Fish Preservation Techniques

A fish is any member of a group of animals that consist of all gill-bearing aquatic craniate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups.

Fish is a very good source of protein, but it is highly perishable; therefore, preservation should start immediately after catch. Fish can be preserved using different methods, and that is what is discussed in this article.

Proper Handling of Fish

Fish is one of the protein foods that needs careful handling. This is because fish spoils easily after capture due to the high tropical temperature, which accelerates the activities of bacteria, enzymes, and chemical oxidation of fat in the fish.

Due to poor handling, about 30–50% of fish harvested are wasted in Nigeria. These losses could be minimized by the application of proper handling, processing, and preservation techniques.

The purpose of processing and preserving fish is to get fish to an ultimate consumer in good, usable condition. The steps necessary to accomplish this begin before the fishing expedition starts and do not end until the fish is eaten or processed into oil, meal, or feed.

Fish begins to spoil as soon as it is caught, perhaps even before it is taken out of the water. Therefore, the key to delivering a high-quality product is close attention to small details throughout the entire process of preparation, catching, landing, handling, storage, and transport.

Fish that becomes spoiled or putrid is obviously unusable. Fish that is poorly cared for may not be so obviously bad, but it loses value because of off-flavors, mushy texture, or bad color that discourage a potential purchaser from buying.

1. Assessing Freshness of Fish

Freshness 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.

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Causes of Fish Spoilage

1. Factors Contributing to Fish Spoilage

Spoilage and freshness are the two qualities that have to be clearly defined. A fresh product is defined as one whose original characters remain unchanged. Spoilage, therefore, is indicative of post-harvest change. This change may be graded as the change from absolute freshness to limits of acceptability to unacceptability.

Spoilage is usually accompanied by change in physical characteristics. Change in color, odor, texture, color of eyes, color of gills, and softness of the muscle are some of the characteristics observed in spoiled fish. Spoilage is caused by the action of enzymes, bacteria, and chemicals present in the fish. In addition, the following factors contribute to spoilage of fish:

1. High moisture content
2. High fat content
3. High protein content
4. Weak muscle tissue
5. Ambient temperature
6. Unhygienic handling

2. Types of Fish Spoilage

i. Enzymatic Spoilage

Shortly after capture, chemical and biological changes take place in dead fish due to enzymatic breakdown of major fish molecules. The digestive enzymes cause extensive autolysis, which results in meat softening, rupture of the belly wall, and drain out of the blood and water, which contains both protein and oil.

A number of proteolytic enzymes are found in muscle and viscera of the fish after catch. These enzymes contribute to post-mortem degradation in fish muscle and fish products during storage and processing. During improper storage of whole fish, proteolysis is responsible for degradation of proteins and is followed by a process of solubilization. Belly bursting is caused by leakage of proteolytic enzymes from pyloric caeca and intestine to the ventral muscle.

ii. Microbial Spoilage

Composition of the microflora on newly caught fish depends on the microbial contents of the water in which the fish live. Microbial growth and metabolism is a major cause of fish spoilage. For unpreserved fish, spoilage is a result of Gram-negative, fermentative bacteria (such as Vibrionaceae), whereas psychrotolerant Gram-negative bacteria (such as Pseudomonas spp. and Shewanella spp.) tend to spoil chilled fish.

iii. Chemical Spoilage

Lipid oxidation is a major cause of deterioration and spoilage for pelagic fish species such as mackerel and herring with high oil/fat content stored in their flesh. Lipid oxidation involves a three-stage free radical mechanism: initiation, propagation, and termination.

Initiation involves the formation of lipid free radicals through catalysts such as heat, metal ions, and irradiation. These free radicals react with oxygen to form peroxyl radicals. During propagation, the peroxyl radicals react with other lipid molecules to form hydroperoxides and a new free radical.

Termination occurs when a buildup of these free radicals interacts to form non-radical products. These non-radical products produce the off-flavor and off-color of oxidized oil.

Oxidation typically involves the reaction of oxygen with the double bonds of fatty acids. Therefore, fish lipids, which consist of polyunsaturated fatty acids, are highly susceptible to oxidation.

The enzymatic hydrolysis of fats by lipases is termed lipolysis (fat deterioration). During this process, lipases split the glycerides, forming free fatty acids, which are responsible for: (a) common off-flavor, frequently referred to as rancidity, and (b) reducing the oil quality. The enzymes involved are the lipases present in the skin, blood, and tissue.

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Methods of Fish Preservation

1. Chilling for Fish Preservation

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. This is obtained by covering the fish with layers of ice. Ice is effective for short-term preservation, such as is needed to transport landed fish to nearby markets or to canning factories, etc.

It reduces the growth rate of bacteria by reducing the temperature of the fish and 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.

2. Salting for Fish Preservation

There are many different kinds of salt, some being better than others for fish curing. A distinction must be made between the two chief techniques of salting: wet salting and dry salting.

i. Wet Salting: The principle is to keep the fish for a long time in brine. The equipment needed consists of a watertight container, which can be a tin, drum, canoe, barrel, etc. To make the brine, one takes four parts of clean water (sea or fresh water) and mixes with one part of salt. If the salt is coarse, it has to be ground or pounded first.

The next step depends on what kind of fish one wants to salt. It is best first to cut off the head, and gut and clean the fish, though small fish can also be salted whole. Large fish must be cut open, and it is preferable to take out the backbone. Fish with a heavy armor of scales must be scaled.

Very large fish should be cut in thin fillets. After the fish has been prepared according to its size, it must be cleaned and put in the brine.

ii. Dry Salting: In this method, the fish is salted, but the juices, slime, and brine are allowed to flow away. For two parts of fish, one needs one part of salt. Layers of fish must be separated by layers of salt. Some people like the salty taste of fish prepared in this way, but it is always possible to wash the salt away by soaking it in fresh water before use.

3. Drying for Fish Preservation

Very small and thin fish can be dried straight away in the sun if they are brought in early enough in the morning (and if, of course, the sun is shining). If fish are laid on mats or other material to dry, it is best to turn them over every two hours so that they will dry quickly and not become maggotty. In the case of large fish, hanging is better if they are merely split.

4. Smoking for Fish Preservation

Any kind of fish can be smoked. There are three main methods of smoking: (a) smoking and roasting; (b) hot smoking; (c) long smoking.

i. Smoking and Roasting: This is a simple method of preservation, for consumption either directly after curing or within twelve hours.

5. Canning for Fish Preservation

This is a process involving heat treatment of fish in sealed containers made of tin plates, aluminum cans, or glass, until the product has been fully sterilized. During canning, heat treatment should be sufficient to destroy all heat-sensitive bacteria and spores, inactivate the enzymes, and cook the fish so that the product remains acceptable to the consumer after prolonged storage.

The canned fish is also prevented from contamination by pathogenic organisms by storing them in a virtually airtight package. If heat treatment is properly carried out, canned fish may remain in storage for several years without refrigeration.

Traditional canned fish are obtained from small pelagic fish species such as herrings (Clupea spp.), sardines (Sardinella sp.), mackerels (Scomberomorus sp.), anchovies (Engraulis sp.), tuna (Thunnus sp.), and bonga (Ethmalosa sp.)

Processed Fish Products

1. Fish Mince

This can be defined as flesh separated in a comminuted form, from the frames, scales, bones, and fins of fish. Fish mince can be prepared either mechanically by the use of a flesh-bone separator or non-mechanically.

A flesh/bone (or meat/bone) separator, also called a deboning machine, can be used to retrieve flesh attached to bones and frames of fish and thus make them better utilized instead of discarding them as waste.

Minced fish is obtained from filleting leftovers to headed and gutted fish using a bone/flesh (meat-bone) separator to remove bones from the edible flesh. Fish mince is very versatile and can be used to make a variety of products such as fish portions, fish fingers, fish cakes, fish sausage, and fish cheese.

2. Fish Sauce

This is an amber-colored liquid extracted from the fermentation of fish with sea salt. It is used as a condiment in various cuisines. Fish sauce is a staple ingredient in numerous cultures in Southeast Asia and the coastal regions of East Asia.

In addition to being added to dishes during the cooking process, fish sauce is also used as a base for a dipping condiment, prepared in many different ways in each country, for fish, shrimp, pork, and chicken.

In parts of southern China, it is used as an ingredient for soups and casseroles. Fish sauce, and its derivatives, impart an umami flavor to food due to their glutamate content.

3. Fish Meal

Fish meal is a commercial product mostly made from fish that are not generally used for human consumption but for animal feed; a small portion is made from the bones and offal left over from processing fish used for human consumption, while the larger percentage is manufactured from sustainable, managed, and monitored fish stocks of wild-caught, small marine fish.

It is a powder or cake obtained by drying the fish or fish trimmings, often after cooking, and then grinding it. If the fish used is a fatty fish, it is first pressed to extract most of the fish oil.

Fish is an important source of protein and healthy fat, but it is highly perishable. Fish should be preserved as soon as it is caught. Preservation methods for fish include chilling, salting, smoking, canning, etc. Fish can also be processed into other products like fish mince, fish sauce, fish meal, etc.

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