Ice Slurry: Advanced Fish Chilling and Preservation Technology
The rapid increase of the world population in the past few decades has put added pressure on the fishing and aquaculture industries to improve the production, quality, and shelf life of fish. One focus is to implement advanced means of fish preservation for all after-catch operations.
Seafood is highly perishable. It begins to deteriorate as soon as it dies, in water or onboard a fishing vessel. Spoilage, coupled with loss of freshness, takes place through bacterial, enzymatic, or chemical action. Bacteria that do little harm to a healthy, live fish invade the fish body on its death and begin weakening the chemical structure. Enzymes in the living fish remain active after death and cause unwanted flavor changes during the early stages of storage. Chemical reactions between oxygen in the air and fat in the fish create additional undesirable rancid flavors. Fish spoil much faster through these reactions at higher surrounding temperatures. For example, Atlantic cod Gadus morhua from the Norwegian seas kept at 0°C has a shelf life of 12–16 d, whereas the shelf life reduces to 7–9 d at 4°C, 5–6 d at 8°C, and 3–4 d at 12°C (Magnussen and Nordtvedt 1999). Similar trends are reported for crab claws, salmon, and sea bream (Huss 1995). The deterioration process may accelerate once fish is bruised, cut, scraped, or contaminated. The key to seafood preservation is therefore immediate chilling at harvest to a temperature slightly above the freezing point and maintenance of this temperature until further processing. This also applies to seafood to be frozen, because the freezing process takes time to accomplish and only slows down or stabilizes the spoilage.
Traditional fish chilling and preservation methods include refrigerated seawater (RSW); ice; ice–water mixture; and freezing by plate, air-blast, or brine-type freezers. RSW usually provides a temporary means of refrigeration, but it takes a long time to bring the temperature of fish down to the desired level, and salt uptake is always a concern. Deep freezing is used if storage of many weeks or months is required. For high-value fresh fish, deep freezing is always avoided.
Ice has been used to control seafood spoilage for more than a century. The latent heat of fusion of ice (335 kJ/kg) provides high cooling capacity. Ice keeps seafood not only cold but also moist and glossy. It also prevents dehydration. Block or crushed block ice, tube ice, flake ice, and shell or plate ice are ice types that have been used for decades. One common and key problem using traditional ice is the low chilling speed, no matter whether ice is directly applied to fish or mixed with water, then applied to fish. In addition, traditional ice particles have irregular shapes with sharp edges that may bruise the fish skin. The supercooled flake ice may partially freeze seafood, reducing the market value of the fresh product. Large energy consumption and space requirement of the ice equipment are also concerns associated with the traditional technology.
A new kind of ice with the trade name of “DeepChill” was developed by Sunwell in the late 1970s. Since then, it has been successfully applied in a form of either dry ice crystals or pumpable ice slurry in fisheries. Demands for ice slurry system come from almost every aspect of fishing and fishprocessing operations.