Parboiling of Rice

Parboiling of Rice/Paddy

1. Parboiling of Rice

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

7. Parboiling Process Flow

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 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 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. Thank You Tushar Bhardwaj 17/MFS/025 Ishwar Singh Tamwar 17/MFS/008