Fish Products and Byproducts Technology

Unit 3 - Salting and Drying

3.2.3.d. Contact dryers
Vacuum dryers
Hollow shelves, through which the heating medium is circulated, are fitted in the chamber. The material to be dried is placed inside the chamber in metal trays, which rests on the shelf. Vacuum will be drawn and drying will proceed under vacuum. This is an expensive method, but suitable for drying fatty fishes where oxidation and rancidity fat can be minimized.
Rotary dryers
These dryers are used mainly for fishmeal. The source of heat is hot air, which is circulated through the shell or it can be supplied from a steam heat jacket. Rotary drum dryer, direct rotary dryer, indirect rotary dryer and steam dryer are some of the type of dryers used for drying purposes.
Spray dryer
Spray driers are generally used for drying foods which are in the form of liquids or suspensions. In principle a food in a liquid are paste form is atomized and dispersed as minute droplets which are suspended in a stream of hot air in a chamber where it gets rapidly dried. The dry particle suspended in the air stream flow into separation equipment where they are separated from the air, collected and packaged In the application of spray drying to fish products it is limited to products like fish protein hydrolysates and fish powders. The features of a simple spray drier are given below.
Better retention of colour, flavour and nutritive value are the advantages associated with spray dried foods.
Drum Dryer
Drum drier is an example where heat transfer to the product takes place through a solid surface. Drum drier is used for drying fluid materials. The food product in the form of slurry is deposited as a thin film on the drum. The drum is heated, generally by steam; while it is being rotated. Drying can be done keeping the drum open to the atmosphere. If the material is desired to be dried under vacuum, the drier can be enclosed in a vacuum chamber. The product when dry is removed from the surface of the drum used a scraper blade.
Drum driers are classified as single drum, double drum and twin drum types. Single drum drier comprises only one roll. Double drum comprises two drums rotating towards each other. Twin drum is similar to double drum, but rotate away from each other.
Vacuum shelf dryer
Vacuum shelf drier consists of a vacuum tight chamber of heavy construction with access door and outlet for gases and vapours. Hollow shelves, through which the heating medium is circulated, are fitted inside the chamber. The material to be dried is spread in fairly thin layers in metal trays which rest on these shelves. Alternately the material can be spread directly on the shelves. Heating is done by circulating hot oil, steam or any other suitable heating medium. Vacuum will be drawn in the chamber through the vapour outlet, and drying will proceed under vacuum. The initial drying rate will be high; however, as the drying proceeds the material will shrink and tend to curve away from the trays. This will reduce the effective are of contact of the material with the heating surface which will cause a decline of heat transfer to the material thus slowing down the drying rate.
Vacuum drying is considered an expensive process. However, it is quite suitable for drying fatty fishes where the probability of fat oxidation and rancidity in the product can be minimized.