GRADING, Systems of grading : Static and Dynamic

1. UNIT-4
GRADING
-B. Pavan Kumar Naik
Grading
The produce is separated into two or more grades on the basis of the surface colour, shape,
size, weight, soundness, firmness, cleanliness, maturity & free from foreign matter /diseases
insect damage /mechanical injury.
For eg.: Apple I. Extra Fancy II. Fancy III. Standard IV. Cull (for processing).
Grading may be done manually or mechanically. It consists of sorting product in grades or
categories based on weight/size.
Systems of grading : Static and Dynamic.
A. Static systems - are common in tender and/or high value crops. Here the product is placed
on an inspection table where sorters remove units which do not meet the requirements for the
grade or quality category (fig.12).
B. The dynamic system - here product moves along a belt in front of the sorters who remove
units with defects (fig.13). Main flow is the highest quality grade. Often second and third
grade quality units are removed and placed onto other belts. It is much more efficient in terms
of volume sorted per unit of time. However, personnel should be well trained. This is because
every unit remains only a few seconds in the worker's area of vision. Eg. Onion grading
There are two types of common mistakes: removing good quality units from the main flow
and more frequently, not removing produce of doubtful quality
New Innovation in grading systems
i. Computerized weight grader – operate on the basis of tipping buckets that drops to release
the pre weighed item at a particular position. – Apples, citrus
ii. Video image capture &analysis–used for size, colour &external defect grading-coffee
bean, apple
iii. NIR Spectrometers – to assess the TSS non – destructively in apple and stone fruits
iv. X-ray imaging and Computer aided tomography
v. MRI - Magnetic Resonances Imaging
vi. Spectroscopy

2. vii. Acoustic methods
viii. Volatile emission analysis
OR
Grading refers to quality separation of food product. The term ‘quality’ has different
conditions for different commodities and relative importance of the component material
properties. Actually, grading involves overall balanced assessment of all those properties of a
material, which affects its acceptance as a food product and as working substance for the
processor. Mostly, grading involves simultaneous evaluation of multiple properties which
makes complexity in mechanical grading and hence manual grading is preferred. The
separation may be based on size, shape, colour of the food product, which is done to make
different qualities.
Grading method: In general the properties of food product which determine its quality may
be shape, size, maturity, texture, flavour, and colour and contaminants. Grading may be done
manually or with the help of specialized machines, which are discussed below:
Manual grading: Most of the grading of fruits, vegetables, eggs or such food product is
carried out manually. The grader forms a balanced judgement of the overall quality and
physically separates the food into quality categories. Apples may be graded using colour
cards making various grades in terms of proportion of the surface showing characteristics of
fruit colour, extent of rusting, surface imperfections etc. Cherries are compared against
models made from spheres of plastic, fitted with nylon stems. Eggs are graded manually by
candling. It is a non-destructive test of egg content. In this type of test, the translucent shell of
the egg is spun in front of a powerful light in a darkness booth. The presentation of each egg
is automatic and candling operator can separate several thousand eggs in one hour into 3-4
grades.
There are some disadvantages of manual grading. It is a labour intensive job, which requires
a lot of labour to grade fruit and vegetables. But still most of the grading of fruits like,
mango, orange, kinnow and vegetables are done manually in India.
Machine grading: Grading machines are available for cereals and fruits and vegetables
separately. Machine grading for cereals is of two types viz. rotary screen grain cleaner and
grader and spiral separator.
Grading machines for fruits and vegetables: There are four types of grading machines for
fruits and vegetables. They are screens, roller grader, and diverging belt grader and weight
grader.
Screens: There is variety of fruits and vegetables which are graded with the help of vibrating
screens made of copper, stainless steel or plastic, which do not react chemically with the

3. products. The material to be graded is passed over to the vibrating or rotary screen. This
screen is perforated to pass the smallest material at the beginning then medium and lastly the
largest material. In this way, it makes different grades of fruits like, apple, orange, kinow, and
vegetables like potato, tomato etc. The material that passes the top of the screen can be
rescreened, and the new fractions can be derived.
Roller grader: This type of graders is fast, accurate and causes little damage to fruit. These
are extensively used in fruit industry. Each roller rotates in a counter clockwise direction. The
fruit is continuously rotated so that each piece has an opportunity to register its minimum
dimension with the space in the grader. Roller conveyers with fixed space between the rolls
are used for removing small fruit, twigs and leaves.
Diverging belt grader: It is widely used grader, which consists of two belts. These belts
diverge as they move. The fruit is carried on and between the belts. Since the distance
between two belts increases gradually and systematically, the smaller pieces will drop
between the belts at the beginning of travel whereas the larger pieces will be carried further
and will be dropped later.
Weight grader: The grading is also done on the basis of weight of product. This method is
accurate, fast and there is minimum damage to fruits or vegetables. This can be used for large
size products like apples, oranges, kinnow, mango, potato, tomato, eggs etc. These are
specially adapted for sorting material, which cannot be handled by other methods, due to their
texture or shape. The material to be graded is placed in individual cups through automatic
feed, which is then passed through the sorter where it gets indexed with the help of spring
loaded trips. The spring tension is progressively weaker from beginning to end of movement.
The heavier fraction discharge in the beginning whereas lighter fraction next and lightest at
the end. This type of grading is not dependent on shape or size of material.
Expanding pitch rubber spool potato sizer: It consists of two driving rollers with helical
grooves of progressively increasing pitch that moves the rods with rubber spool. The
advantage of such mechanism is that as many grades of potatoes can be obtained as desired.
The rods are carried together with two link chains and rubber spools are carried on them.
Wrapping a mild steel strip of varying thickness over a GI pipe makes the expanding pitch
helix. The length of sizing bed is 1.83 m to provide three sizing bed of 0.61 m each or four
sizing bed of 0.46 m each. The frame is made up of angle iron sections and is designed to
support two parallels running driving helix on the sides, an endless sizing conveyor and an
arrangement to drive the sizing conveyor (Fig. 1). Two conveyors i.e. elevator feed conveyor
and intermediate receiving conveyor convey the material from the receiving end and
discharge it to the sizing conveyor. An inclined collecting platform facilitates the flow of
material. The portions of platform can be adjusted to suit the requirements of different grades
of potatoes. The machine can be operated by one horsepower motor and can grade 2-3 tonnes
of potatoes in one hour. The mechanical damage to tubers is negligible.

4. Root crop combines
Self-propelled potato combines are also available commercially for harvesting of potatoes.
The machine consists of a pointed blade to split the ridge, an elevator conveyer to lift the
material and shake off soil, agitator sprockets to provide adequate agitation to material being
carried over elevator conveyer, a rotary cage wheel for elevating and cleaning of tubers and
bagging platform. It can dig the potato tubers, separate, clean and fill in the bags in one
operation. It can cover about 0.6 ha/day. Losses are very heavy with this machine and ranges
from 15-20% or even some times more. Potato tubers harvested at lower soil moisture suffer
less damage than those harvested at higher soil moisture. Also potato tubers harvested in
sandy loam soil suffers less damage as compared to those harvested in heavy soils.