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Rice parboiling improves quality
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By Dr M. Farooq, Dr Shahzad
M.A. Basra & Dr Abdul Wahid
PARBOILING is partial cooking of rice to gelatinise the starch
and harden the endosperm, making it translucent. There is
also a slight change in flavour which some people prefer.
The toughening process makes the seed more resistant to
insect attack and to shattering during husking. It also
helps prevent absorption of moisture from the air during
storage.
 Parboiling consists of three steps, soaking, steaming and
drying.
SOAKING: The main objective of soaking is to achieve
quick and uniform water absorption. The lower the water
temperature, the slower the soaking process. However,
temperature should not exceed the gelatinisation temperature
or the paddy will be cooked. Gelatinisation is the process
by which starch granules change to a gelatinous or jelly
form, filling the voids and cementing the fissures in the
grain. Soaking time can be reduced by first subjecting paddy
to a vacuum for a few minutes or by soaking it under
pressure in hot water.
Rice soaked in water at ambient temperature (20-30°C) will
take 36 to 48 hours to reach 30 per cent moisture content.
In hot water (60-65°C), it will takes only two to four
hours. If soaking time is too long, part of rice dissolves
in water, the seed begins to germinate, and starch
fermentation occurs.
Water temperature and length of soaking time affect the
solubility of substances in rice as well as colour, smell,
and taste. The mineral content, sulphides , and pH of the
soaking water also affect the results of the process. During
cold soaking, starch fermentation occurs because of
respiration of paddy grains and the release of carbon
dioxide. Fermentation can cause an off-flavour and can lead
to excessive development of fungi and other micro organisms
in paddy.
During hot water soaking (60-65°C), the grain absorbs
moisture faster and reaches a moisture level of 30-35 per
cent in two to four hours depending on the variety.
Respiration and fermentation are restricted. The shortest
period of soaking time is achieved by maintaining a constant
water temperature. This requires continuous recirculation
and reheating of water. Hot soaking keeps the grain at a
higher temperature which will reduce the steaming time
needed to complete the process.
After soaking is completed, water is drained with moisture
around 30 per cent for either cold or hot soaked paddy. The
amount of water required for soaking paddy is about 1.3
times the weight of paddy.
STEAMING: Use of steam for gelatinising the starch is
preferred to other methods of heating as it does not remove
any moisture from rice. Condensation adds water and
increases the total quantity of water absorbed. The moisture
content of paddy increases to about 38 per cent during
steaming.
When heating paddy with non-pressurised small variations are
found in colour, quantity of soluble starch, and the amount
of swelling of the milled parboiled rice. Heating has a
considerable effect on colour. When the steaming temperature
exceeds 100°C, the colour becomes considerably deeper and
the grain becomes harder. Longer steaming times also cause
rice to be harder and darker. Keeping steamed paddy in a
heap on the drying floor is equivalent to prolonged steaming
and induces the same effect. Saturated steam at a pressure
of one to five kg/cm is normally used for steaming. Steaming
duration depends on the steaming arrangement.
DRYING: Parboiled paddy should be dried to 14 per
cent moisture for safe storage or milling. Parboiled paddy
is more difficult to dry and requires more energy than field
paddy because its moisture content is much higher. However,
higher air temperature help reduce the drying time. If
drying is done too fast, internal stresses develop in the
grain and cause breakage during milling. After drying is
completed, paddy should be allowed to stand for at least
several hours -- preferably for one or two days -- before it
is milled, to permit internal moisture differences and
stresses to equalise.
Moisture reduction takes place rapidly during the first part
of drying from 36 to 18 per cent moisture level, but is slow
from 18 to 14 per cent. The drying process should be stopped
at about 18 per cent moisture to allow paddy to temper or
equalise for several hours before continuing the drying to
14 per cent.
Most parboiled paddy is sun-dried on large drying floors
close to rice mill. A large number of workers are required
to constantly turn and mix the paddy to achieve rapid,
uniform drying. For best results, paddy should be spread
about 2.5 cm thick over the floor.
In contrast with field paddy, parboiled paddy requires air
temperatures of up to 100°C during the first drying period.
During the second period air temperature should be kept
below 75°C. Maintaining higher air temperature will not
decrease the drying time but will result in increased drying
cost and more damage to milled rice quality. The first
drying period takes about three hours including dryer
loading and unloading time. After tempering, the second
drying period takes about two hours.
Continuous-flow dryers are available in many sizes to match
the capacity of the parboiling system. A 24-5/day parboiling
plant needs an eight ton (holding capacity) dryer.
Parboiling causes physical and chemical changes and modifies
the appearance of rice.
Change Description__Taste and Texture_Change in taste and
texture of the rice, preferred by some consumers and
disliked by others.__Gelatinisation of Starch_Gelatinization
of starch making the grain translucent, hard, and resistant
to breakage during milling which increases milling recovery
for head rice and total white rice yields.__Enzyme
Inactivation_Inactivation of all enzymes which stops
biological processes and fungus growth.__Milling_Easier
removal of the hull during milling but more difficult bran
removal.__Cooking_More rice swelling during cooking and less
starch in the cooking water.__
The following table provides potential advantages and
disadvantages associated with parboiling:
Rice millers in Pakistan are still reluctant to adapt this
useful technology. Rice mill owners and exporters should
realise the situation and should evaluate the usefulness of
parboiling the rice. We are confident hat the storage life
and export quality of premium quality basmati rice will be
improved by opting to this technology.
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Courtesy: The DAWN
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