Steaming
Use of steam for gelatinizing the starch is preferred to other methods of heating as it does not remove any moisture from the rice. Condensation adds water and increases the total quantity of water absorbed. The moisture content of the paddy increases to about 38% during steaming. Steam has other advantages as:
- Its high heat can be applied at a constant temperature,
- It is relatively easy to handle,
- it gives relatively high degree of control of the paddy temperature,
- it can be stopped instantly, and
- it has a higher heat transfer rate than other media (such as hot water).
When heating paddy with non-pressurized small variations are found in color, quantity of soluble starch, and the amount of swelling of the milled parboiled rice. Heating has a considerable effect on color. When the steaming temperature exceeds 100oC, the color becomes considerably deeper and the grain becomes harder. Longer steaming times also cause the 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 1 to 5 kg/cm2 is normally used for steaming. Steaming duration depends on the steaming arrangement. For small batches, steaming takes 2 to 3 minutes while batches of 6 ton may take 20 to 30 minutes. While splitting of the husk indicates completion of the steaming process, it is not a necessary condition and paddy can be completely parboiled without any splitting.