Paddy Parboiling Revisited

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

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

Process

Step 1: Pre Steaming (First Steaming)

Step 2: Soaking

Step 3: Post Steaming (Second Steaming)

Step 4: Drying and Tempering

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

Parboiling process flow

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

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

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

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

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

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

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

Double-pass drying of parboiled paddy with and without tempering