Cooking As A Science: cooking is an
important characters but-determines the properties of the cooked texture (Katum et
al, 20CS). Cooking
quality of rice
is mainly determined by water uptake/ volume expansion, kernel elogation (Farua, 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 of cooking is still considered to be good quality milled rice by the working class
people who do not care whether the expansion is lengthwise or crosswise.. 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 when water is present at sufficiently
high temperature, the starch undergoes a gelatiniation
reaction (Takeuchi 1984) for temperature below 501V , 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%c 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 hydralion of the
starch granules (Whistler 1984).
Rice starch is usually digested quite rapidly compared to other foods such as a noodles, sweet potato or
cassava. 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 amylase
content
of milled rice has been found to correlate positively with hardness values of
cooked rice and negatively with stickness value (Perez et al
between cooked rice that is moist and sticky or cooked rice that is dry or flaky. Milled rice
amylose content can be
classified into:
High amylp content: If the
nee grain will show^lugh
volume expansion (not necessarily/elogation) and ajtfgh degree of flakiness, with
stickness value (Perez et al between cooked rice that is moist and sticky or cocked rice
that is dry or flaky. MillecUice amylose content can be classified into:
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. <*
Intermidate amylose content
20-30 percent amylose
is classified intimidate amylose
content, which is preferred in most rice growing areas of the world. <* Low amylose
content 10-20 percent amylose content is classified as low. (www.knowledgebank.irriorg)
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 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)
Elatinization
Temperature: Gelatinization tmperature 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 gelatinization temperature-! milled rice is
classified as low, intermediate and high.
Low
gelatinization temperature: Varieties
with gelatinization temperature of 70°c have low
gelatinization temperature
Intermediate gelatinization temperature of between 70-74°c
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 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 leaS loss of thiamin 30-50%, riboflourin 25-35% and niacin 25-50% (Saunders, 1979). This is to show that however nutritional a new volume displaced by the sample was carefully and quantitatively transferred from the measuring cylinder to the flask for cooking of the grain.
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 leaS loss of thiamin 30-50%, riboflourin 25-35% and niacin 25-50% (Saunders, 1979). This is to show that however nutritional a new volume displaced by the sample was carefully and quantitatively transferred from the measuring cylinder to the flask for cooking of the grain.
The cooking was
done on an electric stove. The time at which cooking
started was recorded at this point the gelatinization time as
temperature was
recorded.
The grain are choeked closely to know when property cooked by feeling
the grains texture in between two fingers when the grains are soft to press, very
tender in texture than, the time was recorded as the cooking
time.
DETERMINATION OF STARCH CONTENT
Starch determination
was done using (Onwuka 2005)
method. About 2.5 g rice flour sample was mixed with 50ml cold water and
allowed to stand for Ihr. with the addition
of 20ml concentrated HCL and 150ml distilled water, and then reflex for 2hr in
a 250ml round bottomed flask, it was cool and neutralize with 5N NaoH make up to the mark with distilled water the glucose content is determined using anthrone
reagent prepare series of glucose
solution such the 1ml contains 0.04-0.2mg use these to calibrate the glucose standard curve to 1ml of
each of glucose standard solution and
a test sample in test tubes add 5mls of anthrone reagent and mix properly cover the tubes and boil in water
bath for 20 minutes for the colour to develop cool the tubes and read the absorbance at 620nm against a blank containing
only 1ml of water and 5ml of anthrone
reagent and the concentration of the test sample is obtained from the
absorbance by interpolation. The mass of the glucose is obtained by calculation
involving the concentrations and dilutions
made. The mass of starch is consequently obtained from the mass of glucose
using the relation.
Mass of glucose x
Q.9 Mass
of starch
DETERMINATION OF PROTEIN
Protein determination
was done using Kjeldaahl method. Abut 2g
of grinded rice was
weighted in metier weighing balance followed by 5g of Naso4 Ig of
CusO4, 25ml of H2so4 concentration and one tabulate of kjldahl (serenium catalytic tabulate) The
whole regeant and grinded sample was allowed
to digest by heating on a heating mantle until the
41
whole mixtures turn
green in colour. During the stage, it was shaked to obtain the homogeneity in
the flask. After the digest/ the sample was dissolved with distilled water make up to
250ml volumetric flask.
However, during the distillation stage boric acid solution indicator was prepared by 4g in
100ml of distilled water pluse 2 drops of methyl red which turned to pink colour. Also
NaoH solution was prepare by 60g in 100ml of distilled water (60%) then 5ml of
digested sample plus 5 of NaoH were poured in boiling flask manted on serial
heating matle at a temperature of 200°c for 5-7 minutes until the bubles stop
coming up which
is to be absorbed in a 5m of boric acid solution containing in a conical flask.
Finally, the boric
acid plus NOs was titrated with dilute Hcl which turns pink colour of boric acid into orange colour the reading were
recorded from
burette. % Nitrogen = Vs x N acid x 0.041 x 100
W x
1 Vs =volume (ml) of acid required to titrate
sample
N =Normality of acid =0.1N W
=weight of sample in grams
Therefore,
conversation factor = 6.25.