Fish & Seafood Rapid Defrosting with Radio Frequency

Posted in Fish Defroster, Freezing & Thawing, High Speed Defrosting

In the fish processing industry, more than half of the raw material utilised is frozen: squid, mackerel, shrimps, prawns, mussels, calamari, …etc.

Seasonal fluctuations in the availability of fresh fish are a serious impediment to the continuous operation of processing plants: it is possible in principle to regulate fish supply discontinuities by freezing, without significantly impairing the quality of the final product.

When fish has to be processed, it is often first tempered or defrosted using conventional methods available. These methods introduce a number of difficulties directly related to the heat transfer mechanisms:

• heat transfer is an intrinsically slow mechanism and, the larger the size of product, the longer is the time necessary for the defrosting process; this introduces a long delay between the removal of the fish from the cold stores and the next processing stage;
• since bacteria survive at negative storage temperatures, in the slow defrosting process there is a considerable opportunity for bacteria growth in the product;
• proportionally high drip losses may result from lengthy defrosting time, causing changes in the product texture and leading to an immediate and significant economic loss;
• it is not possible to speed up the defrosting process by increasing the heating fluid temperature, as this could cause severe deterioration of the product surface;
• a long defrosting time makes it unavoidable to carry out the process discontinuously (batch operation), resulting in high handling costs; besides, breakage, bruising and other damages to the product may result from such handling;

The drawbacks of conventional defrosting methods described above can be avoided using radio frequency (RF) fields to rapidly generate heat volumetrically within the product. The heating process is uniform and controlled, resulting in a significant reduction of drip losses. RF heating method offers flexibility in the production scheduling and is the ideal solution for many tempering, softening and defrosting processes.

HOW DOES THE RADIO FREQUENCY DEFROSTING METHOD WORK?

The fish blocks or other packages are placed on the machine’s conveyor belt and are transferred through the RF unit (tunnel) passing between an upper and lower metallic plates. These plates (also called electrodes) form an electrical capacitor and the product in between the plates becomes the dielectric element of that capacitor. The electrode plates are connected to a radio frequency generator oscillating at a frequency of about 27 million cycles per second.
When the RF generator supplies high frequency alternating voltage between the capacitor plates, the dipolar water molecules of the frozen fish will vibrate and rotate in the attempt to align themselves according to the fast changing opposite plates polarities. This phenomenon causes intermolecular friction, which will in turn generate heat rapidly and uniformly within the whole product mass regardless of its size, weight, shape and thermal conductivity.
The amount of heat generated inside the product and defrosting time are accurately controlled through the voltage applied on the electrode plates and the speed of the conveyor belt.

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MACHINE CHARACTERISTIC

• Radio Frequency at 27,12 MHz.
• Construction in AISI 304 & 316 stainless steel submitted to anticorrosion treatments
  (passivation, pickling) and shot-peening finish.
• Protection level IP65: the external protection boxes of the RF generator are made of insulating
  sandwich-type panels with sheeting and bearing frame in AISI 304 stainless steel.
• Wide conveyor belt (up to 180 cm) made of certified food-grade rigid polyethylene modules or solid-surface reinforced polyester, depending on the product to be processed.
• Built-in conveyor belt & tunnel washing facilities and full internal access for cleaning through the multiple side panel doors.
• PLC control system for multiple product recipes.
• Modular Construction: STALAM RF defrosters are available in module sizes from 3 kW to 105 kW. Multiple modules can be combined to increase the production capacity.

PRODUCTION CAPACITIES

Production capacities can vary depending upon the type of product to be defrosted and the final
temperature required. Here below an example:
Fish:                                   frozen squid to be tempered from –18°C to –2°C
Size of machine:              single module 75 kWRF.
Max power output:         75kWRF
Max power input:           120 kVA
Production Capacity:      up to 1250 kg/hr