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Rice parboiling improves quality                           Home
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 (20-30C) will take 36 to 48 hours to reach 30 per cent moisture content. In hot water (60-65C), 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 (60-65C), 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 100C, 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 100C during the first drying period. During the second period air temperature should be kept below 75C. 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.

Courtesy: The DAWN;

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