2. Introduction
It is a hydrothermal treatment resulting partial cooking (gelatinization of starch) of paddy
(rough rice)
During parboiling irreversible swelling and fusion of starch granules occurs and changes
starch from crystalline to amorphous form
About 60% of total production of paddy is parboiled in India
Parboiling reduce breakage and loss of minerals & vitamins
3. Advantages
Overall increase in total rice recovery is about 6 %
High nutrients compared to raw rice
25% more oil in the bran compared to raw rice bran
Dehusking is easier and costs less (Higher productivity of rubber rolls compared to raw
paddy de-hulling)
Germination is no longer possible and the endo-sperm has a compact texture making it
resistant to attack by insect and microorganism
4. Disadvantages
More difficult bran removal
A great amount of water is absorbed during cooking causing the
rice to swell
5. Process
Step 1: Pre Steaming (First Steaming)
Step 2: Soaking
Step 3: Post Steaming (Second Steaming)
Step 4: Drying and Tempering
6. Soaking time and temperature
Steaming pressure, velocity, time and stages
Tempering time and drying time
Drying temperature and stages
Factors Affecting Quality of
Parboiled Rice
8. Paddy Pre steaming (First
steaming) Strategy
Pre steaming before soaking softens the paddy, to
achieve desired color of par-boiled rice
Increases the temperature of the paddy and reduces
the soaking time of the paddy
Increases the capacity of par-boiled rice to absorb
water during cooking
Industrial practice of pre-steaming time is 2 to 4
minutes
Steam temperature is 100 – 110 oC during the
first steaming
Energy required in steaming 1000 Kg of paddy:
24.42 kWh (21,000 kcal)
Use of Steam Flow Meters will improve energy
efficiency
To get uniform quality and high efficiency
steaming to be automated
9. Paddy Soaking
Soaking is a diffusion process
The diffusion pressure of dry paddy is zero and porosity is about 48%
Difference in vapor pressure between paddy and water results movement of water into Paddy during
soaking
Step 1: Water molecules first adhere to the husk surface and then penetrate through microspores of husk
Step 2: Water is retained in the voids of rice kernel due to capillary absorption
Step 3: Water molecules will be absorbed in starch granules and held as water of hydration
10. Paddy Soaking
Commonly practiced soaking paddy in hot water 6-10 h at 70 oC
1.3 m3 of water is consumed per metric ton of paddy
Moisture content of soaked paddy is about 30%-35%
Energy required in soaking is used in raising the temperature of water from 25 °C–85 °C:
83.72 kWh Heat energy
Soaking at elevated temperature prevents mycotoxins and also reduce the steaming time
11. Strategy for Efficient Paddy
Soaking
Paddy grain size should be uniform
Continuous recirculation, reheating coupled with aeration reduce the soaking time.
Optimum time-temperature combination for single stage soaking: 6h-65 oC
Accelerated can be achieved at soaking water pH (11)
Two stage soaking (67% time reduction compared to single-stage soaking)
Stage 1: Soaking at 80 °C/ 45 min and moisture content of 35.0% d.b.
Stage 2: Soaking at 70°C/ 195 min and moisture content of 42.7% d.b.
Heat from effluent after soaking can be used for preheating the fresh soaking water
12. Post Steaming (Second
Steaming) of paddy
Steaming is carried out to gelatinize starch
Saturated steam at a pressure of 1-5 kg/cm2 is
used for steaming of soaked paddy
Splitting of husk is indication of completion of
steaming
60 kg of steam is required to steam 1 ton of
paddy
Moisture content of paddy increases to 38%
(w.b.) during steaming
24.42 kWh (21,000 kcal) of energy required in steaming
1000 Kg of paddy
Improves grain length and uniforms the color of Par-
boiled rice
Steam temperature has a considerable effect on the
color of parboiled rice.
The color of rice is due to pigments of husk, bran and
absorption of reducing sugar that react with amino acid
13. Post Steaming(Second Steaming)
Strategy
Keeping steamed paddy in a heap on the drying floor is equivalent to prolonged
steaming and results in dark color of rice
Steaming of paddy completes once steam emerges after for 1 to 2 min steaming of
the batch
Steam at a pressure of 0.5 atm is used for the second steaming of paddy
Timeof gelatinization can be shortened by increasing superficial superheated-steam
velocity
Energy efficiency in steaming can be achieved by use of low pressure steam
14. Drying of paddy
Drying of parboiled paddy from 45-50% (db) to 14-16% (db) needs to be done for proper milling
and storage
The first drying period takes about three hours including dryer loading and unloading time. After
tempering, the second drying period takes about two hours.
159.30 kWh of energy required in drying 1000 kg of Paddy at 80 °C and air flow of 50 m3/min for
drying 4 h
One of the main reasons for rice kernel breakage is fissures which is developed due to Moisture
Content gradients occurred during drying
15. Drying Strategy
Paddy to be dried slowly in two passes instead of continuous drying. In first pass, paddy moisture content
should be brought down to 18-20% and rest in second pass.
Double-pass drying with tempering saves up to 45% of drying time compared to single stage drying
Tempering of 120 min at 50 °C reduced fissuring by 32 to 50%
Use of online moisture meters will help in quality control of rice
The efficiency of dryer blowers should be studied
Heat recovery from dryer condensate
Pressure drop across the dryer to be avoided
