Bin Master Engineering

Parboiled paddy should be dried to 14% 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 temperatures 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, the paddy should be allowed to stand for at least several hours - preferably for 1 or 2 days - before it is milled, to permit internal moisture differences and stresses to equalize.

Moisture reduction takes place rapidly during the first part of drying from 36 to 18% moisture level, but is slow from 18 to 14%. The drying process should be stopped at about 18% moisture to allow the paddy to temper or equalize for several hours before continuing the drying to 14%.

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 gelatinization temperature or the paddy will be cooked. Gelatinization 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 the paddy to a vacuum for a few minutes or by soaking it under pressure in hot water.

Paddy soaked in water at ambient temperature (20-30°C) will take 36 to 48 hours to reach 30% moisture content. In hot water (60-65°C), it will takes only 2 to 4 hours. If soaking time is too long, part of the rice dissolves in the water, the seed begins to germinate, and starch fermentation occurs.

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).