Cooking is a science: cooking is an
important characters that determines consumers preference, these properties determines the cooked
texture
Khatum
et al, (2003).
Cooking quality of rice is
mainly determined by water uptake/ volume expansion, kernel elogation Becker (2003) cooking quality of rice depends on the amylose content and
gelatinization temperature Juliano (1979) However, still on preference, high volume expansion and lengthwise expansion of kernel during cooking are more desirable traits of good quality rice high volume expansion on cooking is still considered to be good
quality
of milled rice by the working
class people who do not care whether the expansion is lengthwise or crosswise.
Sood and Sadiq (1979). Urban people on the other hand prefers the varieties that expand more
in length than in breath. Choudlum
(1972).
Therefore the economic important of rice depends on its
cooking and processing quality. When water is present at
sufficiently high temperature, the starch undergoes a gelatiniation reaction Takeuchi (1984), for temperature below 500C,
the grains absorb a limited amount of
water up to approximately 30% moisture
content (Wet basis). The resulting grains are not cooked because the starch has
not undergone gelatinization from human experience with small sample, it is known that soaking rice grain in
water at 25% for about one hour is required before cooking at a temperature above 70°c for 20 minutes or more. As
water is taken up by a rice particle, the starch undergo a
gelatinization reaction. The term used to
describe the swelling and hydration of the starch granules (Whistler 1984).
Rice starch is usually digested quite rapidly compared to
other foods Starch foods such as
a noodles, sweet potato or cassava, this leads to a prompt and pronounced increased of the
blood glucose level (high glycemic index ) after the ingestion of rice Frei and
Beaker (2004) .
Rapid
starch digestion can cause a sensation
of hunger only shortly after the ingestion of rice and the energy released is quickly used, farmers cultivating rice land races in the Philippines reported a relatively long feeling of
sensation after the ingestion of certain varieties Oko et al
(2012)
Thus far, most of
the published research on cooking quality in rice has focused on physico-chemical properties i.e Amylose
content (AC) Gel consistency (GC) and gelatiruzation
temperature (GT) which are directly related to
rice cooking quality. (Perez 1987)
Amylose content
Amylose content is apparently, reported for rice samples
to ranges from 24.6-28.8% and the level vary within
each location. Amylose consists of linearly linked glucose molecules
and is relatively resistant to digestion, hence
the term "resistance starch" This means that rice with greater proportion of starch in the form of amylose tend
to have a lower glycemic index. Amylose content of milled rice
has been found to correlate positively with hardness values of
cooked rice and negatively with stickness value
(Perez et al 1987) amylase content also makes difference between
cooked rice that is moist and sticky or cooked
rice that is dry or flaky. Tester and
Morrison (1990) reported that Milled
rice amylose content can be classified into:
v High amylose content:
If the rice
grain will show
high volume expansion (not necessarily elogation) and a high degree of
flakiness, the rice grain cooks dry, they are less tender and becomes hard upon
cooking 25-30 percent amylose content is
classified as high.
v Intermidate
amylose content 20-30 percent
amylose is classified as intermidate
amylose content, which is preferred in most rice growing areas of the world.
v Low amylose content 10-20 percent amylose content is
classified as low. (www.knowledgebank.irriorg)
2.6.2
Gel consistency
The gel consistence measures the tendency of the cooked
rice to harden after cooking, it was developed to index
cooked rice hardeness (Juliano 1973) Research has
provide that amylopectin contributed more than amylose
to starch gel consistency and viscosity (Juliano 1993) If the gel of the cooked rice is hard, the cooked rice tends to be
sticky, harder gel consistency is associated
with harder cooked rice and thus future is
particularly evidence in high amylose rice, also
if the gel constancy is soft , the cooked rice has a higher degree of tenderness, this is a preferred characteristic
by most consumers (www.knowledgebankirri.org)
Gelatinization temperature
Gelatinization temperature determines the time required
for cooking in rice. Where as Juliano (1993) refers to final gelatiniazation
temperature of starch as the water
temperature at which, at least 90% of the starch
granules have gelatinized or swollen irreversibly in hot water. Based on this
gelatinization temperature of milled rice is classified as low, intermediate and high. Tester, and Morrison (1990).
v Low gelatinization temperature: Varieties
with gelatinization
temperature of 70°c have low gelatinization temperature
v Intermediate
gelatinization temperature of between 70-74°c
v High gelatinization temperature: varieties with gelatinization temperature
greater than 74 °c.
However if gelatinization temperature is very high, than
the rice will becomes excessively soft and disintegrates when over cooked. It
will also requires more cooking water than rice
with lower gelatinization temperature (www.knowledgebankirri.org).
The difference in quality which are mainly attributed to
difference in colloidal structure and the extent of swelling of any variety of
rice on cooking have always been used as index of
its quality Oko et al. (2012).
Therefore, milled rice nutrients are lost through some
cooking method such as washing before cooking which
is normally done to remove dirt but affects composition of cooked rice and
cooking in excess water (Kernedy 2004). Washing rice
prior to cooking is estimated to lead to losses of protein
(2-7%), potassium (14-20%), Thiamine (22-59%),
riborflavin (11-26%) and miacin (20-60%) (Juliano 1993), losses from washing and cooking methods used in Indian
calculated as follows protein 10%, iron 75%,
calcium and phosphorous 50% (Grist 1986). Cooking in
excess water than is desired can lead to loss of thiamin 30-50%,
riboflourin 25-35% and niacin 25-50% (Saunders,
1979). This is to show that however nutritional a food material may be, it will
not be accepted by the consumer unless It satisfied specific culinary
characteristics. Thus the grains of
cooked rice should be non-sticky as
well separated qualities that can be
Judge by swelling number water uptake and
amylase content (pushpamma and
Uma Reddy 1979). Cooking improves their digestibility, promotes
palatability as well as making the rice soft
to eat. Rice starch has to be gelatinize during cooking or else
rice is too firm to be eaten comfortable
USA Rice Federation,( 2002).Heat also
help to sterilize the food by killing harmful bacteria and other micro
organisms and it increase the availability of nutrient.
These nutrient may be lost during cooking in two ways, first
by degradation, which can occur by destruction or by other
chemical changes such as oxidation or
secondly by leaching into the
cooking medium. Vitamins are susceptible
to both losses while minerals are affected
only by leaching. Rice varieties with high protein tend to have worse flavor, and less
tender, less cohesive when cooked in
some amount of water and longer
cooking times are needed because of water
absorption USA Rice Federation ( 2002). Since consumers choice of rice
varieties are largely based on grain and
cooking quality, the high rate of influx
of hybrid varieties and other new rice
varieties to local farmers in the three
zones of Ebonyi state who abandon their hitherto cherished
indigenous varieties without
critical comparison is
unacceptable.