Freezing Technology

2.4.1.1. Refrigerated Sea Water (RSW)

Seawater has a salt content of around 3 to 3.5%. At 3.5% salt, seawater has a freezing point of about –2^oC. Thus, if seawater is refrigerated, it is possible to reduce the temperature so that a storage temperature of –1^oC can be achieved. The most important advantage of RSW over icing is the ease of handling and storage onboard, with a resultant saving of labour. In a purse - seiner, the fish can be brailed from the net direct into the tank and brailed out of the tank at the landing point. Pumping systems have been developed for unloading fish as large as 10kg each at a rate of more than 25 tonnes/hour. In the most sophisticated systems, the vessel can be equipped with a pump which pumps the fish out of the sea into holding tank and the same pumps can be used for unloading. RSW is popular on large vessels, while CSW can be applied even on small boats. For RSW, larger capital investment is required for the mechanical refrigeration system. In order to avoid inefficient, too slow chilling of the catch, not more than 800kg of fish should be packed per kilo litre of the brine. RSW may also be used to cool the catch by spraying over the top surface of ungutted fish or shrimp in the hold. Rapid chilling and holding of the catch in RSW is a popular practice on board large vessels which catch shrimps.

 Chilling of the catch in RSW before freezing on board freezer trawlers is a very effective method of preventing rapid quality loss of the fish on subtropical and tropical grounds. The shelf life and quality seafoods chilled or stored in CSW or RSW depends upon the species characteristics and the condition of chilling.

Generally, it is a few days longer than that of iced fish. Some species are vulnerable to loss of appearance due to bleaching or leaching. The uptake of salt may significantly decrease the market value of some seafoods, like shrimps, snappers. The advantages of RSW are

1. Labour saving in stowage
2. Rapid chilling, 2 to 3 times as fast as with ice
3. Elimination of crushing and ice pitting
4. Lower chill temperatures
5. Washing effect
6. Ease of unloading