Freezing Technology

4.1.1.1 Thawing in air

a)      Natural convection:

It is sometime convenient to thaw at room condition, if conditions are suitable.  Temperatures above 18^ºC should not be used.  Lot of space and labour is required for thawing on a large scale by this method and the introduction of humidification to prevent slight drying of the fish is difficult.  

b)      Air blast:

In this method, a current of air is directed on the frozen block and the increased air velocity reduces thawing time.  The rate of thawing is mainly dependent on the resistances of the fish itself to the flow of heat.  At the air velocity of 6m/sec, there is very little resistance, which is optimum.  In modern air-blast thawers with the recirculation of humid air at 20^oC( the maximum, temperature that should be employed), the 10cm block of frozen fish requires about 5 hours for thawing before they can be filleted.  The fish can be separated easily after about 3 hours, which reduces the thawing time to about 4 hours and gives more even thawing.  Complete humidification of the air is necessary in order to prevent drying of the fish and to achieve ideal condition for heat transfer.  Weight losses in the fish normally do not exceed 1%.  The problem of cleaning is greater with the air blast thawers when compared with most other types.  Two types of air blast thawers, i) continuous ii) batch.  

i) Continuous:

Continuous air-blast thawers have been built in two designs.  Cross flow and parallel flow.  In the cross flow design, the direction of air flow is across the conveyor at right angle.  Typically there are five conveyors, one above the other.  The fish travels horizontally along the top conveyor until when it reaches the end, it falls to the conveyor below which is traveling in the opposite direction and so on.   In order to achieve high velocity, capacity of the fan should be high with this arrangement and use of  fan power is more than 30 KW for a unit fish.  Thus a large portion of the heat for thawing is obtained from the fan. 

In the parallel flow thawers, the air travels in the same direction as the conveyor.  With humid air, most of the heat transfer to the fish takes place by condensation at water vapour and an air temperature drop at 30⁰C can be tolerated.  The fan capacity is relatively low and therefore, the fan power is about 7.5 KW for the 1 ton/hour unit and the face area of the heater and humidifier in conveyor is, in a single line occupying a greater floor area, than a cross flow thawers.  In the other, the conveyor is divided into three sections, one above the other.

ii) Batch:

            The largest model of batch air-blast thawers has five compartments each holding 1 ton of 10cm blocks of whole fish on two trolleys with suitably- spaced shelves, giving an output at about 1 ton/hour.  Air flow is in series through the five compartments in a line.  Periodic reversal at air flow can be employed to even out differences between compartments.  The thawers can also be used on a semi continuous basis by inserting and withdrawing trolleys at intervals of 1 hours.  More labour for loading and unloading is required than for continuous thawers, but the batch thawer is simple and compact. The 20 KW fan heaters and humidifier are located above the fish compartments.  Batch thawers with small fans giving moderate velocities, in which blocks of fish are placed on racks or shelves, have been used for thawing frozen fish for over periods of up to 10 hours.