Post-harvest processing for quality rice
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
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
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
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
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
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
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
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
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
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