Drying rice

Drying reduces grain moisture content to a safe level for storage. It is the most critical operation after harvesting a rice crop.

Drying and storage are related processes. Storage of incompletely dried grain with a higher than acceptable moisture content will lead to failure regardless of what storage facility is used. In addition, the longer the grain is to be stored, the lower the required grain moisture content must be.

At harvest time rice grain contains a lot of moisture, typically between 20−25%. At such high grain moisture contents (MC) there is increased natural respiration in the grain that causes deterioration of the rice.

The purpose of drying is to reduce the moisture content of rough rice to a safe level for storage. As even short term storage of high moisture paddy rice can cause quality deterioration, it is important to dry rice grain as soon as possible after harvesting - ideally within 24 hours.

Basics

Drying of grain involves exposing grain to ambient air with low relative humidity or to heated air. This will evaporate the moisture from the grain and then the drying air will remove the moisture from the grain bulk.

Since drying practices can have a big impact on grain quality or seed quality, it is important to understand some fundamentals of grain drying:

Moisture contentMoisture content (MC) is the amount of water in the rice grain. 

Rice is usually harvested at 20−25% MC while 14% or less is considered safe for storing grains, 12% or less for storing seeds. Paddy should be dried to safe moisture content within 24 hours after harvesting to avoid damage and deterioration. Improper drying and storage practices lead to low grain or seed quality.

Grain and air properties

Rice is a hygroscopic material.

• When dry rice is exposed to air with high relative humidity (RH) the rice grains will absorb water from the air (re-wetting).
• When wet rice is exposed to air with low RH the rice grains will release water to the air (drying).

The equilibrium moisture content (EMC) is the final moisture content of the grain or seed after being stored for some time with sorrounding air of a certain temperature and RH.

During storage, the final moisture content of grain will be determined by the temperature and RH of the air that has surrounds the grain. If the grain is not protected against the humidity in the air, particularly during the rainy season when the RH is very high, the grain moisture content will rise and this will lead to deterioration in both grain and seed quality.

Drying process

Drying practices can have a big impact on grain or seed quality. Drying of grain involves exposing grain to air with low relative humidity which will lead to evaporation of the moisture in the grain and then the moisture’s removal away from the grain.  

The process includes:

• moisture removal
• monitoring drying periods (pre-heating, constant rate, falling rate)
• increasing drying rate at 18% MC
• uniform drying
• tempering

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