Fish caught at sea are primarily frozen immediately after harvest using sophisticated on-board technologies such as brine immersion, air blast, and plate freezing methods to ensure maximum freshness and preserve quality for consumers.
Freezing fish at sea, often referred to as "freezer trawling" or "on-board freezing," is a critical practice in the modern fishing industry. It allows vessels to extend their trips, maintain the quality of their catch, and provide high-quality seafood to markets worldwide. The immediate freezing process halts spoilage and maintains the taste, texture, and nutritional value of the fish.
Why Freeze Fish On-Board?
Freezing fish directly on the vessel offers significant advantages over bringing fresh fish back to shore:
- Preservation of Quality: Rapid freezing at ultra-low temperatures prevents the formation of large ice crystals that can damage fish tissue, preserving the delicate texture and flavor.
- Extended Shelf Life: Freezing extends the usability of the catch significantly, allowing for longer voyages and broader distribution.
- Reduced Spoilage: Immediate processing minimizes bacterial growth and enzymatic activity, which are the primary causes of fish spoilage.
- Market Flexibility: Frozen fish can be stored and transported efficiently, allowing producers to meet market demand more flexibly.
Primary Freezing Methods Used at Sea
Different freezing methods are employed depending on the type and size of fish, as well as the capacity of the vessel.
1. Brine Immersion Freezing
This method involves submerging fish directly into a super-cooled salt solution (brine). Brine freezing is extremely fast due to the high heat transfer rate of liquids.
- Process: Fish are placed in baskets or directly into tanks filled with brine cooled to temperatures as low as -20°C (-4°F) or even lower for specific applications.
- Application: It is particularly effective for smaller fish, crustaceans, and mollusks like shrimp or sardines. Historically, brine immersion freezers were also used for freezing large fish, such as tuna, due to their speed.
- Considerations: While fast, this method can sometimes lead to increased salt uptake in the fish, which may affect taste or require desalinization.
2. Air Blast Freezing
Air blast freezers circulate high-velocity, extremely cold air around the fish. This method is highly versatile and widely used across the industry.
- Process: Fish are placed on trays or conveyors within an insulated chamber where powerful fans circulate air cooled to temperatures ranging from -30°C to -60°C (-22°F to -76°F).
- Application: This method is excellent for various sizes and types of fish, including fillets, whole fish, and blocks of fish. Significantly, for large fish like tuna, which are often frozen individually, there has been a recent trend toward using air blast freezers due to their efficiency and ability to maintain product quality.
- Benefits: Air blast freezing results in good product appearance and minimizes dehydration, making it a preferred choice for high-value fish.
3. Plate Freezing (Contact Freezing)
Plate freezers work by bringing the fish into direct contact with refrigerated metal plates. This method is very efficient for uniformly shaped products.
- Process: Fish, often in blocks or trays, are pressed between hollow metal plates through which a refrigerant circulates. The direct contact ensures rapid and even heat transfer.
- Application: This method is ideal for freezing fish fillets, blocks of minced fish, or smaller whole fish that can be packed uniformly. It's commonly used for species like cod, pollock, and hake.
- Efficiency: Plate freezers are energy-efficient and produce compact, uniformly frozen blocks that are easy to stack, store, and transport.
Choosing the Right Method
The selection of a freezing method on a fishing vessel depends on several factors:
- Type of Fish: Smaller fish might suit brine freezing, while large individual fish benefit from air blast.
- Processing Form: Fillets or blocks are ideal for plate freezing.
- Vessel Size and Capacity: Larger vessels can accommodate more complex and varied freezing systems.
- Market Requirements: The desired quality, appearance, and specific characteristics of the end product influence the choice.
Table of On-Board Freezing Methods
| Freezing Method | Principle | Best Suited For | Key Characteristics |
|---|---|---|---|
| Brine Immersion | Fish submerged in super-cooled salt solution | Smaller fish, shellfish; Historically large fish (e.g., tuna) | Very fast, but can lead to salt uptake; Less common for large fish now |
| Air Blast | High-velocity cold air circulated around fish | Large fish (e.g., tuna) frozen individually, various sizes | Excellent for individual freezing, good quality preservation, versatile |
| Plate (Contact) | Fish pressed between refrigerated metal plates | Fillets, blocks of fish, whole smaller fish (uniform shapes) | Efficient for uniform products, compact blocks, good for bulk freezing |
By utilizing these advanced freezing technologies, fishing vessels ensure that seafood reaches consumers around the globe at peak freshness, contributing significantly to the global supply of high-quality protein.
