Fish Products and Byproducts Technology

Unit 3 - Salting and Drying

3.2.3.c. Artificial /Mechanical dryers
In artificial driers, removal of water from the fish is achieved by an external input of thermal energy. This is an expensive method since there is need of fuel for heating and maintenance of the temperature. The drying chamber consists of a long tunnel in which the washed and cleaned fish is placed on trays or racks. A blast of hot air is passed over the material to be dried. After the required degree of drying the product is removed from the drier and packed.
These can be broadly classified into two types.

• The heat is transferred into the product through a hot gas, usually eg. Kiln dryers, cabinet dryers, tunnel dryers and fluidized bed dryers.
• The heat is transferred in to the product through a solid surface, which may also be used as the cabinet for the product to be dried. eg. Drum dryer, vacuum dryer.

Hot air dryers
In these dryers the air is heated by steam / electrically. The hot air is blown over the product by the help of a fan or blower. The different parameters like temperature, air velocity, and humidity are controlled. Different types of dryers in this type are
• Cabinet dryer
• Tunnel dryer
• Multideck dryer
• Fluidized bed dryer
Cabinet dryer
This is a simple batch operated model used for relatively small scale operations. A typical cabinet dryer consists of an insulated or non-insulated framed structure. Materials to be dried are spread uniformly on trays placed on permanent supports provided in the drier. A fan located inside the drier will blow air through a heat source, which pass across or through the material loaded in trays.
Kiln dryer
Kiln drier is a batch drier. A typical drying kiln will consist of a two story buildings. The floor of the upper story is slotted or may be composed of narrow slats on which the material can be spread. This story serves as drying room. The burners or furnace product hot gas is located in the lower floor. The hot gas passes through the product by natural conduction; often forced circulation with the help of a fan also may become necessary. The material being dried has to be tuned and stirred frequently to ensure uniform drying.
Tunnel dryer
This type of dryer is most commonly used for drying fish. These are made in the form of long tunnels, 10-15m long. Trolleys loaded with fish are moved at a predetermined schedule through the tunnel. Temperature and air velocity controls are provided. Hot air blown over the material is circulated with the help of fans. The movement is of air is maintained in either direction in relation to the flow of the material. If the air movement is in the same direction as that of product movement, dryer is called a parallel flow drying tunnel. In such dryers the hottest air comes in contact with the wettest fish. Therefore, higher temperatures can be used for drying. One serious disadvantage of the system is that the air towards the outlet side will become cool and highly humid; therefore the finished product may not be sufficiently dried.
If the movement of air is in the direction opposite to the movement of the product it is known as counter flow drying tunnel. Here, the hot dry air comes first in contact with the driest material, so that the finished product obtained will be sufficiently dry. However, the disadvantage is that in prolonged drying schedules the fish from one end of the tunnel may remain in humid air for long periods without getting sufficiently dry and result in spoilage.
Tunnel dryers should be designed based on the concept of hot air recirculation or by allowing part of the hot air passing over the fish to escape to the atmosphere. The recirculating air will become humid quickly and slow down the drying process, hence provision for de humidification and humidity control should be provided.
Multi deck tunnel dryer
In this type of dryer several tunnels are placed one above the other and air is blown in a zigzag manner thrice after which it is exhausted. Here all the controls, heater, air blower and exhaust fans are placed on one side.
Fluidized Bed dryer
The product is passed on a perforated conveyor bed and air is passed below, so that the product is partially lifted and dried. This is mainly used for fishmeal and fish powder.