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The Journal of Food Technology in Africa, Vol. 6, No. 4, Oct-Dec, 2001 pp. 130-132 Effect of parboiling on physico-chemical qualities of two local rice varieties in Nigeria
1B. O. Otegbayo*, B.Sc. M.Sc, 2F. Osamuel, B. Sc. Msc., 3 J. B. Fashakin, B. Sc., M.Sc. Ph.D
1 Department of Food Technology, University of Ibadan, Ibadan, Nigeria Code Number: ft01034
Summary
Two varieties of local rice in paddy form were used for this study. The rice were collected from local farmers at Okemesi Ekiti in Ekiti State of Nigeria (western Nigeria). The rice were divided into halves, one half was processed by Parboiling, drying and milling, the other half was processed by drying and milling only.
The result from the study showed that parboiling affected the physico-chemical qualities of the rice varieties. There were differences in the physical dimension, appearance, colour, water absorption, cooking time, amylose, protein, fat and carbohydrate contents of the parboiled and non-parboiled rice samples. Varietal differences also exist between the rice samples.
Parboiling reduced the breakage, fat, protein and amylose content of the rice while the cooking time, water uptake and thiamine contents were increased. It can therefore be inferred that parboil mg which has been the means of processing rice in Nigeria can be a way of improving vitamin content and milling properties of rice and should aifract the interest of food technologists and food processors to develop the rice industry. Introduction
Rice is one of the leading food crops in the world and a favoured cereal in the diet, appearing often in different forms. It plays a very important role in the diet of many Nigerians because it is a good source of carbohydrate. The protein content of rice is low but studies have shown that it is comparable to that of wheat, while its digestibility is high compared to other cereals (Ngoddy and Ihekoronye, 1985). Rice as an economic crop is also important in household food security, nutritional diversification, income generation and employment, hence it helps in poverty alleviation.
In Nigeria the variety commonly cultivated is Oryza glaberrima steud, it is utilised mostly at household level where it is consumed as boiled rice, fried rice, ground rice and tuwo shinkafa. At the industrial level it is used as brewers rice in brewery, where it is used as an adjunct and as component of weaning diet. It is mainly processed by parboiling, parboiling is the hydrothermal treatment of paddy or pre-cooking of the rice within the hull. It has been reported by many authors (Juliano, 1970, Bhattarcharya and Ah, 1985 and Adeyemi et al 1986 respectively) that parboiling affects the physico-chemical and cooking qualities of rice. Since many varieties of rice exist and there are variations in its utilization and preference depending on the consumer, there will be differences in the effects of parboiling on the different varieties. The objectives of this study therefore are to study the effect of parboiling on the physico-chemical composition and cooking quality of two local rice varieties in Nigeria and determine the nutritional changes that may occur due to processing. Materials and Methods
25 kg each of two local rice varieties in paddy form with local names
Offada (white variety) and Alasoosun (Brown variety) were collected
from local farmers at Okemesi-Bkiti, Nigeria. The rice varieties had been stored
for 180 days after harvesting. Half of each variety was parboiled while the
other half was not parboiled. Cleaned paddy were soaked in water at room temperature
(below the gelatinisation temperature of the rice to minimise the splitting
of the grains), to hydrate the grains in a steeping tank for 5-8. Parboiling
was done in the laboratory by the pressure parboiling method of Inenga et
al (1980). In this method the paddy rice was not saturated with water but
briefly exposed to steam under high pressure using an autoclave. The paddy rice
was then exposed to steam to gelatinise the starch for 15 minutes at 15 p.s.i
g.pressure (1210c). The parboiled paddy rice was then tempered for
30min to cool and air-dried in a cabinet dryer at 450c for
8 hours. Both the raw and the parboiled rice samples were milled in a grantex
mill. Samples for physicochemical analysis were subsequently ground in a hammer
mill and sieved in a sinon laboratory shaker, using 10 Nylon ~71m aperture)
sieve. Flour fractions ( Physical dimension (length, breadth and shape) was determined by randomly picking
twenty whole grains and measured by means of verner callipers, the mean was
then calculated. Percentage breakage was determined by the method of Dimopolus
and Muller (1972) in which percentage breakage is defined as the amount of grains
with less than 3/4 size per 1 OOg of milled rice. Degree of parboiling was determined
by the method of Luh and Shun Lu (1991). Cooking time and water absorption were
determined by the method of Bhaffarcharya (1979). Moisture content, Crude protein
(%Nx5.95), Fat, Crude fibre and Ash were determined on triplicate samples
by AOAC (1997) methods. Amylose was determined by the method of Williams, (1970).
Thiamine was determined by the gravimetric method of Bessot, (1940) which is
based on precipitation of thiamine as thiamine hydrochloride with tungtosilicic
acid.
Results and Discussion
The parboiled rice kernel became translucent and glassy unlike the non-parboiled
kernel that is white and opaque. This occurrence is likely due to the to gelatinisation
of starch and disruption of protein bodies which expanded and occupied all the
air spaces in the endosperm during parboiling as reported by Rhagavendra, Rao
and Juliano (1970). The opaque and white belly caused by loose arrangement of
starch granules therefore disappeared making the kernel translucent.
The rice samples are long grain type according Webb and stermer (1972) method
of grain classification. However the parboiled rice kernel has a shorter length
and broader breadth when compared with the non-parboiled rice sample. This is
in agreement with the result of Rhaghavendra and Juliano (1970) that parboiled
rice expanded less in length but more in breadth.
The degree of parboil mg which is a measure of severity of the heat treatment
the rice was subjected to can be said to be higher in the white parboiled samples
than the brown parboiled samples. This is measured by the percentage of the
rice grain that disintegrated in dilute alkali. The greater the disintegration
the higher the degree of parboiling. This may also explain the reason why the
percentage breakage in the brown rice variety (parboiled) is higher than that
of the white variety. It may be due to incomplete parboiling which results in
a "white belly" kernel, which causes increase breakage during milling.
The percentage breakage in the parboiled rice samples is lower than that of
the non-parboiled samples. This is as a result of hardening of the grains after
parboiling which reduces the breakage and the milling quality.
The cooking time of the parboiled rice samples was between 52-56 mm while for
the non-parboiled rice it was 45-49 mm. The two varieties of rice generally
have a high cooking time when compared with 10-25 mm reported by Adeyemi et
al (1986) and Rhagavendra and Juliano (1970). The longer cooking time of
the parboiled rice samples compared to the non parboiled samples may be due
to the strong cohesion between the endosperm cells which are tightly packed.
This makes the starch grains to hydrate at a slower rate, which leads to a decrease
in water penetration into the grains, hence a longer cooking time. Juliano and
Perez, (1986) found that the higher the protein content of rice, the higher
the gelatinisation temperature hence, cooking time. This is ostensibly due the
hydrophobic nature of proteins, which act as a barrier to inward diffi~sion
of water into the cooking grain, and hence raise the gelatinisation temperature.
The water absorption of the parboiled samples were higher than that of the non-parboiled
samples, while the water absorption of the white variety was higher than that
of the brown variety. Mustapha (1979) in his study on physico- chemical qualities
of rice stated that parboiled rice has higher water absorption, which may be
as a result the steaming pressure during parboiling which in turn affects starch
gelatinisation.
There is decrease in protein content of the parboiled rice samples compared
to the non-parboiled samples, which may be due to leaching of protein substances
during soaking and rupturing that occurs in the molecules due to steaming. The
process of parboiling makes the protein bodies to sink into the compact mass
of gelatinised starch grains, making it less extractable hence a decrease in
the protein content. There is no soaking or steaming process for the non-parboiled
samples though little loss in protein content may occur during mi~ling, but
this is incomparable to what happens during parboiling, hence it has a higher
protein content than the parboiled samples. However the white variety has a
higher protein content than the brown variety. The parboiled rice samples also
have lower fat content than the non-parboiled samples. This may be explained
in terms of leaching and rupturing of the oil globules that occur due to increase
in temperature and steaming pressure that occurs during the parboiling process.
The carbohydrate content of the parboiled rice samples was higher than that
of the non-parboiled samples. This may be as a result of starch gelatinisation,
which makes the grain to expand, thus filling up the surrounding air spaces.
Starch re-associatiort, increase in some carbohydrate components like reducing
sugars, change in molecular size and partial dextrinisation of starch which
have been known to occur during parboiling. (Rhaghavendra and Juliano, (1970).
There was slight increase in the thiamine content of the parboiled rice samples.
This agreed with the findings of Gariboldi, (1974) that it may be due to the
fact that during steaming, water soluble vitamins are spread throughout the
grain, thus altering their distribution and concentration, with inflision of
thiamine from the germ into the starchy endo sperm. There is also a decrease
in Amylose content of the parboiled rice compared with the non-parboiled rice
samples. This is because of starch solubilisation and leaching of the amylose
molecules into the surrounding water during soaking and subsequent steaming
during parboiling. The differences in chemical composition of the two rice varieties
may be caused by variety and environmental factors such as location of field,
planting season, time and rate of nitrogen fertiliser Conclusion
Parboiling as a means of rice processing affects both the physical and chemical
properties of the grain. It improves milling and cooking qualities of the rice
grains in a positive manner which has been found to influence consumers demand
and acceptability The increase in thiamine content of the parboiled rice can
also serve as an effective means of improving the thiamine intake in peoples
diet thereby enhancing their nutritional status. The two rice varieties used
have protein content between 6.86-8.75%, this therefore means that the
rice could be a major source of protein in the diets of Nigerians if consumption
is adequate.
References Copyright 2001 The Journal of Food Technology in Africa, Nairobi |
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