Fish is an extremely nutritious food than meat and egg and it’s extremely perishable because of its high moisture content that is about 80th. Fish preservation is important immediately after catch to increase the shelf life of fish. Preservation methods helps to maintain the quality of fish for longer period of time, prevents spoilage and decomposition, retains its original nutritional contents and makes transportation and storage of fish easier. Fish preservation techniques vary with type, nature, size and condition of fish. Improper handling and process of fish results in immediate spoilage of fish ends up in poor quality. Conventional preservation techniques like chilling, freezing, drying and chemical preservation are widely being employed for fish preservation throughout the globe. Among the various preservation techniques drying of fish is the oldest preservation technique and drying means that preservation of fish by removing water from it through heating. Drying removes the moisture content up to a certain level to prevent microbial growth thereby provides greater shelf life, reduction in weight, volume, transportation and storage space. Two commonly used drying ways are natural and artificial drying. Natural drying includes sun drying, solar drying, wherever artificial drying includes microwave, fluidized bed, spouted bed, infrared, connective drying, desiccant drying, freeze drying, osmotic, vacuum drying, pulsed electric field, high hydrostatic pressure, superheated steam drying, heat pump and spray drying etc.
Natural drying methods are associated with disadvantages like contamination and damage by dirt, insects, rodents, birds and animals. Sun drying of fish often results in low quality products since drying is slow normally it takes five to seven days. Therefore it is necessary to choose an advanced method of drying to obtain good quality product. Artificial drying methods have advantages like less drying time, good quality drying, better process control, operational safety and higher capacity.
Drying involves the evaporation of moisture from the surface of the fish and the migration of moisture from inside the fish to the surface. Drying is affected by the movement of air over the surface of the fish as well as the temperature and humidity of that air. Sun drying of fish, with or without the addition of salt, is practiced in many tropical countries, and is a low cost form of preservation.
Advanced drying methods
Solar Drying
Solar energy has been used all around the world to dry food products. Solar drying is use of equipment to collect the sun’s radiation in order to harness the radioactive. Energy for drying applications Good product quality can be retained with the control of radioactive heat. It is mainly used to dry products like grains, fruits, vegetables, meat and fish.
Fluidized Bed Drying
In fluidized bed drying (FBD) system, air is allowed to pass through the bed of solid material in the upward direction with the velocity greater than the settling rate of solid particles. It is mainly working on the fluidization of solid materials. Since hot air is introduced from bottom of the system at high pressure the solid particles which have to be dried will be in suspended state in a stream of air. Heat transfer is accomplished by direct contact between the solid material and hot air. Vaporized liquid is carried away by the hot air.
Infrared Drying
IR drying can be considered to be an artificial sun drying method and it can sustain throughout the day. Advantages of using IR for drying include: flexibility of operation, simplicity of the required equipment, fast response of heating and drying, easy installation to any drying chamber and low capital cost. It can be used for various food materials like grains, flour, vegetables, pasta, meat and fish.
Vacuum Drying
Vacuum drying is a process in which materials are dried in a reduced pressure environment, which lowers the temperature required for rapid drying. Major advantages of vacuum drying are as follows: less energy is needed for drying, it is highly suitable for heat sensitive food materials, faster method than other drying methods, it retains integrity of materials etc. In general, vacuum drying is performed in combination with other drying techniques.
Superheated Steam Drying
In a superheated steam drying, the drying gas in a convective dryer is replaced with superheated steam. Superheated steam at certain pressure enters in drying chamber and removes the moisture from wet foods and the exhaust from the dryer is also superheated steam with a lower specific enthalpy.
Freeze Drying
Freeze drying or lyophilization is a dehydration process used to preserve a material and make it into more convenient for transport. It is a method of water removal from material by sublimation. This drying process is divided into three stages: pre-freezing of wet material, primary drying (sublimation of frozen water under vacuum) and secondary drying stage (desorption of residual found water from material). Freeze drying is initially freezing the material and then reducing the surrounding pressure to allow the frozen water in the material to sublimate directly from the solid phase to the gas phase. It is one of the best methods of water removal and results in a final product of much higher quality compared to any other drying technique.
Heat Pump Drying
A heat pump is a device that transports energy from a low temperature source to a higher temperature sink. This transfer requires an input of work which may be supplied mechanically as in a Vapour-compression cycle.
Dielectric Drying
Electromagnetic energy of microwave and radio frequency (RF) can directly interact with foods to quickly raise center temperature since most of the food materials are dielectric materials and can store electric energy and convert it into heat. It is volumetric heating and quick raise of temperature is possible.
Advantages of dried fish:
- Dried fish processed through sun drying or dehydration is highly concentrated fish compared to other preserved form of fish.
- As water content becomes reduced so microbial activity cannot run at normal rate thus reduce the spoilage of fish.
- Less expensive method and comparatively simple procedure.
- Reduced water content, enzymatic and many chemical processes which are responsible for fish spoilage retarded.
- In this method, complicated machinery and equipment are not required.
- Dried fish remain stable at most ambient temperature.
Conclusion
Drying is an important process to preserve food materials and to extend the shelf life. Different drying methods are available for drying of foods and each has its own advantages and disadvantages. Traditional drying methods (sun, solar, hot air oven drying) are simple to use but have low energy efficiency and longer drying time. Thus it negatively affects the color, flavor and nutrient content of dried products. Some advanced drying methods (freeze drying, microwave, heat pump and vacuum drying) offer a wide scope for the production of best quality dried products. But usages of these methods for drying are restricted due to its high cost. Therefore cost-effective alternative systems such as combined/hybrid drying can be used for the drying of products with minimum cost and simple technologies. Combination drying with an initial conventional drying process followed by microwave/vacuum or simultaneously two methods hot air with infrared/microwave/vacuum has proven to reduce drying time with improved product quality and minimizing energy requirements.
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