Freezing // Freeze Drying // Cooking // Chilled // Live Seafood // Canned // Retort Pouched // Dried // Surimi

Freezing

Freezing

Freezing basically consists in removing heat from a substance until it attains sub zero temperatures and assumes a hard consistency by the solidification of its water content, both free and combined and hardening of the organic tissues.

1. Quick Freezing

It is the process of rapidly freezing products.  In fast freezing, small ice crystals form and good quality is maintained.  When meat is frozen slowly, large ice crystals form that break the fibers and cells.  Upon defrosting, these meats lose more juice and the texture is not good.  Although the meat loses quality, it is safe to eat.  Temperature just below 00 C is the critical zone for spoilage by protein denaturation.  Quick freezing recommended that all the fish should be reduced from a temperature of 00 C to -50 C in 2h or less. 

2. Factors which affect freezing time:

The following 10 factors affect freezing time of products.

  1. Freezer type
  2. Freezer operating temperature
  3. Refrigeration system and operating condition
  4. Air speed in an air blast freezer
  5. Product temperature
  6. Product thickness
  7. Product shape
  8. Product contact area and density
  9. Product packing
  10. Species of fish

Types of Freezers:

Air Blast Freezers : Blowing a continuous stream of cold air over the fish
Plate Freezers : Direct contact between the fish and a refrigerated surface
Immersion or Spray Freezers: Immersion in or spraying with a refrigerated liquid

a. Air Blast Freezers

 In an air blast freezer, fish is frozen in a stream of high velocity cold air either in a batch or continuously, typically in a duct or tunnel in which a stream of cold air is guided over the product on shelves (batch) or on a conveyor (continuous air blast freezer); also called blast freezer, freezing tunnel, tunnel freezer.  The advantage of the blast freezer is its versatility.  It can cope with  a variety of irregularly shaped products and whenever there is a wide range of shapes and sizes can be frozen. 

Continuous air blast freezers:  In this type of air blast freezer, the fish are conveyed through the freezer (trolleys or they may be loaded on a continuously moving belt or conveyor) usually entering at one and leaving at the other.

Batch air blast freezers  Batch air blast freezers use pallets, trolleys or shelf arrangements for loading the product.  The freezer is fully loaded, and when freezing is complete, the freezer is emptied and reloaded for a further batch freeze.

b. Plate freezers

 In a contact freezer or plate freezer the fish is frozen by direct contact with a refrigerated surface, typically between two hollow metal plates cooled by a refrigerant, such that the distance between the plates can be varied up to 100 mm or more.  One type in which fish, especially in flat packs such as laminated blocks, is frozen between two or more hollow, horizontal, parallel plates through which refrigerant passes is known as a horizontal plate freezer.  In a vertical plate freezer, the refrigerated, parallel plates are vertical and it is used mainly at sea or onshore for freezing large 25 or 50 kg blocks of whole, gutted, or headed and gutted fish.

1.  Horizontal plate freezers

The two main uses for this type of freezer are the freezing of pre packed cartons of fish and fish products for retain sale and the formation of homogeneousrectangular blocks.  An HPE will only operate correctly if good contact is made on both the top and bottoms surfaces of the pack or tray to be frozen. 

2.  Vertical plate freezers

The main advantage of this type of freezer is that fish can be frozen in bulk without the requirement to package or arrange on trays.  The plates form what is in effect in bin with an open top and fish are loaded directly into this space.  This type of freezer is therefore particularly suitable for bulk freezing and it has also been extensively used for freezing whole fish at sea in factory vessels.  In most cases, fish can be loaded between the plates without wrappers and water need not be added either to strengthen the frozen block or improve the contact with the plates.

3. Immersion freezer or spray freezer

It is mostly used for IQF products of small size frozen within minutes.  The fish product to be frozen is in direct contact with the refrigerated liquid (freezant).  One type is brine freezing by immersing fish such as tuna in a tank of re-circulating cold sodium chloride brine at -150 C in a brine immersion freezer that may take about 3 days to freeze a large tuna completely. 

Freeze Drying

Accelerated Freeze Drying (AFD):

It is the process by which water (and other solvents) are removed from a frozen material by sublimation and processes: freezing, primary drying (sublimation), and secondary drying (desorption)

Accelerated Freeze drying (also known as lyophilization) is a dehydration process typically used to preserve a perishable material or make the material more convenient for transport. Freeze-drying works by freezing the material and then reducing the surrounding pressure to allow the frozen water in the material to sublime directly from the solid phase to the gas phase.

The three major steps in a freeze drying include freezing, primary drying(sublimation), and secondary drying (desorption). Each step is critical in producing the required quality end.

Cooking

Fish is cooked in order to produce changes in the texture and flavour of the product and to kill pathogenic microorganisms. Heating fish to an internal temperature above 66 °C or 150 °F (i.e., pasteurization conditions) is sufficient to kill the most resistant microorganisms. The cooking time must be closely regulated in order to prevent excessive loss of nutrients by heat degradation, oxidation, or leaching (the loss of water-soluble nutrients into the cooking liquid).

Chilling

Harvested fish must be immediately stored in a low-temperature environment such as ice or refrigerated seawater. This chilling process slows the growth of microorganisms that live in fish and inhibits the activity of enzymes. Because fish have a lower body temperature, softer texture, and less connective tissue than land animals, they are much more susceptible to microbial contamination and structural degradation.

Ice cooling and holding normally requires a one-to-one or one-to-two weight ratio of ice to fish, depending on the specific geographic location and the time it takes to transport the fish to the processing plant. Refrigerated seawater cooling and holding causes less bruising and other structural damage to the fish carcasses than ice cooling.

Live seafood

Live fish is preferred by many markets especially in the South East Asia and China. Many consumers are ready to pay a premium price if the seafood is in live condition. Live and fresh fish and seafood are extremely dependent on the transit environment to maintain their optimum selling condition and avoid mortality. Special packing systems should be employed to avoid temperature abuse and counteract rough handling. Selecting high quality product for air shipment adds extra assurance that the product will arrive in a condition acceptable to the buyer. Product handling that assures high quality should begin on the fishing vessel or at the fish farm. Quality levels will also depend on natural factors including seasonal changes, particular harvest areas, feeding behavior, sex and spawning condition of the fish and seafood. Harvest methods and onboard handling factor greatly in maintaining product quality and freshness. When shipping live fish and seafood, there are a number of species specific factors that must be considered to ensure that the product arrives alive. These factors include: 1) respiratory rates; 2) ability to handle stress; 3) excretory functions, and; 4) temperature tolerance ranges.

Canning

Canning is a method of preserving food, and provides a typical shelf life ranging from one to five years. A means of obtaining long-term microbiological stability for non-dried foods without the use of refrigeration, by prolonged heating in hermetically sealed containers, such as cans to render the contents of the container sterile.

Fish have a low acidity at levels where microbes can flourish. From a public safety point of view, foods with low acidity (a pH more than 4.6) need sterilization under high temperature (116-130 °C). To achieve temperatures above the boiling point requires a method of pressurized cooking which is provided by the containment within the can. After sterilization, the containing can prevents microorganisms from entering and proliferating inside. Other than sterilization, no method is perfectly dependable as a preservative. For example, the microorganism Clostridium botulinum (which causes botulism), can only be eliminated at temperatures above the boiling point.

Such preservation techniques are needed to prevent fish spoilage and lengthen shelf life. They are designed to inhibit the activity of spoilage bacteria and the metabolic changes that result in the loss of fish quality. Spoilage bacteria are the specific bacteria that produce the unpleasant odours and flavours associated with spoiled fish.

Retort Pouched

A retort pouch is a type of food packaging created by aseptic processing, made from multiple layers of flexible laminate, allowing for the sterile packaging of a wide variety of food and drink, ranging from water to fully-cooked, thermo stabilized (heat-treated) high-caloric (1300 on average) meals such as Meals, Ready-to-Eat (MREs), which can be eaten cold, warmed by submersing in hot water.

A retort pouch is constructed from a flexible metal-plastic laminate, which is able to withstand thermal processing via sterilization. The food is first prepared, even raw or semi-cooked, and then sealed into the retort pouch. The pouch is then heated to 240-250 0 F (116-1210 C) for several minutes under high pressure, inside retort or autoclave machines. The food inside is cooked, similar to pressure cooking. This process reliably kills all commonly occurring micro organisms (particularly Clostridium botulinum), preventing it from spoiling. The packaging process is very similar to canning, except that the package itself is flexible The lamination structure doesn't allow permeation of gases from outside into the pouch. The particular retort pouches used for several products consists of four layers bonded in a lamination machine.

Dried

The principal methods of drying, or dehydrating, fish are by sun drying, forced-air drying, vacuum drying, or vacuum freeze-drying. Each of these methods involves adding heat to aid in the removal of water from the fish product. During the initial stages of drying, known as the constant-rate period, water is evaporated from the surface of the product and the temperature of the product remains constant. In the final stages of drying, known as the falling-rate period, the temperature of the product increases, causing water to move from the interior to the surface for evaporation.

Surimi

A washed, refined, and stabilized fish mince, a process originally developed in Japan to provide an intermediate and relatively stable frozen material suitable for the subsequent production of traditional kamaboko products. Preparation of surimi is a means of intensifying the gelling properties of fish protein and of preserving these properties during frozen storage.