Grading sorting

Grading is dividing a food item into different categories, which is done based on a measurable physical property and should be done as soon as possible to ensure the uniformity of the product in the next processing. This work is done in the food industry by grading machines or sorting machines. The main physical characteristics, which are used in food grading, are: size, shape, weight, color, and density, which the devices are designed based on.

The shape of some foods is important in determining their processing capability or commercial value. Sorting in terms of size, which is called sieving or granulation, is the division of solid materials into two or more components based on their size difference. The devices that perform grading based on this are: A- Sieves with fixed pores Two types of sieves with fixed pores are more common: flat sieve and cylindrical sieve The multi-layer flat sieve is equipped with a number of horizontal or inclined mesh plates, which have different openings from 20 mm to 12.5 cm, which are located inside an oscillating frame. Pieces of food whose size is smaller than the mesh of the upper sieve (the first sieve from the top) pass through the sieve under the influence of gravity and this process continues until they finally reach the mesh on which they remain. br>

How to distribute the size and shape of parts Sieve construction materials Amplitude and frequency of sieve vibration Effectiveness of the methods used to prevent blockage (or blinding) of the sieve mesh. The capacity of a sieve is the amount of food that passes through each square meter in one second. Flat sieves are widely used in dry food categories such as flour, sugar and spices.

blinding or blockage of the sieve; Especially if the seeds and grid are almost the same size. The very high speed of the flow of food, which leads to the accumulation of excess load on the sieve network and causes larger particles to exit from the pores. Large grains that block the pores of the sieve Wet parts or high humidity of the environment that cause small particles to stick to each other or to the pores of the sieve and produce larger particles that do not pass through the sieve. Often, the vibration of the sieve alone is not enough to completely separate the parts from each other. For this reason, it is necessary to use a rotating movement to spread the food all over the surface of the sieve and a pulsating movement in the vertical direction to break the parts stuck together and prevent blinding. Different types of cylindrical sieves are used to classify some foods that are in the form of small grains (such as peas and beans) and have sufficient mechanical resistance to withstand twisting movements inside the sieve. Cylindrical sieves are made of metal cylinders with holes or grids that are placed in an almost horizontal position (with an angle of 5 to 10 degrees). The cylinders may be concentric (one inside the other), parallel (food exits one cylinder and enters the next cylinder), or stacked (a single cylinder consisting of several sieve areas with different sizes). The total capacity of cylindrical sieves is more than that of flat bed sieves. The capacity of cylindrical sieves increases with increasing rotation speed until it reaches a critical point. Above the critical speed, the centrifugal force prevents the food material from passing through the sieve and the separation does not take place properly. B - Roller sorting device In this type of devices, a roller is used, that's why this method is called roller grading. In this way, the rollers are placed two by two parallel to each other and their distance is small at the beginning and gradually increases towards the end. The rollers are used at different speeds to rotate and align the pieces of food and as a result, the food is placed on the smallest side of the sieve. A step increase in the pore size is provided by adjusting the distance between the guide rollers and an inclined conveyor belt. The piece of food rotates and is placed on the pores from the smallest dimension, and the smallest dimension is the basis of classification.

The classification based on weight is more accurate than other methods, and for this reason, it is used for more expensive foods such as eggs, meat, etc. The weighing sorting machine consists of a conveyor with special houses that transfer the eggs to the arm-shaped pans of a weighing scale. The conveyor works intermittently and when it is stopped, the arms move upwards and weigh the eggs. Heavy eggs are poured into a duct made of soft material, and lighter eggs are transferred back to the conveyor to be taken to the next weighing machines.

The specific weight of a product changes with the type of handling and health. Separation is done in solutions with controlled specific gravity, heavier products fall to the bottom and lighter products float on top. This method, which is known as immersion method, is used to classify green peas, corn, potatoes, etc.

This method is based on the diameter, length, surface damage, the way the parts are placed on the conveyor and the color of the food. This method is used in the classification of corn clusters. The ears of corn pass in front of three video cameras that are positioned at an angle of 120 degrees to each other and on top of a conveyor belt. The images of the clusters are recorded and stored in the memory of a microprocessor. Then the resulting information is analyzed and compared with the product features that have already been pre-programmed to the system. As a result, each of the corn clusters is added or returned to groups with similar characteristics.

With the use of microprocessors in the control of color classification devices, the possibility of quick classification of fine grain food is possible. Seeds follow each other into a channel. The angle of the shape and material of the channel layer can be changed to adjust the speed of the seeds passing through a photometer. The background color and light intensity required to brighten the surface of the food is carefully controlled for each type of product, the light meter measures the light reflected from each piece and compares it with the predetermined control colors. Defective foods are separated from the rest by short-term blowing of compressed air. This classification method is used in peanuts, beans, rice, corn seeds and small fruits.