Rice processing
is of great importance as this affects the quality of rice. The technique
employed effects product quality in terms of colour, breakages, conveniences, product
yield, time and equipment used.rice is harvested as a coated caryosis. The moisture
content of rice at harvest is usually 18-25% and drying is necessary before storage
and milling.
This drying may be achieved using air currents through ducts in
the floor blown through the rice in drying sheets. Heated air is used in some
countries while the use of sun or wind drying is also widespread. During
drying, moisture content is reduced to 12% over a period varying from 4-5 days up
to 2 months, depending on the drying method used. Open air drying by the sun
allows contamination with dirt and stones and a subsequent loss of quality and
yield.
Rice milling
This is shown in
Figure 2.2. The first step is the removal of the hull. The grain is fed in
between two stone discs, separated by slightly more than the length of grain,
with the top disc capable of rotation. Another type of dehuller uses a
horizontal endless rubber belt which passes beneath a rubber roller. This
produces a shearing action on the grain and thus removes the hulls. During this
step the hulls are split and fall from the kernel. The grain and hulls are then
readily separated by air currents. Methods involving crushing the hulls by
winnowing are also used. The grain thus obtained is the bran. The next step is
the removal of the bran by either a wet brushing process or a dry abrasive
method. The product of this step is unpolished milled rice. The grain is still
enclosed by the aleurone layer. The final step is polishing the grain using a
brushing process which removes the aleurone layers and any other particles
adhering to the grain. The polished rice may be further treated by coating with
sugar syrup and talc and tumbling in a machine called a rice tumbol to produce
a brighter shine on the grains. The unpolished milled rice has a tendency to
lipid rancidity which is greatly reduced by the polishing process.
Whole rice grains
|
Pass through shellers: machine
contains abrasive discs which rub off the outer layer of hull
|
Air separation to remove hulls
|
Brown rice
|
Pass through milling machines:
abrasive machine removes inner layers of bran and dislodges the germ
|
Air separation to remove bran and
germ
|
Endosperm
|
Pass intact grain through rice
polisher to obtain a white high gloss finish
Figure: schematic diagram for producing milled rice
Source:
Ihekoronye and Ngody 1985
Parboiling
The process of
parboiling involves soaking of the rough or paddy rice with water, followed by
drying back to 12% moisture. The parboiled material is then dehulled and milled
as described above.
Parboiling
of rice is an ancient tradition in most countries of the tropics; the original
purpose was to loosen the hulls, however, in addition, there are a number of
other advantages relating to the increased nutritive value of parboiled rice.
Water dissolves vitamins from the hull band bran case of wheat, the bran of
rice contains much higher levels of vitamins than does the inner part of the
grain. Rice hulls are also higher in vitamin content than the inner part of the
grain.
The
parboiled grain resists breakage during milling to a greater extent than
non-parboiled rice, and parboiled rice is less liable to insect attack; it
keeps better than milled rice; and it is more resistant to rancidity. Although
parboiled rice is nutritionally better, its acceptance in many countries is
limited because of its colour and flavour.
Small scale milling – the system approach
Too
frequency countries use a piecemeal approach to improving their post harvest
operations. That is, in one program the nation attempts to better its storage
facilities, and so on. Although such efforts are beneficial, greater savings
are possible when the entire post harvest system is attacked. A systems approach
to cutting losses and improving efficiency, for example, many include matching
the cleaning and drying of the paddy to the purchasing program; adjusting the
storage capacities to the receiving and milling schedules, or matching the
milling capacities and facilities to the storage system and to the rice
distribution requirements. This sort of approach provides maximum utilization
of existing facilities, and minimum investment in new facilities.
A
complete technological package for rice processing for small-scale rural
facilities in the capacity range of 0.5 tonne of paddy per hour has been
developed by the department of Food Science and Technology, University of
Nigeria, Nsukka. The system consists of the following components:
1.
Boilerless Parboiling Unit (Figure
2.3)
· Rapid soaking
and steaming within a total of 4 hours
· Uses solid fuel or rice husk as fuel
· Conceived in modular units of 100 kg/unit; soaking
and steaming are possible on the
same unit
· Designed and
built for high thermal efficiency
· integral
material handling using overhead trolley/ chain block hoisting arrangement
· Local
fabrication at low- cost
2. Self – Contained Heated – Air Dryer
· Flat- bed design
· Tonne capacity
per batch
· Self- propelled
with diesel engine driven centrifugal fan and dieselized air heating
· Large component
of local fabrication and assembly at comparatively low-cost.