Abstract
Parboiling is the hydrothermic treatment given to ro
soaking, steaming and drying. Due to parboiling treatment starch gelatinization is
taken place, which brings about the changes in the physicochemical properties of
rice. More than 70% of world's production of parboiled rice is produced in the
Southeast Asia, where small-scale production is popular. Parboiling is done using a
big vessel or locally-make small boiler having about 100 and 200 kg capacity per
batch. For improper distribution of steam a heterogeneous mixture of parboiled rice
is produced. Due to lack of suitable processing equipments and methods quality of
parboiled rice varies from plant to plant. Lower quality of parboiled rice loses the
market value and consumer's acceptability. Therefore, for wider acceptance
processing equipment and method should be developed to produce the improved
quality of parboiled rice.
The quality of parboiled rice was evaluated with the physical properties,
namely, maximum viscosity, hardness of brown rice, hardness and adhesion of
cooked rice, volume expansion ratio and solid content. It was found that severely
treated rough rice produced the worse quality product. A first-order model
predicted well the effect of processing conditions on the maximum viscosity and
hardness of parboiled brown rice, indicating the quality index and the rate of
change of quality with their respective final and rate constant values. It was
suggested that processing equipments and methods should be developed for
producing the improved quality of parboiled rice.
A laboratory-scale experimental setup was fabricated to produce the parboiled
rice at lower temperatures (80 to 1000C) to identify the suitable processing
conditions for producing the improved quality product. The change of the material
(rough rice) temperature in the steaming vessel was investigated for different
sample sizes. A sample size of 200 g was found to be adequate for producing the
parboiled rice. The physical properties of parboiled rice produced using the
experimental setup were compared with that of produced at higher temperatures
(110 and 1200C). Parboiled rice was prepared at different processing conditions.
The quality Of parboiled rice was affected significantly with the processing
conditions. The quality index was identified with the final value of the quality
indicators using a first-order model. The improved quality of parboiled rice was
achieved at the lower temperatures of 80 to 1000C than that of at higher
temperatures of 110 and 1200C. The processing conditions of 800C-40 min, 900C-20
to 30 min and 1000C-15 to 20 min were recommended for producing the improved
quality of parboiled rice. The quality index of 58.0 to 63.5 and 17.0 to 21.0 for
lightness and color values, the reasonable lightness and color values, 4.0 to 9.5%
higher milling yield having 50 to 80 N hardness value were achieved for the above
processing conditions comparing with the untreated sample.
As those of physicochemical properties the thermal properties, i.e.
gelatinization parameters of parboiled rice are also affected with the parboiling
treatment, which ultimately affects the cooking qualities. The effect of processing
conditions on the gelatinization parameters was studied. Gelatinization parameters,
namely, onset (To), peak (T p) and completion (Tc) temperatures, and the residual
gelatinization enthalpy (A1-I) were investigated. The To, Tp and Tc increased, and the
A1-I decreased due to increasing the severity of the heat treatment during parboiling.
The peak temperature, which represents the half conversion temperature of the
sample melting, was assumed to be a suitable quality indicator to identify the
severity of the heat treatment during the parboiling process. The quantitative peak
ii
temperature values of 77.4 to 79.30C can be considered as the index for the
improved quality of parboiled rice. It was assumed that gelatinization parameters
might be used to get information about the cooking behavior of parboiled rice.
The effect of the processing conditions on the cooking qualities of parboiled
rice was studied. The cooking qualities, such as, water uptake, solid content and
cooking time were investigated. The water uptake and solid content decreased, and
the cooking time increased according to the severity of the heat treatment. Cooking
time of parboiled rice can be determined on the basis of water uptake value of 2.5.
The higher transition temperatures were responsible for the cooking behavior of
parboiled rice. The improved quality of parboiled rice produced using the proposed
processing method needed about 20 to 24 min cooking time, which possessed 6 to
7% solid content comparing to the untreated one having cooking time and solid
content of 20 min and 6%, respectively.
The major problem for the small-scale parboiling plant in the developing
countries is improper heat distribution in the parboiling vessel, which affects the
quality of the final product. The above problem can be minimized using the
proposed parboiling vessel having the perforated bottom. The construction of the
vessel is very simple with .the locally available material. The proposed
experimental setup should be scaled-up for adapting in household and commercial
purposes. The qualities of parboiled rice produced using the proposed experimental
setup and processing conditions were closure to the untreated sample. This implies
that the proposed parboiling setup and processing conditions are quite applicable
for the quality improvement of parboiled rice.