Post production: Drying

The importance of drying

To define drying and to explain its importance.

Dry before storage

What is drying?

Drying is when you reduce the moisture in the grain until it becomes safe to store the grain. Dry grain can be stored much longer than wet grain. Remember that paddy is usually harvested at 20-25% moisture content (MC). With "wet grain" we mean any grain with moisture content above 14%.

Grain needs to be dried before you can safely store it

Poor drying gives low quality grain

Why is drying so important?

If you wait too long before drying the grain,
if you don't dry the grain long enough, or
if you dry the grain in a poor way,

--> the quality of the grain will decrease fast.

What happens if the moisture content is high?

Why is "wet" grain such a problem?

At harvest, the rice grain still contains a lot of moisture.

Too much moisture:

	gives natural respiration in the grain that causes fast deterioration of the rice.
	promotes the development of insects and molds that are harmful to the grain.
	lowers the germination rate of rice that is intended for seed.

Therefore, drying of rice is very important to prevent insect infestation and quality deterioration of grain and seed.

Grain that has changed color because of deterioration.

Dry harvested grain within 12 hours!

Moisture Content = MC

Drying for storage

It is important to dry rice grain as soon as possible after harvesting. This means within 24, better within 12 hours!

You must know how long you want to store the grain. The longer you want to store the grain, the lower the moisture content needs to be so the more you have to dry the grain first!

The purpose of storage is to provide the dried grain with protection against insects, molds, rodents and birds, and to prevent moisture from re-entering the grain. BUT if the drying is not done properly, you will have losses no matter how good the storage is.

The following table shows the best moisture content (MC) for storage of paddy grain and seed, and possible problems when the moisture content is not right at storage:

Longer storage needs a lower MC

Moisture content	purpose	Possible problems during storage	Range on IRRI moisture meter
> 14%	Unsave, dry within 24h after harvesting	Rapid temperature increase, yellowing	red
14% or less	Weeks to a few months of storage	Molds, discoloration, respiration loss, insect damage, moistre adsorption	green
13% or less	8 to 12 months storage	Insect damage	green
12% or less	Farmers’ seeds	Loss of Germination	yellow
9% or less	Storage for more than 1 year	Loss of seed viability

Next lesson

There are 2 drying systems: Traditional drying (sun drying) and mechanical drying. First, we have a look at sun drying.