There are various size reduction equipment and have different principle. The common working principles in food industries are slicing (as in preparing potato chips), dicing (cutting into cubes), shredding (shear force as in wheat milling), grinding (impact force as in making spice powder), crushing and pulping (as in fruit juice extraction). However, size reduction equipment are not only used in food processing industries. It is also widely used in construction industries. Common examples are; Jaw crusher and Gyratory crusher.
Jaw crushers: Jaw crushers are used to bring very big particle into reasonably small size of crushing of ores; lime, stone etc. The moving jaw angle is 20 – 30° against fixed jaw. One jaw is fixed and another moves in backward and forward movement which helps in crushing. Clearance gap is more on the top and gradually decreases down. The jaw opens and close 250 – 400 times / min. Gyratory crushers: They are heavy crushers to break huge lumps of ore; lime and stone into reasonably small size. They have circular jaws between which materials are crushed. They consist of conical crushing head which gyrates within casing and the bottom of crushing head moves towards and then away from the stationary walls. The speed of crushing head is 125 to 425 gyrates/min.
Commonly used size reduction equipment in food industries:
Crushing rolls: Two heavy steel cylinders revolve toward each other. The compressive force between rolls break or crush food particles. Some machines use differential speed between the rolls break or crush the food particles. Some machines use differential speed between the rolls so that shearing force also arises. The output of this unit is governed by roller length, diameter, and speed of rotation. For large diameter rolls, speed of rolls varies from 50 – 300 rpm. But for small diameter rolls speed rolls are very less. Generally size reduction ratio of these rolls are below 5 (i.e. coarse grinding). The diameter of these rolls and speed between rolls can be varied according to feed size and output rate required.
To prevent damage due to overload, compression spring is used. In these types of rolls, hard foreign substances should be avoided. Crushing rolls are used in milling of wheat and refining of cocoa powder. Rolls surfaces may also be added with friction nip so as to control the speed. Single roll crushers with stationary breaker plate can also be used for crushing friable materials.
Hammer mill: This is impact type grinder. A high speed rotor carries a base bearing number of hammers around its periphery. When the rotor turns, the hammer head swings through a circular path inside a closed fitting casing containing a tough breaker plate. Reduction in size is due to impact forces mainly but shearing or attrition forces also play role in breaking.Hammers are often replaced by cutters or blades. Hammer mill can handle hard crystalline solids, fibrous materials like vegetables, sticky materials like gum, milk powders if hammers are replaced by cutters or blades. In food industries, it is generally used for grinding pepper and other spices, sugar crystals and dried milk (after drum drying).
Disc and attrition mill: Disc and attrition mill can be further classified as following.
a) Single disc mill (SDM): The feed is passed between a high speed rotating grooved disk and a stationary casing. The shearing action results in size reduction. The gap between disk and casing is adjustable depending upon feed size and product requirement. b) Double disc mill (DDM): In this type of disc mill, generally both discs are rotating. Discs rotate in opposite direction so there is greater shearing force than single disc mill. This type of disc attrition mill is used in cereal products like corn and rice milling.c) Buhr mill: This is older type of disc attrition mill used in milling of flour. Two circular stones are mounted on vertical axis. This mill also uses shearing force. In this type of mill, upper stone rotates and the product exits between the edges of two stones. These are generally used in wet milling of corn, pulses and separate starch from hull or husk.
Tumbling mill: Tumbling mills can be further classified as following.
a) Ball mill: both shearing and impact forces are used. It consist of horizontal slow speed rotating cylinder which contains steel balls or stones. As cylinder rotates, balls move up and down and strikes the feed which breaks the feed. Balls are usually of the size 25 to 150 mm i.e. 1 – 6 inch depending on the type of feed.
At low speed, balls tumble over each other where shear force predominates but at high speed, balls are lifted and impact force increases. If speed is very high, central force may act resulting in collision to the balls. So for efficient size reduction a critical speed should not exceed. Critical speed is the speed at which balls inside the mill just begin to centrifuge given by formula
Nc = 42.3/D1/2 where,
Nc = Speed in rpm
D = diameter of mill in meter
Practically, optimum operating speed is about 75 % of critical speed. The balls occupy 50 % of total volume.
b) Rod mill: in rod mills, balls are replaced by high carbon steel rods. Here, both impact and shearing force act but the effect of impact force is less than shearing force. Rod mills are recommended for the use with sticky substances where balls are trapped in their mass. The rods occupy 50 % of total volume.
Name : Pratiksha Shrestha
Ms. Shrestha holds masters degree in food engineering and bioprocess technology from Asian Institute of Technology (AIT) Thailand. She is currently working for Government of Nepal at Department of Food Technology and Quality Control (DFTQC), Kathmandu. She is also a teaching faculty in College of Applied food and Dairy Technology (CAFODAT) affiliated to Purbanchal university, Nepal.