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Post Harvest Management

Post Harvest Management
26
Aug

Direct and Indirect Heating

1. In direct heating the combustion products

are mixed with the drying air meaning that they come in contact with the paddy. 

2. In western countries this is only allowed for products used to feed animals.

3. In SE Asia direct fired heaters are not considered as problem because the flue gases will only pollute the rice hull, which is not considered a problem since the hull is removed during the milling process.

File Courtesy: 
http://www.riceindia.net/RKBank/Training_Manual_Paddy_Drying.pdf
26
Aug

Fixed-Bed Batch Dryers

1. Fixed bed batch dryers usually have rectangular

bins with plenum chamber underneath or circular bins with central duct.

2. The most common fixed bed dryers are flat bed dryers which have a very simple design. Grains are laid out on a perforated screen, and dried by forcing air from below. 

3. The air fan that provides the drying air is usually a simple axial flow fan, powered by a diesel engine or by an electric motor. 

File Courtesy: 
http://www.knowledgebank.irri.org/rkb/index.php/paddy-drying-methods/heated-air-drying
26
Aug

Low-temperature drying

1.In low-temperature drying the objective of the dryer management is to keep the RH of the drying air at the equilibrium relative humidity (ERH) corresponding to the desired final moisture content of the grain, or the equilibrium moisture content (EMC). 

2. The effect of the temperature is minimal compared to the RH. For example a final MC of 14% is desired one should target an RH of the drying air of around 75%. 

File Courtesy: 
http://www.knowledgebank.irri.org/rkb/index.php/drying-basics/heatedair-and-low-temperature-drying
26
Aug

Heated-air fixed-bed batch dryers

1. In heated-air fixed-bed batch dryers, the hot drying air enters the grain bulk at the inlet, moves through the grain while absorbing water and exits the grain bulk at the outlet. 

2. The grain at the inlet dries faster because of the drying air has the highest water absorbing capacity. Because of the shallow bed and relatively high airflow rates, drying occurs fastest in all layers of the grain bulk at the inlet and slowest at the outlet. As a result a moisture gradient develops, which is still present at the end of drying.

File Courtesy: 
http://www.knowledgebank.irri.org/rkb/index.php/drying-basics/heated-air-and-low-temperature-drying
26
Aug

Additional factors affecting the drying rate

Besides layer depth and mixing interval the drying rate of sun drying depends on other factors which are usually out of control of the operator:

1.Temperature and humidity of ambient air: The rate at which rice dried is affected significantly by the temperature and humidity of the air which move over or through the grain. 

2. For this reason, in most Tropical and Humid climates, Sun drying is only successful during a few hours in the mid-day. Initial moisture content of grain: Wet grains dry more rapidly than comparatively dry grains.

File Courtesy: 
http://www.knowledgebank.irri.org/rkb/index.php/paddy-drying-methods/sun-drying
26
Aug

Protection during Sun Drying

1. During hot days the grain temperature may rise above 50-60ºC. If that is the case cover the grain at mid-day to prevent over-heating;

2.Cover the grain immediately if it starts raining. Re-wetting of grain causes fissured grains and high grain breakage during milling;

3.Prevent contamination of grain with other materials and keep animals off the grain; and

monitor grain moisture content and grain temperature.

File Courtesy: 
http://www.knowledgebank.irri.org/rkb/Index.php/paddy-drying-methods/sun-drying
26
Aug

Mixing while Sun Drying

1. During good weather conditions mixing or turning the grain is the most important activity for maintaining good quality.

2. Turn or stir the grain at least once per hour, better every 30 minutes to achieve uniform moisture content. 

3. Variation in moisture content within the grain causes re-wetting and subsequent grain cracking of dried kernels.

File Courtesy: 
http://www.knowledgebank.irri.org/rkb/index.php/paddy-drying-method s/sun-drying
26
Aug

Layer thickness while Sun drying

1. Rice millers invariably use concrete floors for drying. Such drying results in sun-cracks and contamination on quality of rice is little realised as much of the milling is in hullers which by themselves contribute to breakage.

2. In case of sun drying the produce is spread on hard floor or threshing yard around 10-cm thickness, and is allowed to dry by heat from the Sun.

3. Spread the grains in thin layers, ideally 2-4 cm.

File Courtesy: 
http://www.ikisan.com/CropSpecific/Eng/links/ap_riceDrying.shtml
26
Aug

Recommendations for Sun Drying

Recommendations for Sun Drying include

1.Layer Thickness

2. Mixing

3. Protection

4. Additional factors affecting the drying rate

File Courtesy: 
http://www.ikisan.com/CropSpecific/Eng/links/ap_riceDrying.shtml
26
Aug

Technology options to improve Sun drying

1. Sun drying will be in practice (vogue) as long as

there is a market for low quality paddy, because it is the cheapest drying method.

2. As long as there is no quality-incentive for better quality rice it will be the preferred method whenever the weather allows. Traditional sun drying can be improved using simple tools and monitoring equipment.

File Courtesy: 
http://www.knowledgebank.irri.org/rkb/index.php/paddy-dryingmethods/sun-drying
26
Aug

Pavement drying

1. It is useful for medium to large scale

farmers, traders and millers.

2. Place threshed grain on pavements made specifically for drying. Mixing and collecting can be partially mechanization. Due to large size, labour can be more effectively used.

3. System can be improved by using pavements with elevations for the grains and drainage channels for rainwater.

4. Usually manual tools are used for mixing and grain collection

File Courtesy: 
http://www.fao.org/teca/content/rice-sun-drying-method
26
Aug

Drying on nets mats or canvas

1. It is useful for small to medium scale

farmers and also has multiple uses.

2. The drying of the grains on the mats and plastic sheets is the most hygienic method and that allows quick collection and mixing.

3. Paddy when dried on mats or canvas does not contain stones and other dirt, often found in paddy that was dried on roads.

File Courtesy: 
http://www.fao.org/teca/content/rice-sun-drying-method
25
Aug

Panicle drying

1. Drying of paddy grains that are still attached

to the panicles is a traditional method for drying small amounts of paddy.

2. The panicles are harvested with a small knife, bound together and carried to the drying location.

3. The dried panicles are then stored in farmers’ houses, for example by hanging them under the roof for protection from rodents.

4.For drying, the tied bundles are placed on pavements or mats or hung from frames.

File Courtesy: 
http://www.knowledgebank.irri.org/rkb/index.php/paddy-drying-m ethods/sun-drying
25
Aug

Options of sun drying

Options of sun drying include 

1.Panicle drying

2.Drying on nets

3.Mats or canvas

4. Pavement drying

File Courtesy: 
http://www.knowledgebank.irri.org/rkb/index.php/paddy-drying-methods/sun-drying
25
Aug

The limitations of sun drying

Limitations of Sun Drying: 

1.There is uncontrollable non-uniform drying which result in sun checks or cracks in kernels. Due to this when dried grains milled, give larger quantity of broken grains.

2. This process is dependent on availability of sun energy normally which is not available in monsoon season.

3. Large number of unskilled labours is required with no mechanical energy.

4. During the process quantity of 1 to 2 per cent of paddy is lost due to birds, insects, rats and other rodents.

File Courtesy: 
http://www.indiaagronet.com/indiaagronet/post_harvest/CONTENTS/rice.htm
25
Aug

Why Sun drying?

1. The cost of drying per tonne of paddy is relatively low as compared to mechanical drying as it requires no fuel or mechanical energy.

2. The rate of drying in the sun depends upon solar insulation, moisture content of paddy.

3. The time required for sun drying depends on the weather.

4. Size of the floor or mat required for drying is determined by matching the drying rate with the amount of paddy grains to be dried.

5. Sun drying is environment friendly.

File Courtesy: 
http://books.google.co.in
25
Aug

Sun drying

1. Sun drying is the most preferred method in India.

2. In this method spread the grains in thin layers on the floor, mix the grains for every 30 minutes and monitor the grain temperature.

File Courtesy: 
http://www.fao.org/teca/content/rice-paddy-drying-systems
25
Aug

Field drying

1. The crop is harvested at higher moisture

content and is left in the field or on bounds of field till it has dried to proper moisture content.

2. Another improved method followed in Japan is to dry harvested crop on racks. There the harvested plants are found in a bundle near the ears and are hung on a rope exposing the ears to the sun.

File Courtesy: 
http://www.indiaagronet.com/indiaagronet/post_harvest/CONTENTS/rice.htm
25
Aug

Drying Methods

In India, three methods are used for drying the paddy grains namely:

1.Sun drying

2.Mechanical drying

3.Chemical drying

File Courtesy: 
http://www.indiaagronet.com/indiaagronet/post_harvest/CONTENTS/rice.htm
25
Aug

Tempering

1. When the drying of grain is temporarily stopped the moisture within the grain equalizes due to diffusion.

2. When drying is restarted, the drying rate becomes higher compared to continuous drying. The process of stopping intermittently is called tempering.

3. In addition during tempering the moisture differences between grains equalize.

4. Tempering therefore also ensures that moisture gradients in the grain bulk that develop during drying in certain dryer types are minimized.

File Courtesy: 
http://www.knowledgebank.irri.org/rkb/index.php/drying-basics/the-drying-process
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