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Post-harvest processing for quality rice

 
The level of efficiency of field management operations with timeframe of weather, pest and disease incidence at different crop stages and post-harvest processing have a bearing on the yield and quality of paddy and eventually the amount, quality and cost of milled rice and the by-products..

Huge grain losses occur at the time of early or late harvest. Early harvesting results in broken kernels and low milling recovery while late harvested crop is prone to more insect, rodent and bird pests in addition to increased risks of lodging and grain shattering. Therefore, the best time for harvesting the crop is when the variety has reached the particular date of maturity.

The field should be drained 7-10 days before the maturity date when 80 per cent of grains or the upper portion of the panicle has changed from green to straw colour; at least 20 per cent of the kernels at the base are already in hard dough stage and the grain contains 21-24 per cent moisture.

During the sequence of manual harvesting, field drying, bundling and stacking in traditional systems can result in 2-7 per cent losses: 0.11-0.35 per cent during field transport, 0.11-0.76 per cent during field stacking even with greater percentage when stacking is left in the field with high grain moisture contents.

Among rice harvesting methods the traditional long stalk cutting by sickle is the most widely used method. In this method stalk is cut 10-15 cm above the ground or with stalk length of about 60-70 cm for ease of bundling and threshing.

Among mechanised harvesting methods due to scarce labour include combine harvester mostly adopted in the country. However, low income, small land holding, excessive moisture contents at harvesting, uneven ripening, and severe lodging with high shattering might hinder the adoption of mechanised farming.

Transportation: Transportation of paddy from the field is mainly by means of human and animal power and sometimes by mechanical power.

Threshing paddy in the field and transporting the grain in bags minimises grain losses if sun drying of paddy is done in the house yard instead of drying it on stalk in the field. Surplus paddy is usually sold fresh to traders or directly to rice mills.

Transportation of paddy can be partially mechanised, that is, paddy bags are brought from the field to the roadside manually or by animal power, then transported to the drying area or rice mills on tricycles, tractors with trailers, trucks and lorries. .

Threshing: This operation should be done at the earliest after harvest. Quick threshing can be done only with powered threshers. Mostly, threshing is done manually accomplished by either treading, beating the panicles on drums, threshing board or rack, or beating the panicles with stick. Power threshers like axial-flow thresher are also common.

Drying: Paddy is usually harvested with about 24-26 per cent of moisture contents, higher during the rainy season and lower during the dry season. At harvest with these moisture contents, paddy has a high respiration rate and also susceptible to micro-organisms, insects and pests attack. Hence quantitative loss and qualitative deterioration may increase. Grains become rancid, moldy, yellowish, insects and pest infested.
 

Newly harvested grain with high moisture content must therefore be dried within 24 hours to about l4 per cent for safe storage and milling or to at most 18 per cent for temporary storage of up to two weeks.

A heated air temperature of 43oC is recommended in drying paddy for seeds or for food grain milling. High drying air temperature will affect grain viability and the quantity and quality of milled rice during milling. Drying is the most important step in minimising post-harvest losses, since it directly affects safe storage, transportation, distribution and processing quality. Improper and over-drying as normally happens in sun drying, which is difficult to control, may reduce head rice yield and aroma.

Field drying is generally practiced by farmers. This method is resorted to during rainy season or harvesting immediately after the rains to remove surface moisture on the cut panicles and grains. Shade drying is also practiced by farmers particularly for grains intended for seeds. Sun or solar drying of threshed grain, being the cheapest method, is practiced by all sectors. Almost 70-90 per cent of field harvest retained in the farm is sun dried.

The use of mechanical dryers eliminates the problems associated to sun drying. Mechanical drying offers the advantage of timeliness in drying, reducing handling losses, maintaining grain quality, and better control over the drying process. The choice of the mechanical dryer for a particular drying operation depends on several factors such as the drying capacity needed, ease in installation and operation, portability, fuel heat source and the initial cost of acquisition.

Cleaning: It involves the separation of undesirable foreign matter from the grain for storage and processing. Where threshing is done manually, paddy is contaminated by foreign matter. Rough paddy cleaning is accomplished in the field right after the threshing operation by hand raking and sifting the bits of straw, chaff and other large and dense materials from small piles of paddy.

The second cleaning stage is after storage prior to milling process to remove the remaining foreign matter that could damage the milling machinery and affect the grain quality or grade of milled rice and therefore, its market value. Cleaning devices may include vibrating or rotating sieves, aspirators, de-stoners and magnetic separators.

Harvested paddy must be dried, cleaned and stored as a source of food supply until the next harvest. Decisions on how much to retain is influenced by previous cash commitments, labour scarcity at harvest, transport difficulty, weather conditions, and lack of handling and storage facilities, current prices and immediate source of cash during emergencies.

In small-scale production and processing, field-threshed and partially cleaned paddy is bagged in jute or propylene sacks for handling purposes in transporting paddy from the field to the roadside or to the house. Milled rice is packaged in polyethylene, propylene or jute sacks in weights ranging from 1 kg to 1000 kg depending upon whether the market is for retail or wholesale or for export.

Higher quality rice normally sold in groceries and in supermarkets is packed in attractively labeled packages made of polyethylene, propylene, jute and paper bags or cardboard boxes.

Storage: Rice in either milled or paddy form, is stored to provide a buffer stock of the staple, the amount, form, and sophistication of which depend upon whether the storage is household or own consumption, rice mill, wholesaler, retailer or distributor and government logistics. At any level and scale of storage, drying of paddy to the moisture content level of about 14 per cent is a basic requirement to prevent spoilage.

Uniform drying and prevention of moisture spots and moisture migration inside the grain mass by means of aeration is essential especially in large-scale storage where metal or concrete silos are used. Proper storage for seed purposes is necessary to maintain its viability. Hence, the storage structure must protect the paddy from extreme heat or cold, moisture levels at which the seed will spoil and be subjected to microbial or fungal attacks, insect pests and rodent consumption or damage.

Grain used for seed requires different treatments. Paddy is usually stored in bags which are stacked inside warehouses. Some are stored in bulk using bins or silo associated with the drying and milling operations.

Fumigation, aeration and the maintenance of clean warehouses are considered good warehouse management practices. Stored paddy is lost due to moisture, vertebrate pests, insects, mites, spillage and possibly theft or pilferage. Storage containers must be free from large and small openings to prevent vertebrate pests and insects to gain access to the stored grain.

It must be located in a cool dry place with minimal temperature variation within the container to prevent moisture migration within the stored grains.


Courtesy: The DAWN

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