Theoretical and industrial aspects of paddy parboiling

1. Paddy Parboiling Revisited

2. Parboiling
• 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 of Parboiling
• 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
Disadvantages
• More difficult bran removal
• A great amount of water is absorbed during cooking causing the rice to
swell
• High BOD (1350-1800 ppm) of Effluent water

4. Process
Step 1: Pre Steaming (First Steaming)
Step 2: Soaking
Step 3: Post Steaming (Second Steaming)
Step 4: Drying and Tempering

5. Factors controlling the quality of parboiled rice
• Soaking time and temperature
• Steaming pressure, velocity, time and stages
• Tempering time and drying time
• Drying temperature and stages

6. Parboiling process flow

7. 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

8. 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
• 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

10. 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
• Time of 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 of 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

16. Double-pass drying of parboiled paddy with and without tempering