Drying

postharvest-drying-manual-2Drying reduces grain moisture content to a safe level for storage. It is the most critical operation after harvesting a rice crop.

Delays in drying, incomplete drying or ineffective drying will reduce grain quality and result in losses.

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:

postharvest-16-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.

Open in new window: Moisture content

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.

Open in new window: Grain and air properties

postharvest-drying-processDrying 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

Open in new window: Drying process

Heated air drying and low-temperature drying (also referred to as near-ambient drying or in-store dyring) employ two fundamentally different drying principles.

Both have their own advantages and disadvantages and are sometimes used in combination e.g., in two stage drying systems.

  • Heated air drying employs high temperatures for rapid drying and the drying process is terminated when the average moisture content (MC) reaches the desired final MC.
  • In low-temperature drying the objective is to control the relative humidity (RH) rather than the temperature of the drying air so that all grain layers in the deep bed reach equilibrium moisture content (EMC).

Open in new window: Drying principles

Methods

There are many different methods used for drying rice. This involves various drying technologies of different scale and complexity. There is no ideal dryer for drying rice since each drying method has its own inherent advantages and disadvantages. 

In many traditional harvesting systems farmers leave their harvested rice in the field for extended time because they are either waiting for the thresher or because they want to pre-dry the paddy.

postharvest-stackingIn this practice, the rice plants are either left lying in the field or are also often stacked in piles with the panicles inside to protect them from rain, birds and rodents. This practice can lead to massive heat build up inside the stacks. As a result molds grow quickly and infest the grains and discoloration usually develops within the first day of field drying. Another unwanted effect is that grains that are already dry often absorb water from the wetter straw, which leads to fissuring of the these grains and thus reduces the potential head rice recovery.

It is impossible to produce good quality grains with field drying practices; field drying should therefore be avoided.

Open in new window: Field drying and stacking

postharvest-sundryingSun drying is the traditional method for reducing the moisture content (MC) of paddy by spreading the grains in the sun. The solar radiation heats up the grains as well as the surrounding air and thus increases the rate of water evaporating from the grains.

It is the most common drying method in Asia because of its low cost compared to mechanical drying. It requires little investment and is environmentally friendly since it uses the sun as the heat source and therefore produces no CO2.

Fact sheet: How to sun dry

Open in new window: Sun drying

postharvest-lowcost-dryerCompared with sun drying, heated air drying or mechanical drying allows for suitable drying air conditions to be set. In this method, drying can be carried out any time of the day or night.

Use of mechanical drying may also reduce the labor costs, especially if some form of mechanical turning or stirring of grain is practiced, as in the case of re-circulating dryers. Grain re-circulation allows to uniformly dry grains, and the automatic drying air temperature control to maximize the drying rate and at the same time reduce over-heating or over-drying.

In general, mechanically dried grain will produce better quality rice compared to sun drying. Mechanical drying will lead to more uniform drying of grain and higher milling yield and head rice recovery.

Since rice quality is becoming more important to rice consumers, medium-sized grain dryers have become a common sight throughout Asia. For production of premium quality rice or seed, mechanical drying with heated air dryers is highly recommended.

Open in new window: Heated air drying

postharvest-korean-inbin-drying-storageIn in-store drying, the grain only dries until it reaches the equilibrium moisture content and thus over drying of the bottom layer is minimized.

The drying process is slow and can take several days to weeks depending on the depth of the grain bulk.

In-store drying is the ideal second stage drying method because the slow and gentle drying process maintains the grain quality and low energy requirement leads to low energy cost. In addition, if moisture contents increase in storage, the storage facilities can easily be aerated again.

Open in new window: In-store drying

FAQs

Click on the slider for the answers.

Since solar energy is free, would it not be great to have a solar rice dryer?

Using solar collectors to generate heat for rice drying is not economically feasible and there are technical constraints.

Some facts

  • Assuming an average solar radiation of 500W/m³ and 70% collector efficiency, one square meter of solar panel can generate 1, 260 kJ/h or roughly 15, 000 kJ/day.
  • The desired drying rate for optimum dryer use and grain quality is 1% moisture content per hour. To achieve this, around 1m³ of air with a temperature of 43°C needs to be moved through each ton of grain. For each ton dryer capacity this requires 0.7-1 kW motor power for the blower and around 50 kJ/h energy for heating the air.

Advantages of solar energy

  • Solar energy is freely available during the day and is environmentally friendly.

Disadvantages

  • Most heat for drying is needed when it rains or at night when solar radiation is low.
  • High investment cost and space requirement, the solar collector area needed is around 10 times the area of the drying bin.
  • Temperature control is a major problem.

Conclusion

While the technical problems can be sorted out the specific investment cost (cost per ton installed capacity) and the space requirement remains a major constraint to using solar power for heating the drying air. There are other options for using renewable energy sources such as rice husk.


Fact sheet: Energy Options for Rice Drying

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There is a lot of fly ash or black sooth in my paddy which reduces it's market value. How can I reduce it?

Most paddy dryers are direct fired, which means they don't use a heat exchanger. If the furnace - fan combination is not designed properly the fan can suck ash from the furnace and blow it into the air distribution system of the dryer. If the air velocity is high enough the ash or sooth even gets carried into the grain bulk.

Most furnaces have a fly ash separation device, either using a set of baffles that forces the air to turn sharply and separates the ash trough centrifugal force. Other furnaces use a circular air flow like in a cyclone for as separation. Check the following:

Is the as separation device designed according to the specifications?

Has fly ash accumulated in the as separation device? If so, clean it.


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The drying time of my flat-bed dryer is longer than 8 hours, what could be the reason?

A properly designed and well maintained flat bed dryer should have a drying rate of around 1% moisture reduction per hour. That means that if paddy is harvested at 22% moisture content and the final moisture content after drying is 14% it would take around 8 hours for drying. If the drying time is longer there are three possible reasons:


 The paddy is very wet

If the paddy was very wet because of harvesting after rain or early harvest before physiological maturity obviously the drying also takes longer since the drying rate is fixed by the design of the flat-bed dryer.

 The dryer performed well when newly installed but only recently started having longer drying time

Some operating parameters must have changed. Check for the following:

  1. Check the air temperature. It should be around 43ºC. Adjust if neccessary.
  2. Check the airflow rate, a sheet of paper should float on top of the drying bin at each location. If not:
  • Check the fan rotor speed, sometimes operators reduce the speed to extend the engine life or to safe fuel
  • Check for slippage of the fan belt or any other
  1. Check the grain layer depth, was the dryer overloaded?
  2. How dirty was the grain? A lot of dirt and dust between the grains can hinder airflow.
  3. Check whether fly ash has accumulatedin the plenum chamber or in the air distribution system

 The dryer always had a long drying time

If the dryer has never achieved a drying rate of 1%/h there is something wrong with the desing. Possible causes are:

  1. Most likely the fan does not perform according the the specifications.  Often fans are not properly designed and deliver only a fraction of the required air flow. Fan design is hard-core engineering and any tinkering will lead to failure.
  2. The air ducts might be too small limiting air flow.
  3. The plenum chamber below the drying bin might be too small.
  4. The furnace might not provide sufficient heat, check the drying air temperature inside the plenum chamber.
  5. The drying bin might be too big for the blower. Often manufacturers and users use a bigger drying bin in order to increase capacity but maintain the blower designed for a smaller bin. While the bin holding capacity increases the drying capacity expressen in t dried per hour might actually decrease.
    Example:
    In South Sumatra users doubled the drying bin capacity from 3.3 to 8 t but tripled the drying time from 8h to 24h. In reality they reduced the dryer capacity from 0.41t/h to 0.33t/h.
  6. Check all other design parameters.

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Drying strategies

Drying strategies

When developing a drying strategy, it is important to consider other activities in rice post production system, and look into the economic criteria

Economic aspects of drying

Economic aspects of drying

A guide to site-specific assessment on the potential of mechanical drying

Safety considerations in drying

Safety considerations in drying

Important considerations that have to be taken into account when doing a site-specific assessment on the potential of mechanical drying.