese rice scientists asserted
that rice might have originated from south or south east Asia, including India,
china, Thailand and Indonesia.
Thermolumine scene and carbon 14
test of the pottery shards with the imprints of rice grains in Thailand
indicates that rice could be dated back to at least B.C.E (IRR, 1997). The
second oldest pieces of archacological evidences for rice origin are from
maharaga of India (6500-4500 B.C.E) and pentonshan of china (7150-6250 B.C.E)
(Abro et al., 1999.
World wide there are more than
40,000 different varieties of rice, species name Oryza sativa. There are four
major categories of rice world wide indica, japonica, aromatic and glutious.
Improved varieties that have the following essential characteristics are, high
yields, good grains quality and resistant to rice blast disease most improved
varieties are also resistant and tolerant to drought, cool temperature, iron
toxicity in the soil (Odomena, 2006).
2.1.2 Flow
chart on rice processing
(Source
Okaka, 2005)
2.1.3 Nutritional
composition of rice.
|
ENERGY
|
1,527K,
(365K CAL
|
|
Carbohydrates
|
80g
|
|
Sugars
|
0.12g
|
|
Dietary
fiber
|
1.3g
|
|
Fat
|
0.66g
|
|
Protein
|
7.13
|
|
Water
|
11.61g
|
|
Thiamine
(vitamin b1)
|
0.070lmg
(6%)
|
|
Ribo
flavine (vitamine B2)
|
0.0149mg
(1%)
|
|
Niacin
(Vitamin B3)
|
1.62mg
(11%)
|
|
Pantothenic
acid (B5)
|
1.014mg
(20%)
|
|
Vitamin
B6
|
0.164mg
(13g)
|
|
Calcium
|
28mg
(3%)
|
|
Iron
|
0.80mg
(6%)
|
|
Magnesium
|
25mg
(16%)
|
|
Phosphorus
|
115mg
(16%)
|
|
Potassium
|
115mg
(2%)
|
|
Manganese
|
1.088mg
(52%)
|
|
Zinc
|
1.09mg
(11%)
|
Source:
USDA nutrient database
Rice is a good source of protein and a staple food in
many parts of the world, but it is not a complete protein. It does not contain
all the essential amino acid in sufficient amounts for good health.
Rice is rich in starch moderate in
protein, poor in fat, iron and calcium provides about 350 kcal (1.47m) per 100g
day weight (Wadswort 1993) Rice is a complete food, it does not contain some
amount of thiamin, riboflavin and niacin but lacks in vitamin (A,D,C) The level
of vitamin is considerably low in polished rice than in brown rice due to high
content of b- complete vitamin in the bran and germ that are removed during
milling.
2.2 SOURCE AND UTILIZATION OF STARCH
Starch
is one of the most important natural organic compound, abundant in nature. It
is found in the roots or fruits (Duprat et
al, 1980; Buleon et al., 1990).
The most common sources of food starch are corn, potato, wheat, tapioca and
rice (Woolfe, 1992; Henry and Westby, 1998). Developed countries (Canada, USA,
Europe and Japan) have 77% of the global starch market (Sansavani and Verzoni,
1998).
2.2.1 STARCH
Starch is a polysaccharide (meaning
“many sugars”) made up of glucose units linked together to form long chains.
The number of glucose molecules joined in a single starch hundred thousand,
depending on the type of starch. Starch is the storage form of energy for
plants, just as glycogen is the storage form of energy for animal. Starch
exists as granules varying in diameter from 2 to 130 microns. The size and
shape of the granule is characteristic of the plant from which it came and
serves as a way of identifying the source of a particular starch (Swamy 1982).
2.2.2 CHEMICAL
COMPOSITION OF STARCH
Starch is predominantly composed of
two polysaccharides macromolecules, amylose (20-30%) and amylopectin (70-80%).
Amylose is mainly linear with very few branches, while amylopectin is highly
branched. The organization of amylopectin and amylase is the basis for the
semi-crystalline structure of the starch granule (Bell et al., 1998). The ratio
of amylose to amylopectin is one of the key factors determining industrially
important properties.
2.2.3 CHEMICAL
STARCH MODIFICATION
Starch modification is the changing
of starch so that it obtains the characteristics that deviate from the nature
starch. The products are called derivatives. Chemically modified starches are
starches that have been modified with chemicals.
2.2.4 TYPES
OF STARCH MOLECULES
- amylose
- amylopectin
Amylose averages 20 to 30% of the
total amount of starch in most native
starches. There are some starches, such as waxy starch, which contain only
eamylppectin. Others may only contain amylose. Glucose residues united by a 1,4
linkage form the linear chan molecule of amylose. Amylose is the linear
fraction and amylopectin is the branched
fraction. The is because the linear chains can orient parallel to each
other, moving close enough together to bond. Probably due to the ease with
which they can slop past each other in the cooked paste, they do not contribute
significant to viscosity. The branched amylopectin molecule give viscosity to
the cooked paste. This is partially due to the role it serves in maintaining
the swollen granule. Different rice have different relative amounts of amylose
and amylopectin. These different proportions of the two types of starch within
the starch grains of the plant give each starch it’s characteristic properties
in cooking and gel formation.
Rice starch composes approximately
90% of milled rice. It is made up of two
major glucose polymers, amylose which is a slightly branched, primary long
chain and amylopectin, which is highly branched. The amylose content of rice
starch ranges form 0 to 30% (w/w) cooked rice texture and rice starch
functional properties are reported to be primarily impacted by amylose content
(Bhatta charya, soubhagy, Indudhara swamy 1982)
2.2.5 STARCH
PROPERTIES
Because of its structure starch bears some special
properties which make it a unique natural polymer. The following list contains
the most important properties of starch, which are highly dependent on the
origin and the chemical.
Physical modification of the polymer
- thickener
- Rheology enhancer
- Film former
- Sticking – adhesive (cohesive
- Water binding capacity
- Water retention
- Flocculent
- Shear stability
(Muhrbek 1987 )
2.2.6 Starch
components and molecular weights of rice oryzenin
|
Rice
|
Mx105
|
Rice
|
Mx105
|
|
Oryzenin
(protein medium – grainrice)
|
|
Medium
- grainrice
|
|
|
Control
4c
40c
|
1.03
1.12
2.01
|
Control
4c
40c
|
29.9
31.0
32.0
|
|
|
|
|
|
|
Long
grain rice
Control
4c
40c
|
1.22
1.44
2.01
|
Long
grain rice
Control
4c
40c
|
25.2
25.6
31.2
|
|
Amylase
|
|
Starch
|
|
|
Medium-grain
rice
|
|
Medium-gran
rice
|
|
|
Control
|
1.24
|
Control
|
24.9
|
|
4c
|
1.18
|
4c
|
25.3
|
|
40c
|
1.03
|
40c
|
27.5
|
|
|
|
|
|
|
Longer
grain rice
|
|
Long
grain rice
|
|
|
Control
|
1.55
|
Control
|
19.7
|
|
4c
|
1.46
|
Control
|
20.3
|
|
40c
|
1.10
|
40c
|
22.7
|
(sources
J. Chrastil, 1990)
Starch
composition of milled rice largely determines the cooking and eating
characteristics of rice. Starch consists of amylase and amylopectin. Amylase is
a long, straight starch molecule that does not gelatinize during cooking long
grain white rice has the most amylase and the least amylopectin. Amylopectin is
a highly brached molvecue that makes the sticky when it’s released from the
grain during cooking. Medium grain rice has more amylopectin.
Rice can thus be generally
categorized into
1. Waxy
(or glutinous) rice, which is mainly amylopectin and has an opaque endosperm.
These high amylopectin starches endosperm. These high amylopetin starches are
available with a range of viscosity and stability characteristics
2. Non-waxy
rice, which is mainly amylase and usually has a translucent endosperm.
Volume expansion and water
absorption of milled rice during cooking increases with increased amylase
content. Cooked high-amylose rice is less tender, but drier and flakier (not
sticky) than intermediate or low amylase rice, but becomes harder on cooling.
After cooking, low amylase rice remain glossy, stocky and moist (Damion and
Ben, 2006)
2.3 AMYLOSE
CONTENT
This is considered one of the most
important quality factors of rice since it is known that amylose content is a
good index of water absorption and textural properties of rice. The texture of
cooked rice and its gloss are principally determined by the amylase,
amylopectin ratio of the starch. Increasing the amylase content improves the
capacity of the starch granule to absorb water and expand in volume without
collapsing because of the greater capacity of amylose to hydrogen bond or
retrograde. Thus, the amylase content is an index of resistance to
disintegration during cooking. It has been shown that an increase in amylose
content improves the firmness of cooked parboiled rice, the correlation
coefficient ® between amylase content and firmness being 0.62. amylase content
of milled rice is determined by the colourmetric method is the starch-iodine
blue test which is used for screening amylose content of sample with less than
30% amylase. The most widely used method involves digesting 100mg of rice flow with lml, 95% ethanol and
9ml of NaoH heating the sample for 10m in a boiling eater bath and diluting the
gelatinized starch with distilled water, 5ml of the starch solution is pipettes
into a 100ml volumetric flask lml of in acetic acid and 2ml of iodine solution added made up to volume
and absorbance read at 620m. amylose content is determine by reference to a
standard curve.
The
amylase content of some Nigeria rice varieties has been shown to very between
19% and 28%. Various workers have shown that parboiling does not effect the
amylase content of rice (Adeyimi 2006)
2.3.1 AMYLOSE
CONTENT AFFECTS RICE COOKING AND EATING QUALITY
|
Amylase
content
|
Term
|
|
0.2%
|
Waxy
|
|
8.20%
|
Low
amylose
|
|
21.-25%
|
Intermediate
|
|
>25%.
|
High
amylose
|
- 100
amylose rice are moist, skicky, and glossy hen cooked
- They spit and disintegrate when over
cooked
- High
amylase rice cook dry and fluffy but become hard when cool
- Intermediate
amylose rice are fluffy when cooked and remain soft when cool(Andrew 2006)
2.3.2 EFFECT OF COOKING
Cooking quality is one of the
important aspects of food quality. However, nutrition’s a food material may be,
it will not be accepted by the consume unless it satisfies specific culinary
characteristic amylose dispersion is responsible for greater absorption and
retention f water and thus the expansion of rice grain (Chungcharoen 1987).
2.4 GRAIN
SHAPE
Long Grain
The category known as long grain
contains milled rice that is approximately three times longer than it is wide. Conventional
U.S long grain rice has an intermediate gelatinization temperature and from 19
to 23% apparent amylose content (Houston 1972).
After cooking it is firm and fluffy,
consumers in areas of the world such as North and South America, southern
China, Europe, and the middle East often prefer this type of rice. California
long grain rice generally has slightly highly apparent amylose content and
lower gelatinization temperature compared to southern grown U.S long rice.
Medium Grain: The medium grain rice category describes milled rice
that is from 2.1 to 2.5 times longer than it is wide U.S. medium grin rice
after cooking is soft, moist and sticky in texture. This type of rice is in
general preferred by people from Japan, Northern China and North and south
Korea (Houston 1992) medium grain rice is generally lower in amylose content
and has a lower gelatinization temperature compare to U.S conventional long
grain
Short Grain: Rice that is
less then two times longer than it is wide classified as short grain. In
general short grain rice has cooking quality, amylose content and
gelatinization temperature similar to that of rice in the medium grain
category. Because this type o rice is used for making sushi some call it sushi
rice.
2.5 RICE
PRODUCTS
Virtually all the rice produced in
Nigeria is used for human consumption. The increase in consumption has been
predicted to continue at a rate faster than the rate growth in population rice
apart form being eaten as a while grain can be used in food industry in the
processing of some products such as infant foods, break fast foods, snacks,
rice flour and for production of alcoholic beverages such as beer and rice wine
etc (Ihekoronye and Ngoddy 1985)
Break fast food and baby foods” rice
are pre-cooked, dried, flaked formed and expanded and toasted into ready to-eat
break fast foods. The rice flour can be made into baby food since is absolutely
gluten free. Some people are allergic to gluten and may turn rice based product
(wade 1972).
Noodles: The resultant rice flour can equally be made into
noodles, a mixture of rice and egg being
made into dough and expressed through holes. The noodle is improved by the use
of local gums, okra gums and gum Arabic (NIFEST, 1999).
Beverages: Various
aleoholic beverages are manufactured From rice. Rice is also used in the
brewing industry for the manufacture of beer and in the production of wine.
(Yashizawa and Kishi 1985)
2.6 RICE
QUALITY
Rice breeders considers rice quality
as the most important factors after yield, that determine the accept ability
and price of processed rice. The physio-chemical characteristics of rice
determine it quality. The term quality refers to milling quality appearance and
market acceptability, cooking eating qualities. Storage and nutritional
quality. Rice quality changes continuous through the entire post-harvest
system. Some of the changes are negative, but grain milling quality can also
improve during storage (Bradley, 2007).
2.7 FACTOR
AFFECTING QUALITIES OF RICE
Rice qualities are affected by many
factors, which may vary according to the standard on which it will be sold
(grist 1986). The degree of milling, percentage of damage kernel and percent of
broken are all important factors.
Milling degree: Milling degree is an
important factor for quality. This depends on the size and shape, i.e on the
variety, conditions under which it was grow, the degree of ripeness and amount
of exposure. Milling removes the bran layer. Rice bran is about 20% oil and is
subject to very aggressive enzyme activity.
Broken Rice: Broken rice get soft and mushy cooked from a
nutrition standpoint, broken are the same as whole grain. But the texture is poor
and broken tend to have more bran, which affect flavour (Liu et al., 1998).
Nature of Starch: The nature of
the starch affects texture and retro graduation properties of rice. The nature
of rice starch differs depending on variety, growing area, and crop year. There
are two different types of starch. Amylose is a long chained starch without a
lot of branding and amylopectin is a shorter but much more highly branched
chairs of element.
The amylopectin/amylose ratio does
not exist a very important factor.
Age of Rice: Age creates
changes in nature of starch. Rice gets harder with age as if the percentage of
amylose is increasing. The rice gets former and less sticky most older rice
losses moisture and so the cracking problem during cooking occurs if there is
any brain on the surface of the rice due to loose milling, then rancidity
problems causes poor flavour.
Moisture: The moisture
of the rice prior to cooking also affects texture. If the moisture is below 13%
the quick involvement of water will cause cracking. The lower the moisture, the
greater the cracking. Rice that cracks will have a poor cooking texture.
Cracking of Rice: This is
probably the biggest factor affecting the quality of cooked rice. The quick
migration of moisture through rice causes cracking kernel. Quick changes in
temperature can cause cracking. The cracking can occur in milling where milling
temperature gets high or as a result of pressure applied in milling. Also
cracking releases enzymes that affect flour.