Sanitary Handling and Processing of Harvested Fish

Handling of fish is all about preprocessing of fish. This operation prepares the raw material for final processing. It is often performed on shipboard or in a shore-based plant.

The retention of nutritional properties and product quality of fish is dependent on proper handling of the catch after it has been harvested from its aquatic environment.

Handling Operations for Harvested Fish

The handling of fish includes such operations as inspection, washing, sorting, grading, chilling, and butchering of the harvested fish, including the four basic procedures used in the final processing of fish products: heating, freezing, controlling water activity (by drying or adding chemicals), and irradiating.

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Assessing Freshness, Chilling, and Butchering of Fish

1. Evaluating Fish Freshness

Freshness of fish is usually judged in the trade entirely by appearance, odor, and texture of the raw fish. Since assessment depends upon the senses, these factors are known as sensory or organoleptic. The most important things to look for in the freshness of fish are:

1. The general appearance of the fish, including that of the eyes, gills, surface slime, and scales, and the firmness or softness of the flesh.
2. The odor of the gills and belly cavity.
3. The appearance, particularly the presence or absence of discoloration along the underside, of the backbone.
4. The presence or absence of rigor mortis or death stiffening.
5. The appearance of the belly walls.

2. Chilling Techniques for Fish Preservation

Chilling and butchering are the most technical aspects of handling practices of fish. 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.

If immediate chilling is not possible, then the fish must generally be sold and eaten on the day of the harvest.

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 as a cooling technique causes less bruising and other structural damages to the fish than using ice cooling. However, fish cooled in refrigerated seawater absorbs salt from the water.

For this reason, fish that is destined for sale on the fresh or frozen market may be held in refrigerated seawater for only a limited amount of time. The addition of salt during canning or smoking processes is adjusted in order to compensate for any absorbed salt.

3. Butchering Practices for Fish

The butchering of fish involves the removal of nonedible portions such as the viscera, head, tail, and fins. Depending on the butchering process, as much as 30 to 70 percent of the fish may be discarded as waste or reduced to cheap animal feed. The lower figure applies when the fish is canned or sold as “whole.”

The higher figure applies when the fish is filleted or made into other pure meat products; in these cases, the skeleton is discarded with as much as 50 percent of the edible flesh attached. Efforts to utilize this discarded fraction for the production of alternative food products have begun in the fish industry.

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Final Processing Methods for Fish Products

The four basic procedures used in the final processing of fish products are heating, freezing, controlling water activity (by drying or adding chemicals), and irradiating.

All these procedures increase the shelf life of the fish by inhibiting the mechanisms that promote spoilage and degradation. Each of these procedures also has an effect on the nutritional properties of the final product.

1. Heating Processes for Fish

This comprises the following treatments: cooking and canning. Heat treatment can significantly alter the quality and nutritional value of fish. Fish is exposed to heat during both the cooking process and the canning process.

i. Cooking: Fish is cooked in order to produce changes in the texture and flavor 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).

ii. Canning of Fish: The canning process is a sterilization technique that kills microorganisms already present on the fish, prevents further microbial contamination, and inactivates degradative enzymes.

In this process, fish are hermetically sealed in containers and then heated to high temperatures for a given amount of time. Canned fish can be stored for several years.

However, sterilization does not kill all microorganisms, and bacterial growth and gas production may occur if the products are stored at very high temperatures.

Because the severe thermal conditions of canning cause the disintegration and discoloration of the flesh of many species of fish, only a few types of fish are available as canned products. The most common types are tuna, salmon, herring, sardines, and shrimp.

The thermal processing does not have a detrimental effect on the high-quality protein of the fish. In addition, these species are often canned with their bones left intact.

The bones become soft and edible, significantly increasing the level of calcium present in the fish product. Tuna is an exception; because of special handling considerations, the bones of tuna are removed prior to canning.

Tuna is normally caught far offshore and must be frozen and held for some period of time prior to canning. During this freezing and holding period, unsaturated fatty acids are oxidized, causing the tuna to become rancid.

The rancidity is removed by precooking, and the bones are removed at this time in order to facilitate the cutting and preparation of the meat for canning.

2. Freezing Techniques for Fish Preservation

This comprises immediate cooling and rapid freezing. Of the many processing methods used to preserve fish, only freezing can maintain the flavor and quality of fresh fish. Freezing greatly reduces or halts the biochemical reactions in fish flesh.

For instance, in the absence of free water, enzymes cannot react to soften and degrade the flesh. The three steps for freezing fish include immediate cooling and holding, rapid freezing, and cold storage.

If fish is frozen improperly, structural integrity may be compromised because of enzymatic degradation, texture changes, and dehydration.

Once fish is frozen, it must be stored at a constant temperature of −23°C (−10°F) or below in order to maintain a long shelf life and ensure quality. A large portion of fresh fish is water (e.g., oysters are more than 80 percent water).

Because the water in fish contains many dissolved substances, it does not uniformly freeze at the freezing point of pure water. Instead, the free water in fish freezes over a wide range, beginning at approximately −2°C (28°F).

The amount of remaining free water decreases until the product reaches a temperature of approximately −40°C (−40°F). Fish held below that temperature and packaged so as not to allow water loss through sublimation can be stored for an indefinite period.

Unfortunately, there are relatively few commercial freezers capable of storing fish at −40°C because of the tremendous variation in energy costs. Fish are therefore normally stored at −18 to −29°C (0 to −20°F), resulting in a variable shelf life ranging from a few weeks to almost one year.

i. Immediate Cooling and Holding: The rapid cooling and holding of fish at temperatures between 2 and −2°C (36 and 28°F) takes place immediately after the fish have been harvested, and this is called immediate cooling.

ii. Rapid Freezing: The key to freezing is rapid reduction of the temperature to between −2 and −7°C (28 and 20°F). This temperature range represents the zone of maximum ice crystal formation in the cells of the flesh.

If water in the cells freezes quickly, then the ice crystals will remain small and cause minimal damage to the cells. However, slow freezing results in the formation of large ice crystals and the rupturing of the cell membranes.

When slow-frozen flesh is thawed, the ruptured cells release water (called drip) and many compounds that provide certain flavor characteristics of fish, resulting in a dry, tasteless product.

Fish that passes through the zone of maximum ice crystal formation in less than one hour will generally have minimum drip loss upon thawing.

3. The Nigerian Frozen Fish Industry and Marketing Channels

The Nigerian frozen fish industry is based on the industrial fishery and chartered vessel landings. Shrimps by-catches occupy a potentially important position. Because the species are usually from different distant water fishing grounds, they often represent a mixture of local and exotic fish species.

Marketing of fish begins as soon as the vessels land, and so, the post-handling operations constitute essential components of the marketing system. Factors influencing the intensity and efficiency of these operations include the location of cold stores and markets to be supplied in relation to the quay.

While, for instance, some companies have direct access to docking facilities, many others discharge their catch in rented wharf. There has been an upsurge in the degree of mechanization in catch off-loading.

Off-loaded fish in boxes are either arranged in cold rooms immediately after landing or loaded into trucks to be transported to the companies’ main cold stores somewhere inland of the quays.

A clearly-defined system of frozen fish distribution is conducted through a chain of intermediaries who handle products at different levels. Cartooned fish are marketed either directly after off-loading from the vessel or after a period of storage in the cold-rooms.

As soon as the product enters the distribution channel, it exchanges hands a number of times until the retail stage, when it is sold to the ultimate consumer. The frequency at which the product changes hands varies from company to company and from place to place.

The sole distributors at the retail level are the women. They monopolize the marketing of frozen fish through a particularly powerful organization.

Controlling Water Activity in Fish Preservation

Water activity is represented by the symbol aw. It is that amount of water present in a food that supports the growth of microorganisms in that food. Its value is used to predict the type of microorganism that causes the spoilage of that food; because different microorganisms grow at different water activity values. Treatments involved in controlling water activity include drying, curing, and smoking.

Reducing the water activity of fish inhibits the growth of microorganisms and slows the chemical reactions that may be detrimental to the quality of the fish product. The control of water activity in fish is accomplished by drying, adding chemicals, or a combination of both methods.

1. Drying Methods for Fish Preservation

Fish drying carried out under ambient conditions and fish dehydration carried out artificially enhance the storage life of fish by decreasing its water content. In a drying process, moisture content is reduced from about 80% to about 10% and may take several months.

In cold climates, dried fish may last for several years with control of spoilage being due to control of microbial growth and enzyme activity by the low moisture content.

The principal methods of drying, or dehydrating, fish are by 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.

i. Sun-Drying in Nigeria: In the Northern part of Nigeria, sun-drying as a means of preservation is the most important method. It takes advantage of the high ambient temperature and low humidity. In the Lake Chad area, where a lot of fish are sun-dried in the open, split fish is often dipped in approximately 8% Grammalin 20 solution before being exposed to the sun.

It is erroneously believed that this dip prevents insect infestation of the finished product. It must be stressed that this use of insecticide is dangerous and must be discouraged.

The water content of the dried fish ranges between 14-30%, and no salt is used. Clarias sp. are often gutted, haphazardly cleaned, and then bent into a horseshoe manner, the shape being retained by means of a sharp stick that pierces through the caudal region and the head.

This practice is presumably designed to prevent the crumbling of a well-dried product. The product is heavily smoked and attains a shining black luster. Bony fish like Tilapia sp. are split asymmetrically in the longitudinal plane and sun-dried. Big fish such as Gymnarchus sp. and Lates sp. are chopped into lumps, smoked for a short time, and then sun-dried.

This product is the traditional “banda” fish and is in great demand among fish-eating communities. In the Kainji Lake area of Nigeria, Physalia sp. and Clupeids are also sun-dried.

As for the latter, the fish is spread out on mats or polythene sheets in the open. They are then allowed to dry over a period of about one week with regular turning.

ii. Smoking in Southern Nigeria: Smoking is the commonest method of fish processing in the southern part of Nigeria, and they employ kilns of all forms and shapes. Among certain fishing communities where this technology is used, firing is usually aided by means of bellows.

There is no general agreement about gutting. For instance, very big fish are cut into chunks before smoking in certain parts; and within the same locality, they are not. Members of the catfish family are known to be smoked ungutted in some fishing villages.

The fish processors believe that gutting of fish is an economic waste. In Bendel state, small fish are sometimes tied to neatly-woven frames and smoked as such. This prevents crumbling of the fish and improves the drying pattern.

2. Curing Methods for Fish Preservation

Curing reduces water activity through the addition of chemicals, such as salt, sugars, or acids. There are two main types of salt-curing used in the fish industry: dry salting and pickle-curing.

i. Dry Salting: In dry salting, the butchered fish is split along the backbone and buried in salt (called a wet stack). Brine is drained off until the water content of the flesh is reduced to approximately 50 percent (the typical water content of fresh fish is 75 to 80 percent), and the salt content approaches 25 percent.

In heavy or hard-cure salting, an additional step is taken in which warm air is forced over the surface of the fish until the water content is reduced to about 20 percent, and the salt content is increased to approximately 30 percent. Most dry-salted fish products are consumed in warm, humid countries or in areas that have few means of holding products in refrigeration or cold storage.

ii. Pickle-Curing: In pickle-curing, fish are preserved in airtight barrels in a strong pickle solution formed by the dissolving of salt in the body fluids. This curing method is used for fatty fish such as herring.

3. Smoking Techniques for Fish Preservation

Traditionally, smoking was a combination of drying and adding chemicals from the smoke to the fish, therefore preserving and adding flavor to the final product.

However, much of the fish smoked today is exposed to smoke just long enough to provide the desired flavor with little, if any, drying. These products, called kippered fish, have short shelf lives, even under refrigeration, since the water activity remains high enough for spoilage organisms to grow.

The smoking process consists of soaking butchered fish in a 70 to 80 percent brine solution for a few hours to overnight, resulting in a 2 to 3 percent salt content in the fish. The fish are then partially dried on racks.

As the brine on the surface dries, dissolved proteins produce a glossy appearance, which is one of the commercial criteria for quality. Smoking is carried out in kilns or forced-air smokehouses that expose the fish to smoke from smoldering wood or sawdust.

In cold-smoking, the temperature does not exceed 29°C (85°F), and the fish is not cooked during the process. Hot-smoking is more common and is designed to cook the fish as well as to smoke it. Two separate stages are involved in the smoking of fish. The fish is split and eviscerated and steeped in brine composed of 70-80% saturated salt solution.

This reduces the water content of the fish and causes the water surface layer proteins to coagulate. After treatment with brine, the fish is hung on racks in a kiln and exposed to smoke from burning wood. The tar and phenols from the smoke produce a desirable color, flavor, and odor, while methanol provides a preservative effect.

4. Irradiation for Fish Preservation

Irradiation offers a means of pasteurizing or sterilizing a variety of food products. However, the use of this process has not been universally accepted throughout the food industry. Food irradiators utilize radioisotopes, such as cobalt-60 (60Co) or cesium-137 (137Cs), or electron beam generators to provide a source of ionizing radiation.

The irradiation of seafood has been extensively studied since the 1950s. The pasteurization of fresh fish using low-level dosages of ionizing radiation may extend the shelf life of the product up to several weeks. The sensory and nutritional characteristics of the fish are unaffected at these low levels of radiation.

i. Minced Fish Products: Minced fish flesh is used in a wide variety of products. The largest volumes are extruded into formed patties for main dishes and sandwiches. The forming process involves combining the minced flesh with condiments and extruding the mix under pressure to produce the desired product, much like the formation of hamburger patties and sausages.

The formed product may be battered and breaded in a final processing step. Other minced flesh products include nuggets and items used as hors d’oeuvres, fish chowders, and smoked fish sticks.

Preservation Techniques for Fish

The preservation and processing of freshwater and marine fish are largely dependent on the technology developed in any particular area.

Once fish has been preserved, further processing is less sophisticated than the various red meats industry processes and is limited mainly to fresh frozen products, as well as smoked, salted, dried, and some canned products.

The principles involved in preservation include: scrupulous cleanliness and hygiene in handling the fish caught; immediate freezing to at least −10°C; the freezing time not to exceed two hours; glazing to prevent drying; and storage of the frozen fish well below −16°C.

1. Salting for Fish Preservation

Salting reduces the moisture content and discourages the growth of microorganisms which otherwise would cause decay. The process of salting involves interspersing salt between layers of piled, split fish. The pile of fish and salt is left for up to a month.

The high concentration of salt outside the fish causes the liquor inside the fish to be drawn out by plasmolysis, and when it does so, it drains away. This extracted fluid is called green cured or “pickle.”

This fish at this stage is called green cured, and its water content has been reduced from 82% to about 54%. The green cured fish is next exposed on racks to dry in the sun and wind. It may also be hung up and dried over a coal fire.

2. Semi-Preserves for Fish

The semi-preserves involve the treatment of raw fish with ethanoic acid or acid brine. The texture of the flesh becomes less elastic, and the storage life is fairly limited since the concentration of ethanoic acid necessary to prevent microbial growth would make the product unpalatable.

The European product roll mops is an example of a semi-preserve. These products may be given a pasteurization to enhance their storage life. The acid conditions in these products contribute to their storage stability.

2. Canning for Fish Preservation

Canning softens the bones and renders them edible, so they become a useful source of calcium. Fish is a good source of protein and iodine, but the proteins are limited by the sulfur amino acids and contain a relative surplus of lysine.

The vitamin of greatest importance in fish is vitamin D in fatty tissues, and this is stable to canning and subsequent storage, causing negligible damage, but there can be considerable extraction into the canning brine of a source of fat, fat-soluble vitamins, and bone minerals. These are, of course, lost if the liquor is discarded.

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