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Refreezing Seafood. Often it is necessary for a processing plant to receive frozen fish (either whole, butchered, or partially processed portions) for subsequent final processing or reprocessing. This occurs when fish are frozen at sea or other remote areas where total processing is not practical due to limited facilities or economic considerations. A typical example is when fish blocks are shipped to a plant for thawing, trimming, battering and breading, and re-freezing. A similar situation occurs when frozen fish or pre-processed raw materials are held in cold storage until they are processed in response to a specific market demand. For example, if fresh salmon are cut into steaks and frozen, there is no opportunity to sell the fish in the fillet form if the demand or price of fillets takes a sudden increase.

It is often said that seafood cannot be thawed and re-frozen. This is based on the too-often encountered situation whereby the fresh fish has been abused, the products have not been frozen rapidly, or have been held at high or fluctuating cold storage conditions. However, if the fish have been properly frozen and held in cold storage as described above, there will be little cell damage and the thawed product will have minimum water loss and reduction in quality from that of the original fresh raw material. Thus it can be thawed, reprocessed, and refrozen without significantly altering the quality of the finished product from that of the fresh fish.

The increase of high-seas processing vessels encouraged by the 200-mi limit (Fishery Conservation Zone) has greatly accelerated the final processing of frozen products prepared at sea. The future will probably bring an even greater acceleration of this trend as such products as batter and breaded (nondeep-fried) portions, whole fish, and products engineered and formulated from minced flesh or chunk portions are being developed for microwave cooking. These are inexpensive low-fat products with good sensory properties especially acceptable to the increasingly large number of nutrition-conscious consumers.

Commercial Refrigeration Systems

There is a distinct difference between selecting facilities for freezing or for cold storage for a commercial seafood operation.

40 I M I I I I I I I I I I I I I I I I I I I I I 0 10 20 30 40 50 60 70 80 90 100 110 120 Minutes

Figure 17. Freezing curve for blast-plate freezing of chum salmon (4 lb).

40 I M I I I I I I I I I I I I I I I I I I I I I 0 10 20 30 40 50 60 70 80 90 100 110 120 Minutes

Figure 17. Freezing curve for blast-plate freezing of chum salmon (4 lb).

Freezing Facilities. The freezing facility is related to the products that will be processed, the throughput of product, and the space availability in the processing area. The differences between various freezing techniques lie in the control of the type of heat transfer between the product and the refrigerant. The types of freezer as related to the products being frozen are as follows.

Natural Convention Freezing. This facility is a room or chamber in which the product is frozen by natural convection. There are minimum problems with dehydration but the freezing rate is slow. Only products not affected by slow freezing should be frozen in this type of freezer. These would include formulated products or very small items where the water loss or cell damage through slow freezing is not a problem.

Combined Conduction and Natural Convection. The addition of freezer plates on which the product is placed

Figure 18. Remaining free water in frozen food held at different temperatures.

Temperature (°F)

Figure 18. Remaining free water in frozen food held at different temperatures.

Figure 19. Time-temperature relationship of food during cold storage holding.

greatly accelerates the freezing rate and maintains the advantage of minimum hydration during freezing.

Blast Convection Freezing. Blast freezing is a popular method of freezing irregularly shaped seafood items such as whole or partially dressed fish. A major disadvantage of this facility is that considerable loss of water through hydration can cause an unsightly surface condition known as freezer burn. This hydration, if allowed to continue, can remove a considerable amount of water from the flesh, making the fish inedible. Such is the case when a product is frozen and then allowed to remain (actually to be stored) in the freezer for some length of time before being removed.

When unpackaged products are destined for storage in a cold room with fast moving air, they are glazed to protect against dehydration. This consists of immersing the frozen product in cold water and then allowing the water film to freeze on the surface. The layer of ice protects the fish, because the air removes water from the glaze rather than from the product.

Combined Conduction and Blast Convection Freezing. This facility normally consists of refrigerated plates in a forced-air cold room. This freezer has the advantage of fast conduction freezing assisted by convection air on portions of the seafood that are not in contact with the plate surface. The tremendous increase in freezing rate by this combination of freezing techniques as compared to that of a blast freezer were previously discussed and compared in Figures 16 and 17. Normally the freezing rate is high enough to eliminate the problem of surface dehydration.

Conduction Freezing. When small rectangular products or packages with two parallel surfaces are placed between two refrigerated plates in a cold room or chamber, fast freezing takes place with minimum harm to the products. These types of facility have mechanisms that allow the plates to be vertically adjusted. This allows the plates to open, or separate, during loading and then they come in contact with the product for freezing.

Immersion and Cryogenic Freezing. Immersion in a cold liquid that will not affect the safety of the product (eg, brine, liquid nitrogen, or freon) is the fastest method of freezing. Freezing takes place by both convection from the circulating fluid and conduction from being in direct contact with the liquid. Immersion freezing in extremely cold refrigerants such as nitrogen or freon is called cryogenic freezing. Brine freezing utilizes saturated salt solutions that freeze rapidly due to the combined convection and conduction but takes place at about 0°F.

Cryogenic freezing has two limiting factors for extensive use in freezing seafood products. The first is that the freezing is so rapid that it causes extreme internal tension due to the freezing of the fiberous materials at a differential rate. This causes the flesh to rupture. It can be minimized by allowing the product to temper at room temperature before further handling, packaging, and placing in cold storage. The splitting problem can be fairly well overcome for small items such as fillets and steaks but large fish cannot be satisfactorily frozen by this means. Brine, on the other hand does not have the splitting problem and is a major means of freezing whole tuna fish on high-seas catcher vessels.

The second problem with cryogenic freezing is that it is uneconomical to operate unless the freezer is used continuously, because a considerable amount of heat from the cryogen is lost each time a processing unit is shut down and restarted. Most seafood-processing plants do not operate on an extended basis so that the cost per pound for intermittent freezing limits cryogenic freezing to a few fish processing plants.

Modern cryogenic freezing no longer uses liquid immersion true freezing. It is more economical to spray the liquid refrigerant on the food as it moves along on a conveyor belt. This uses less refrigerant and still gives the same advantages as immersion. Furthermore, when extremely cold liquids are used for freezing, especially in immersion freezing, vapor bubbles are formed that insulate the liquid from the product and prevent the rapid heat transfer expected from such a large temperature difference between the product and the liquid.

Cold Storage Facilities. Cold storage facilities must meet the requirements for long-term storage of commercial seafood products ranging from large whole glazed fish to cases of packaged retail products. Of particular importance in purchasing or contracting for such facilities in a plant or choosing a public cold storage for use include the following:

1. A design with proper insulation and construction materials that will insure a minimum of heat loss through the walls, ceiling, and floor.

2. Properly designed protection to minimize heat loss through doors and other openings while the product is being taken into or out of the cold storage room.

3. Refrigeration machinery that will have sufficient capacity to hold the cold storage at the desired temperature.

4. Sufficient refrigeration capacity to insure that the temperature in the cold storage rooms does not fluctuate to the extent of adversely affecting the products being stored. This is probably the major problem encountered with commercial cold storage facilities. So often a low bid is awarded to a contractor who is the lowest due to cutting back on the amount of refrigeration machinery and thus the ability to maintain constant storage temperature during variable and seasonal outside weather conditions.

Cured and Dried Seafood Products

Control of water activity, defined as the ratio of the vapor pressure of water in a product at any given temperature divided by the vapor pressure of pure water at the same temperature, not only applies to smoking and drying but also to salting, pickling, and product formulation. Product stability, the growth of microorganisms, and chemical reactions occurring during processing and storage are all dependent on the water activity, that is, the ability of water to move and interact with other ingredients in the food. The importance of water activity (aw) in foods is shown in Table 10. The many facets of water activity and its relation to the preservation, safety, and shelf life of foods has been summarized by a group of internationally recognized experts (17).

Dehydration. Drying or dehydration is a means of controlling water activity by reducing the water content of a product. Many dried products, such as cereal grains, legumes, and many nuts and fruits, are dried by nature in the field prior to harvesting. In the past humans dried seafood products in the sun, long before they were aware that they were controlling water activity. In fact, today many developing countries located in tropical parts of the world use the sun as a major means of drying seafood and other food products.

The principal cured fishery products produced in the United States are shown in Table 11 (2). It should be noted that these statistics do not differentiate between smoked, salted, dried, and pickled products due to the fact that all of the processes are based on controlling or reducing water content. It is often difficult to distinguish between process classifications. For example, salted and salted-dried fish have a different final moisture content but the mechanism of removing the water and stabilizing the product are the same, control of water activity.

The most efficient means of drying a seafood product is through dehydration by forced-air drying, vacuum drying, or vacuum freeze-drying. In each case the drying mechanism is a combination of adding heat to increase the temperature and vapor-driving forces between the product and the environment. The drying time is divided into two distinct periods: constant drying rate and falling drying rate. During the constant rate period, all of the heat added to the product is used to evaporate water from the surface and near surface of the product. In this case, there is free water in contact with the environment and drying occurs similarly to that of an open container of water. During the falling rate period, part of the heat energy is imparted to the product to cause water to migrate to the surface. Therefore, the product is heated during this period of the drying cycle.

Excellent highly nutritious dried formulated fish-base products can be prepared from the minced flesh of seafood. Shaped into forms such as patties and air dried, these items have a long and stable shelf life (18).

Curing. Whereas dehydration removes sufficient water to inhibit growth of microorganisms, curing consists of adding sufficient chemicals (eg, sodium chloride, sugars, and acetic acid) to prevent degradation of a product by microorganisms. Although sufficient water is not removed to accomplish this objective, the water activity is reduced to the point where growth is prevented.

Curing methods currently practiced include dry salting, where split fish is covered with salt and the brine liquor is allowed to escape, and pickling where products are immersed in a strong brine, or pickle, allowing salt to penetrate the product and water to be exuded into the brine solution. Low-fat white fish such as cod are dry salted by the heavy (hard) cure and fatty fish are cured in airtight barrels by the Gaspe (light) cure. Figure 20 shows the process for the hard cure; the last step is air drying, which results in a salted-dried product with long-term, room temperature shelf life and a water activity of between 0.75 and 0.85. The Gaspe or light-cured product remains edible only a few days in the wet-stack stage (aw = 0.85-0.90) at room temperatures and must be pressed and mechanically dried for longer-term storage.

Smoking. The age-old practice of smoking has changed drastically over the last few decades. The process as originally practiced by Eskimos and American Indians to pre-

Table 10. Importance of Water Activity in Foods

Phenomena Food examples

1.0 Water-rich foods (aw = 0.90-1.0): foods with 40% sucrose or

0.95 7% NaCl, cooked sausages, bread crumbs, and kippered fish

0.90 General lower limit for bacterial growth Foods with 55% sucrose or 12% NaCl, dry ham, medium-

age cheese, and hard-smoked fish

0.85 Lower limit for growth of most yeast Intermediate-moisture foods (aw = 0.55-0.90): foods with

65% sucrose or 15% NaCl, salami, old cheese, and salt fish

0.80 Lower limit for activity of most enzymes Flour, rice (15-17% water), fruitcake, and sweetened condensed milk

0.75 Lower limit for halophilic bacteria Foods with 26% NaCl (satd), marzipan (15-17% water), and jams

0.70 Lower limit for growth of most xerophilic (dry loving) molds

0.65 Maximum velocity of Maillard reactions Rolled oats (10% water)

0.60 Lower limit for growth osmphilic or xerophilic yeasts and molds Dried fruits (15-20% water), toffees, and caramels (8%

water)

0.55 DNA becomes disordered (lower limit for life to continue) Dried foods (aw 0-0.55)

0.50 Noodles (12% water), spices (10% water), and fish protein concentrate (10% water)

0.40 Minimum oxidation velocity Whole egg powder (5% water)

0.30 Crackers and crusts (3-5% water)

0.25 Maximum heat resistance of bacterial spores

0.20 Whole mild powder (2-3% water); dried vegetables (5%

water), and cornflakes (5% water)

0.00 Maximum oxidation velocity

serve fish for the winter months was essentially a drying process whereby heat from a fire was used to reduce the moisture content sufficiently for extended storage. The smoke flavor was somewhat incidental to the process. In fact in some areas the fish were dried by the sun and the natural air currents and the smoke was used to prevent flies and insects from consuming and contaminating the product. This dried smoked fish, which takes days to cure, is known as hard smoked.

Today, most smoked fish is smoked for the flavor and there is relatively little loss of water in the process. The change in processing is a reaction to the consumer, who prefers a soft, moist texture rather than the tough texture of a dried product, and to the processor, who cannot afford to tie up large processing areas for longer-term smoking. Furthermore, the minimizing of moisture loss greatly improves the economics of processing and marketing smoked products. Hence, although smoked fish are considered to be processed in the same manner as fish in which water activity is altered, in reality the modern product is a partially or wholly cooked fish that has smoke added as a condiment (19).

Most of the smoked fish prepared in the United States has little shelf life stability beyond that of a fresh fish. The commercial process of smoking involves splitting and cleaning the fish, salting or brining (soaking in a brine solution) to firm the texture of the meat, draining to remove excess moisture, and smoking. Smoking is carried out as a cold smoke, the temperature of the smoke does not rise above 85°F, or as a hot smoke, the smoke is hot enough (eg, 250°F) to raise the center temperature of the fish to above

140°F. It is also common to smoke the fish with colder smoke and then to raise the smoke and air temperature during the terminal part of the smoking to pasteurize the fish.

Today, much of the smoking is carried out in commercially constructed smoking facilities, or kilns, that have smoke generators (using sawdust), controlled temperature forced air, and humidity control of the air. There are as many specific smoking procedures as there are processors in the business. Each processor has a favorite method, including the use of certain additives in the brine, controlled-temperature drying, smoking, cooking, and cooling. Modern kiln smoking has allowed precise control of the variables in smoking and has removed much of the artisan approach that was so prevalent when all smoking was carried out using open wood fires.

Specialty hard-smoked fish, known as jerky, is prepared by smoking and drying fish to a hard, chewy consistency. This product is popular in the bar and tavern trade and for hikers who wish to carry a meat product that will not spoil during a several-day outing.

Irradiation

The United States is out of step with the rest of the world when considering the use of irradiation for preserving food. During the early 1980s, the World Health Organization gave ionizing radiation its blessing after an extensive review of many years of scientific work and investigation (20). Since that time, many countries in the world have been using irradiation on a wide variety of foods (9). Al-

Table 11. U.S. Production of Principal Cured Fishery Products

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