SEED CLEANING AND UPGRADING
Cleaning
of seeds
Principles
of cleaning seeds
In
the cleaning process, the separation of undesirable material, namely, inert
matter, weed seeds, other crop seeds, light and chaffy seeds, off-size, damaged
or deteriorated seed from desirable material is done on the basis of differences
in physical properties of desirable seed and undesirable matter.
The principal physical differences found in seeds are seed size (length,
width and thickness) density, shape, surface texture, color,
affinity for liquids, and seed conductivity.
If the differences between desirable and undesirable material in regard
to any of these properties exist, separation of undesirable material could be
done with the help of suitable machine/machines designed for the purpose.
Seeds of
different species and inert matter widely differ in regard to the
physical properties. Length, width,
shape, weight, and surface texture differences are quite common in crop species
and forms the basis of seed cleaning operations.
Method
of Cleaning Seeds
Cleaning
of seeds can be conveniently discussed in the following groups:
Pre-conditioning and
Pre-cleaning
Pre conditioning refers to operations such as
shelling, debearding, etc., that prepare seed lots for basic seed cleaning, and
also to the removal of particles such as pieces of trash, stones, clods, etc.,
larger in size than desirable crop seed, from threshed seed lots.
Some pre-cleaners, in addition to removing larger sized particles, also
remove particles that are lighter in weight and smaller in size than the crop
seed. The necessity of both of
these operations, other than shelling, is associated with advanced mechanized
agriculture. No pre-cleaning is
usually required on hand harvested and winnowed seed lots.
Pre-conditioning and pre-cleaning equipment and their use:
The most common equipment used in these operations
are scalpers, debearders, huller-scarifier, Buckhorn machine and maize sheller.
Scalper/Rough
cleaner
Scalpers
are simple devices intended to remove only large trash.
Such units basically consists of a vibrating or rotating screen or sieve.
The screen perforations are large enough to allow the rough seed to pass through
readily while the larger inert material is ‘scalped off’ and removed from
the seed lot. The scalpers
manufactured for pre-cleaning may consist of several screens, or reels, with one
or more controlled air separations. The
single sieve pre-cleaners are called scalpers and the multiple sieve units are
referred to as rough cleaners.
The rough cleaners are essentially the simple air screen seed cleaners
that make possible a separation of light chaff and dust with a controlled air
current; a separation of large trash over a large hole screen; and a separation
of small foreign material through a small hole screen.
Most scalpers are arranged to make the air separation before the seeds
reach the screens.
After
scalping/rough cleaning many kinds of seeds can be cleaned without any further
pre-processing. Seeds of some
crops, however, may require hulling, scarification, etc., after scalping.
Huller-Scarifier
Hullers and scarifiers usually abrade the seeds between two rubber-faced surfaces, or impel seeds against roughened surfaces, such as sandpaper. In a huller-scarifier, the seeds fall from the feed hopper on to a rotating distributing disc, where they are thrown against the hulling and scarifying surface by centrifugal force either once or twice, depending upon the machine. At this point the seed are hulled and/or scarified. After this operation the seeds are moved into a suction chamber where the suction removes the light, fine dust, and the seed discharge at the bottom of the chamber. The severity of abrasion or impact must be controlled accurately to prevent damage. Hulling (removal of an outer coat or husk) and scarification (scratching of the seed coat ) can be done separately or jointly with a huller scarifier.
Debearder
The
debearding machines have a horizontal beater with arms rotating inside a steel
drum. The arms are pitched to move
the seeds through the drum. Stationary
posts, adjustable for clearance, with arms, protrude inward from the drum.
These machines rub the seeds against the arms and against each other. The time, the seeds remain in the machine, is varied by regulating a discharge gate. The degree of action is determined by the processing time, beater clearance and beater speed.
Pebble
Mill
The
pebble mill is used for removing cob-webby hairs from blue grass and similar
seeds. It has a drum rotating about a shaft, inserted off-centre at opposite
ends. The mill is loaded with seeds and smooth half-inch pebbles and turned
at a slow speed until the rubbing action of the pebbles rolls the fuzz
from the seeds into small round balls. The
mixture of pebbles, seeds and matted fuzz is then run over a scalper to remove
the pebbles.
Maize
Sheller
The
maize sheller varies in size from small hand-powered sheller to large
motor-driven sheller with capacities up to ten tonnes per hour.
Small
hand-power sheller consist of a crank, a small feed inlet , a heavy cast iron
fly wheel and burrs that remove the maize seed from the ear.
Seeds drop out to the bottom and into a container, and the cobs are
discharged out from the rear of the sheller. These types of sheller are useful
for small lots of breeder’s seed of inbred lines.
At
processing plants, power sheller is installed to give high capacity shelling.
The power sheller has four main parts, namely, inlet hopper, rotating beating
cylinder, concave, and fan. The
inlet hopper is kept sufficiently large to feed several cobs at a time.
The rate of feeding could be adjusted through the sliding gate inside the
inlet hopper. The rotating beating cylinder has spirally arranged shelling lugs.
On one cylinder a drive pulley is fixed.
Drive pulley can be driven by an oil engine, tractor belt pulley, or an
electric motor. Most indigenous power sheller require a 7.5 h.p motor. The lower
concave is made of a perforated steel sheet to allow the seed to pass through,
but to retain and divert cobs towards the vibrating screen so as to discharge
them outside. Shelled seed coming
out of the lower concave is passed through an air blast to remove small cob
pieces and dust.
For obtaining satisfactory shelling, the cylinder shaft speed should be in the range of 450 to 500 RPM.
Basic Seed Cleaning
Basic cleaning refers to actual cleaning and grading of seeds. Unlike pre-cleaning/pre-conditioning which may or may not be required, basic seed cleaning is an essential process in the seed cleaning operations. Many kinds of seeds can be completely cleaned and made into a finished product by basic cleaning. In other instances however further separations to remove specific contaminants may be necessary or desirable. Regardless of whether further specific separations are made or not made basic seed cleaning for every lot is invariably done.
Equipment for basic seed cleaning: Basic seed cleaning is done over an air screen machine, commonly referred to as an air screen cleaner. It is also the basic equipment in seed processing plants.
Upgrading the Quality of Cleaned seed
The various processing operations conducted after basic cleaning to further improve seed quality are regarded as upgrading operations. The choice of upgrading operation however, shall depend upon the type of contaminants and crop seed.