Fish Preservation and Processing Guide

Proper fish preservation and processing is very important especially since fish is a major source of protein and its harvesting, handling, processing and distribution provide livelihood for millions of people as well as providing foreign exchange earning to many countries.

Appropriate processing of fish enables maximal use of raw material and production of value-added products which is obviously the basis of processing profitability.

Fish processing, like the processing of the other food raw materials should: assure best possible market quality, provide a proper form of semi-processed final product, assure health safety of products, apply the most appropriate processing method and reduce wastes to the barest possible extent.

The development of appropriate fishing machinery and techniques that employed effective production, handling, harvesting, processing and storage, cannot be over-emphasized especially in the age when aquaculture development is fast gathering momentum.

Fish processing is the processing of fish and other seafood’s delivered by fisheries, which are the supplier of the fish products industry. Although the term refers specifically to fish, in practice it is extended to cover all aquatic organisms harvested for commercial purposes, whether harvested from cultured or wild stocks.

Fish processing may be subdivided into two major categories: fish handling (which is initial processing of raw fish) and fish products manufacturing.

Another natural subdivision is into primary processing involved in the filleting and freezing of fresh fish for onward distribution to fresh fish retail and catering outlets, and the secondary processing that produces chilled, frozen and canned products for the retail and catering trades.

Fish processing can take place aboard fishing and fish processing vessels, and at fish processing plants fish.

Fish Value Addition

In general, value addition means “any additional activity that in one way or the other change the nature of a product thus adding to its value at the time of sale.” Value addition is an expanding sector in the fish processing industry, especially in export markets.

Value is added to fish and fishery products depending on the requirement of different markets. Globally a transition period is taking place where cooked products are replacing traditional raw products in consumer preference. For example, there is a demand in urban centers for “ready to eat” and “ready to cook” convenience products in a frozen condition.

There is also a need to divert low value fish to human consumption which can be facilitated by diversifying fishery products to value added products, such as minced meat from low priced fishes.

Characteristics of Fish in the Fresh State

It is very important to know the quality of fresh fish so as to know when spoilage sets in

Colour: in the fresh state, fish colour is bright, shiny and iridescent or lustrous (capable of reflecting different colour). When spoilage sets in, the colour is dull, dirty yellow or brown.

Skin texture: the skin of a fresh fish is firm and elastic. Under spoilage, the skin becomes dry, slack, swollen and inelastic.

Eyes: the eyes of a fish are bright, clear with bulging eye balls. The pupils are black brilliant and the cornea is white and bright. When spoilt, the eye is dull, shrinks and sinks in to the eye socket and the pupil becomes cloudy and milky; the cornea becomes opaque.

Gills: the gills of a fresh fish are bright red or pink in colour; the colour changes to dark red or brownish when spoilt.

Flesh: in the fresh state, the flesh of a fish is “firm”. As spoilage sets in, the flesh becomes gradually soften and when completely spoilt, it can strip of the backbone and exude juice when slightly pressed. It may even leave finger impressions.

Mucous coating on the sin of fresh fish is transparent, free floating and is like a lubricant. In a spoilt fish, the mucous is slimy, becomes thick. Sometimes the colour becomes whitish or creamy in colour and it is no longer transparent.

Smell/odour: Fresh fish is said to have a fresh sea-weedy smell. The colour of spoilt fish is sour, bad and offensive. Spoilt fish is said to have “Off odour/flavour”.

Post-Mortem Changes in Fish

The quality of the freshly caught fish and its usefulness for further utilization in processing is affected by the fish capture method. Unsuitable fishing method does not only cause mechanical damage to the fish, but also creates stress and the conditions which accelerate fish deterioration after death.

Fish is highly susceptible to deterioration without any preservative or processingmeasures. It has been reported that immediately the fish dies, a number of physiological and microbial deterioration set in and thereby degrade the fish.

Spoilage proceeds as a series of complex enzymatic bacterial and chemical changes that begin when the fish is netted or hooked. This process begins as soon as the fish dies. The rate of spoilage is accelerated in warm climates.

The fish’s gut is a rich source of enzymes that allow the living fish to digest its food .Once the fish is dead; these enzymes begin digesting the stomach itself. Eventually the enzymes migrate into the fish flesh and digest it too.

This is why the fish becomes soft and the smell of the fish becomes more noticeable. There are countless bacteria naturally present on the skin of the fish, in the gills, and in the intestines.

Normally, these bacteria are not harmful to a living fish. Shortly after death, however, they begin to multiply, and after two to four days they ingest the flesh of even a well-iced fish as enzymatic digestion begins to soften it.

The bacterial load carried by a fish depends on its health, its environment, and on the way it was caught. Healthy fish, from clean water, will keep better than fish dragged along the bottom of a dirty pond in a trawl net. Both enzymatic digestion and bacterial decomposition involve chemical changes that cause the familiar odors of spoilage.

Oxygen also reacts chemically with oil to cause rancid odors and taste. The aim of fish processing and preservation is to slow down or prevent this enzymatic, bacterial, and chemical deterioration, and to maintain the fish flesh in a condition as near as possible to that of fresh fish.

There are five sequences of events that characterized post-mortem changes in fish and they are listed thus:

Biochemical changes: such as glycolysis which is caused by enzymes action (tissue respiration.

Rigor mortis in the muscles: this is simply the stiffening of the muscles.

Muscle tendering: it results from softening of the muscles cause by proteases

Spoilage: due to the microbial action, once the muscles become tender, micro- organisms attacks it and this is usually followed by the release of mucus.

The process of spoilage is irreversible; once a fish is spoilt nothing can be done to change it quality back to what it initially was at the fresh stage. Therefore, it is important to preserve fish immediately after capture to prevent deterioration and spoilage as improper preservation significant result into post-harvest loss.

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Methods of Fish Preservation

Fish is a highly nutritious food and it is particularly valued for providing protein of high quality better than those of meat and egg.

However, it is one of the most perishable of all the foods because it is a suitable medium for growth of micro-organisms after death. In the tropics at ambient temperature, spoilage is rapid; fish will spoil within 12-20 h depending on species, method of capture.

As soon as fish is caught and dried, certain irreversible spoilage begin to take place, leading to spoilage and deterioration and most subsequent processing or preservation operation are intended to prevent these causes or usually reduce the rate at which they proceed.

Fish preservation is the method of extending the shelf life of fish and other fishery products by applying the principles of chemistry, engineering and other branches of science in order to improve the quality of the products. Preservation methods maintain the quality of fish for a longer period of time.

The basic task of every fishery is to get the catch to the consumer in good, usable condition. The first fish caught were probably eaten raw, on the spot. Communities grew up near enough to productive fishing grounds so the fish could be consumed the day it was caught.

The earliest preserved fish was probably accidentally overcooked, and some observant fisherman saw that dry cooked fish kept for a period of time without spoiling.

Traditionally, air drying, salting, and smoking (or some combination of these three) preserved fish for the short periods required by the fishermen.

Fish preserved in these ways is often tough and stringy, the quantities produced are small, and success is uncertain. Few people will eat fish preserved this way, if they have an alternative. Over time, other, better methods of preservation came into being.

Proper Steps in Handling Fresh Fish

Avoid exposing the fish to sunlight. Keep them in a shaded area.

Ice the fish immediately after they are caught to lower their temperature.

Remove the gills and internal organs.

Avoid soaking the fish too long in the water after death as this easily spoils the fish.

Use mechanical refrigeration if there are facilities.

It is imperative to understand that all processing methods are preservation method but all preservation methods are not processing methods. Smoking or drying is the most common method of fish preservation.

Traditionally, fish is smoked in mud kiln or halved cut drum with wire netting on top and use of wood as source. Hardwoods are preferred to softwoods for fish smoking because the former yield more acid and may therefore produce products that are more bacteriologically stable.

If fish is not sold fresh, preservation methods should be applied to extend shelf-life. These include freezing, smoking, drying and heat treatment (Sterilization, pasteurization, etc.).

Efficient preparation of fish is important when top quality, maximum yield and highest possible profits are to be achieved.

Whenever fish must be kept for several hours or longer before being consumed, they must be treated in some way to prevent spoiling. These are the basic means for preserving fish:

Cooling and icing;

Salting and pickling;

Pastes and sauces;

Canning and bottling;

Air drying and smoking;

Kiln drying;

1. Cooling and Icing

The first and simplest method to both preserve and process fish is to keep it cool. Cool fish keeps longer than uncooled fish, although both will spoil in a matter of hours.

If the market is only a few hours away, and if the fish will be sold promptly, evaporative cooling might suffice. All that is required is some coarse cloth enough to completely cover the fish-and enough water to keeps the cloth damp.

The movement of air over the water causes it to evaporate, and thus keeps the fish much cooler and fresher than fish directly exposed. Wrap the fish completely in the cloth.

Any portion that is exposed to the air will dry and become warm enough to support the rapid growth of bacteria. Splash water on the wrapped fish, keeping the cloth wet but not soaked. How well this will work depends on too many variables to predict, but it is a distinct improvement over uncovered fish.

Most fish caught are preserved with ice at some stage in their processing. Trained taste panels are usually unable to distinguish well-iced fish kept less than six or seven days from fresh fish, and storage life can be extended somewhat if antibiotics are added to the ice.

Ice works in two ways: it reduces the growth rate of bacteria by reducing the temperature of the fish; and it also washes the bacteria and slime away as it melts.

Because of this, it is important to keep melt water drained away from the fish. Fish are usually gutted and stowed mixed with ice. Small flat fish are stowed without gutting. An active fish like salmon is gutted and the belly cavity is packed with ice as it is stowed.

Fish can be iced in bulk, in large quantities, or they can be boxed. Boxing produces a better quality product for several reasons: the bottom fish are not crushed by the weight of the fish on top; and the melt water is better able to drain away. In addition, it seems to be human nature to take better care of a small box than of a pile of fish.

Ice is expensive and begins to melt immediately, so the fishermen are faced with a loss before they even begin. The temptation to get away with as little ice as possible must be avoided.

Within limits, the more ice the better. The box should be lined with ice so the fish does not touch sides or bottom of the box. Layer the fish, avoiding overlaps, and ice each layer as it is boxed. If the catch is large enough that the boxes must be stacked, try to channel the melt water away from the bottom boxes. Keeping the boxes covered with wet cloth will dramatically increase the life of the ice.

There is a wide range of icemakers on the market, ranging from small flake ice machines that produce a couple of tons a day to huge machines that make many tons. They all require electricity and a certain level of technical expertise to operate.

The newer machines are built with the small operator in mind, however, and are practically unbreakable. With these machines, it is possible for small operators to make their own ice.

2. Salting and Pickling of Fish

Salting and pickling, along with various kinds of drying, are the traditional methods for preserving fish. Indeed, Egyptian tomb paintings illustrate fish being prepared for salting and drying, and the process must be many years older than that.

The bacteria that spoil fish need moisture to grow. If the moisture in the fish can be reduced to about 25 percent of its normal level, bacterial activity will cease. Some bacteria are killed at these levels, while others simply go dormant.

The fish will keep for several years as long as the moisture level is not allowed to increase beyond 25 percent. Salt replaces a portion of the water naturally present in the fish, and so reduces the moisture content below the point where bacterial spoilage can occur.

The several salting methods vary mainly in the amount of salt the fish are allowed to take up. “Dry salting” is used to preserve non-fatty fish such as cod.

The split fish are completely buried in salt, and the brine liquid that emerges is allowed to drain away. The fish are finally dried. In the “pickling” process, used for fattier fish such as herring, the fish are packed in salt in airtight containers.

Bacterial decay is reduced or prevented when the salt has replaced enough of the moisture in the fish to inhibit the growth of fish spoilage bacteria.

A combination of coarse grained salt (like rock salt) and a fine grained salt is used. The coarse grains keep the fish separated so as to drain, and the fine grains dissolve quickly into the flesh of the fish.

Salt may be mined from prehistoric deposits, manufactured from partly concentrated brines, or “manufactured” by solar evaporation of shallow ponds of salt water. Any of these may be available to you, as well as salt that is produced expressly for use in salting fish.

Some of the flavor of the finished product depends on the kind of salt used. Impurities in the salt, such as magnesium or calcium, if present at too high a level, impart a bitter taste to the final product; these impurities also interfere with the absorption of salt into the fish.

Some common impurities readily reabsorb moisture from the air, so if the curing salt contains enough of these compounds the fish will become damp again and grow bacteria. On the other hand, small amounts of these same impurities give the salted fish a whiter appearance that is more attractive to some consumers.

For all these reasons, it is important to know what kinds of salt you are using, and what their effects will be.

The fish to be salted are cleaned, and the guts and gills and sometimes heads are removed. Larger fish must be split so they can be opened up and laid flat in the salt. In general, a layer of salt is placed in the bottom of a container and a single layer of fish is placed on it, flesh side down. The first layer of fish is then covered with more salt and another layer of fish is added. The layers of smaller fish like herring are crisscrossed. The process is continued until the container is full.

The same cautions as to cleanliness and care in handling apply. Avoid reintroducing a bacterial load. Use clean processing equipment and keep the work area clean.

Keep guts and offal away from the processed fish and dispose of it in an area removed from the cleaning area and water supply. If drying is the ultimate goal, the water that is withdrawn from the fish by the action of the salt is allowed to run off, and the fish are restacked at frequent intervals, rotating the fish from the top to the bottom of the pile to equalize the cure.

The fish can remain stacked for several months in a cool climate before being dried, but this is not possible in temperate countries.

Fish can be air dried in Norway or Iceland, but in most areas some sort of dryer is generally required. If the fish is to be pickled, it is packed in the same way, in a container that can be sealed. As the fish shrink, the barrels are consolidated, putting fish from the same day’s catch together.

After about ten days, half of the replaced water is drained off, the container is packed full again with fish from the same batch, and the spaces between the fish are filled with the water that was drained off earlier. The container is then sealed and stored.

Salting is a simple process. It does not require much equipment or manpower, but the product has a limited life unless it receives some sort of additional processing such as canning or freezing. Drying, which is explained in the following section, is an alternative to freezing.

3. Air Drying and Smoking of Fish

Even the most heavily salted fish will begin to spoil after a few weeks at warm temperatures. Some additional processing is required to preserve fish in any but the coldest climates.

Moreover, although salt alone will protect against the growth of some bacteria, salt-loving bacteria continue to flourish. A combination of salt and reduced moisture, or salt and no air, will allow fish to be kept for several years.

Bacterial activity ceases when the moisture content is reduced below about 25 percent. Mold will no longer grow at a moisture level of about 15 percent. Fish dried to this level will last several years if not subsequently moistened.

Air drying and kiln drying reduce the moisture content of fish to the point where bacterial action ceases. Smoking dries the fish, and also adds bactericides that are present in the smoke. The process varies from a mild cure that will keep several weeks if chilled, to a hard smoke that will keep indefinitely if not moistened.

Fish preserved by air drying tends to be tough and stringy. Most people will not eat fish preserved this way unless they must. If the weather is dry, fish may be air dried. Take care to keep the fish in shade, exposed to breeze.

Keep flies and insects away! Air drying of fish is an uncertain undertaking. Since it requires a low relative humidity to achieve the necessary degree of dryness, the fish will keep only so long as it is kept dry.

On the other hand, it requires a minimum of equipment and no technology. It is primarily suitable for small quantities for personal use. With a minimum of investment solar dryers can be constructed for the drying of fish.

Solar dryers made from plastic on a wooden frame eliminate contamination by insects and can increase ambient temperature to accelerate drying. They also reduce storage of fish when rain storms interfere with sun drying.

Kiln or tunnel drying of fish is a more complex process, and the final product is much more palatable than natural air dried fish. It requires careful control of many variables, such as relative humidity, air temperature and velocity, and rate of drying. The product will have to be stored in some sort of cold storage because it also will draw moisture and putrify.

In kiln drying, the fish is hung on racks in a tunnel. Dry inlet air is heated, circulated through the tunnel, reheated, and recirculated. A portion of the moisture laden air is vented off and replaced with outside air.

Control of the humidity inside the kiln can be accomplished by venting off more or less of the moisture laden air from the kiln. Midway through the process the kiln is unloaded and the position of the fish is reversed to equalize the drying rate.

4. Fish Sauces and Pastes

In areas where a rice diet predominates, a number of fermented fish products have been developed. If a fairly fixed procedure is followed, the product has a more or less consistent flavor and texture. In areas in which dried or salted fish is impractical because of the high humidity and temperature, fermented sauces or pastes may be an acceptable or preferable alternative.

Small, ungutted fish are mixed with salt (four to five parts salt to six parts fish) and sealed in vats or pots. In a process that requires several months, the fish dissolves and ferments. The result is a clear “pickle” with good keeping properties that is used as a condiment for flavoring rice dishes. Fresh or salt water fish can be used, as well as shrimp. The processes vary as widely as the kinds of fish used.

Fish paste is made from cleaned fish, which is mixed with salt (one part salt to three parts fish) and allowed to digest. Sometimes fermented rice, roasted grains, or bran are added.

The manufacturing methods are complicated and vary considerably from area to area. As a result, the product is seldom standardized. Tastes vary from area to area, so local knowledge is imperative.

5. Bottling and Canning

The bottling and canning of fish requires more precision and expense than the aforementioned methods of preservation. Many nations during their lean fishing seasons import large amounts of canned fish to supply a source of protein.

In such a case, perhaps domestic canning is a viable option. The canning or bottling of fish requires a high quality product at the onset. It should not be employed as a last resort for unsold fish. Consumption of such fish may cause severe illness.

Bottled fish is usually prepared for personal consumption. The bottled fish is usually cooked, boned, and put in a pickling solution, then stored in sterilized jars with rubber sealed lids.

Canning, on the other hand, entails placing the fish in a tin can with a lid, removing the air within the can through heat treatment, sealing the lid entirely, and then heating a second time to a specified degree.

The two most important considerations of this method are the availability and expense of the cans or bottles and strict quality control of the product.

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