Farm Machinery and Equipment-II

LESSON 17. GRADERS AND ROOT CROP COMBINES

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 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.

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.