Microbial spoilage of fish and fish products and its preservation

• Fish is among the most nutrient-dense and highly perishable foodstuffs. The delicate nature of fish is readily apparent in its rapid quality decline soon after harvest, if incorrectly stored and not rapidly handled.
• The inherent composition of fish and contaminations encountered during processing are the primary drivers of the commencement and progression of undesired quality alterations in these products.
• Enzymatic, chemical, and microbiological processes all play a role in the decomposition of fish. In most cases, the proliferation of harmful microorganisms as a result of contamination does not alter the sensory qualities of fish, which increases the difficulty of fish processing.
• To combat the quality and safety issues associated with fish and fisheries products, a number of preservation strategies have been developed and are now being refined. Although any method can yield good results, there is no single method that can simultaneously guarantee safety and halt the progression of rotting.
• The numerous preservation techniques employed in the fish and fishery business can be divided into three categories: physical, chemical, and biological.
• Different mechanisms of action are employed to extend the shelf life of fish and fish products using these techniques.
• Fish flesh contains microconstituents, such as water, proteins, and lipids, as well as minerals, vitamins, and enzymes. Crustaceans and mollusks also contain carbs in the form of glycogen.
• Due to their unique makeup, seafood goods are regarded extremely perishable. The fact that fishing vessels capture seafood typically at great distances from consuming places demands effective preservation to prevent product degradation.
• Consumer desire for high-quality, minimally-processed food with minimal alterations to nutritional and sensory qualities exacerbates this need.
• This also applies to aquacultured seafood species that must be carefully preserved for safe transport to distant locations.
• In addition to traditional seafood preservation methods such as chilling (at 0–1 °C), freezing (at 1 °C), drying, smoking, salting, fermentation, and canning, more recent methods include (1) the use of natural preservatives, (2) high hydrostatic pressure treatment, (3) ozonation, (4) irradiation, (5) pulse light technology, (6) retort pouch processing, and (7) packaging in conjunction with refrigeration or freezing.

Contamination of Fish and Other Seafoods

Contamination of fish and other seafood can occur through various factors throughout the harvesting, handling, and processing stages. Understanding these potential sources of contamination is crucial for ensuring the safety and quality of seafood products. Here are some key factors that contribute to the contamination of fish and other seafood:

1. Environmental factors: The environment in which seafood is harvested can play a significant role in contamination. Factors such as water pollution, harmful algal blooms, industrial discharge, and agricultural runoff can introduce contaminants into aquatic ecosystems, which may subsequently affect fish and other seafood.
2. Equipment used: The equipment used during the fishing, handling, and processing of seafood can act as a potential source of contamination. Catch boxes, bins, holds, dressing surfaces, decks, and cutlery handles should be regularly cleaned and sanitized to prevent the transfer of pathogens or other contaminants onto the seafood.
3. Water used for washing and cleaning: Water plays a crucial role in seafood processing, including washing fish and cleaning equipment. If the water used is contaminated with harmful bacteria, viruses, or chemicals, it can introduce contaminants to the seafood. It is essential to use clean and safe water sources for these purposes.
4. Method of harvesting: The method of harvesting seafood can impact its quality and potential for contamination. Improper handling techniques, such as rough handling or delays in chilling, can promote bacterial growth and compromise the freshness and safety of the seafood.
5. Season handling: Proper handling of seafood during different seasons is crucial to minimize contamination risks. Factors such as temperature control, maintaining appropriate storage conditions, and implementing timely processing and distribution practices are essential to preserve the quality and safety of the seafood.
6. Processing practices: The processing of seafood, such as filleting, shucking, or canning, should be conducted in hygienic conditions to prevent contamination. Proper sanitation protocols, including regular cleaning of equipment and surfaces, adequate training of personnel, and adherence to food safety standards, are essential to minimize the risk of contamination during processing.

To mitigate the contamination of fish and other seafood, various preventive measures can be implemented:

• Implementing proper environmental management practices to reduce pollution and maintain the quality of aquatic ecosystems.
• Regular cleaning and sanitization of fishing equipment, storage facilities, and processing areas.
• Using clean and safe water sources for washing fish and cleaning equipment.
• Following appropriate harvesting and handling techniques, including prompt chilling and maintaining proper storage temperatures.
• Implementing good manufacturing practices and adhering to food safety regulations during seafood processing.
• Conducting regular testing and monitoring for contaminants in seafood products to ensure safety and quality.

By addressing these potential sources of contamination and implementing appropriate preventive measures, the seafood industry can ensure the production of safe and high-quality fish and other seafood products for consumers.

Spoilage of fish 

• Fish contains vital digestive and nutritional proteins, amino acids, minerals, lipid-soluble vitamins, and highly unsaturated fatty acids.
• Its high water activity (0.98–0.99) and high water content (75–85%) make it susceptible to microbial development.
• Three types of fish spoilage exist: oxidation, enzymatic, and microbial.

1. Oxidative spoilage

• Lipid oxidation is a significant factor in the degradation and spoiling of fish with a high oil/fat content in their meat.
• Typically, oxidation includes oxygen reacting with the double bonds of fatty acids.
• Lipid oxidation in fish can occur either enzymatically or non-enzymatically.
• Lipid oxidation induces protein denaturation, protein alteration, electrophoretic profiles, nutrient losses, and loss of endogenous antioxidant systems.
• Lipid hydrolysis and oxidation induce “belly burst” in fish, in which the digestive tract’s enzymes and microbes cause huge gas production.

2. Enzymatic spoilage

• Dead fish undergo biological and chemical alterations as a result of the action of fish-specific enzymes upon capture.
• The digestive enzymes produce significant autolysis, resulting in softening of the fish’s muscles, rupture of the abdominal wall, and blood loss.
• Fish include several proteolytic enzymes that contribute to muscle and product breakdown during storage and processing.
• Proteolysis is responsible for the destruction of proteins that leads to the microbial growth that causes fish to deteriorate.
• Changes in fish induced by the presence of certain enzymes in fish include:

3. Microbial growth

• You can find amino acid molecules like trimethylamine oxide (TMAO), urea, taurine, creatine, free amino acids, and trace glucose, among other things in fish flesh along with the protein, lipids, carbs, water, and so on.
• Most fish internal organs are thought to be completely infertile. The slime layer of the epidermis, the gill surfaces, and the gut all harbour bacteria.
• Microbial growth in fish is the primary cause of fish spoiling, and it results in the formation of compounds with off- and unpleasant odours, such as amines, biogenic amines, organic acids, alcohols, aldehydes, and ketones.
• Since fish has a high water activity and low acidity (pH > 6), it quickly grows bacteria, which alters the fish’s look, texture, flavour, and odour for the worse.
• Bacillus, Clostridium, Escherichia coli, Micrococcus luteus, Proteus vulgaris, Sarcina sp., and Serratia may prevail at room temperature.
• Gram-negative, fermentative bacteria (like Vibrionaceae) are typically to blame for the rotting of fresh fish, whereas psychrotolerant Gram-negative bacteria (like Pseudomonas spp. and Shewanella spp.) are the most common culprits when it comes to the deterioration of cold fish.
• Psychrotrophic, aerobic, or facultative anaerobic Gram-negative bacteria like Pseudomonas, Moraxella, Acinetobacter, Shewanella putrifaciens, Vibrio, Flavobacterium, Photobacterium, and Aeromonas are also present in fish, causing spoilage.
• Under vacuum or CO2 conditions, lactic acid bacteria (LAB) can thrive in fish storage.
• Fish also have a role in the transmission of parasites such the tapeworm (Diphyllobothrium latum), nematodes (Anisakis simplex and Capillaria philippinensis), and trematodes (Opisthorchis and Paragonimus).
• Microorganisms develop spoilage chemicals while fresh fish is being stored.

Spoilage of fish products

1. Dried fish

• The low water activity of fully dried or salted and dried fish prevents the growth of microorganisms.
• However, fungal development is a significant issue, and microbiological changes occur in fish during processing, such as salting and drying.
• Dried fish are commonly infected with Aspergillus niger, Aspergillus flavus, and Penicillium spp.
• This dried fish undergoes a chemical alteration as well. The oxidation of lipid molecules in fatty fish can result in rancidity.

2. Smoked fish

• Hot smoking and cold smoking are the two styles of smoking.
• Smoking reduces gram-negative bacteria, but gram-positive bacteria, specifically micrococci and Corynebacterium, are present in hot-smoked fish, but cold-smoked fish contains gram-negative bacteria. Pseudomonas spoilage occurs.
• As a result of the phenolic chemicals generated during the smoking process, the bacterial burden is reduced.
• These compounds consist of guaiacol, creosol, and pyrogallol, which have a high level of phenol and are effective against Salmonella Typhi and Staphylococcus aureus.

3. Surmi

• Surimi is a deboned or filleted meat product.
• It is predominantly composed of muscle protein fibre.
• Surimi has Moraxella, pseudomonads, and Corynebacterium as microflora.

Preservation of Fish and Other Seafoods

• Fish is the most sensitive of all meat meals to autolysis, oxidation and hydrolysis of lipids, and microbiological deterioration.
• Therefore, its preservation requires rapid treatment with preservative procedures, which are frequently more stringent than those used on meats.
• When fish are caught far from the processing facility, they must be preserved even aboard the fishing vessel.
• To halt the activity of digesting enzymes in the intestines, evisceration must be performed rapidly. The benefits of gutting a fish may be countered by a probable delay in its quick cooling. Rigor mortis is crucial to the preservation of fish because it slows postmortem autolysis and bacterial decomposition.
• Therefore, every technique that prolongs rigour mortis also prolongs storage duration. It is longer if the fish have had less muscle activity before to death and if they have not been handled violently and injured during capture and processing, and it varies by species.
• The final pH of the flesh after death is proportional to the quantity of glycogen present at the time of death. The pH decreases as glycogen concentration increases.
• The greater the quantity of glycogen or the lower the final pH, the less muscle activity occurs before death.
• Temperature reduction will extend the time. Aseptic methods for reducing the contamination of seafood are difficult to implement, but some of the gross contamination prior to processing can be avoided through general cleaning and sanitization of boats, decks, holds, bins, or other containers and processing equipment in the plant, as well as the use of ice with a high bacteriological quality.
• The removal of filth from contaminated surfaces and fish using appropriate cleaning procedures, such as detergent solutions, significantly reduces the microbial burden on fish.
• The removal of organisms is challenging, but the fact that the majority of contamination is on the outer surface of fish and other seafood makes it possible to remove many bacteria by washing off slime and debris.

1. Use of Heat

• To assist removal of crab flesh from the shell, live crabs are roasted in retorts at temperatures as high as 121 degrees Celsius. The practise of selecting and packaging meat by hand is popular.
• Crabmeat processing temperatures and timeframes range from 85.6 to 87.2 degrees Celsius for 92 to 150 minutes.
• These procedures are referred to as pasteurisation, and cans are preserved by refrigeration. Some seafood, such as oysters, are “canned” by packing into cans or jars; however, they are not heat-processed and are instead preserved by refrigeration.
• The majority of canned seafood, however, is sterile or at least “commercially sterile” as a result of heat processing.
• As with meats, seafoods are low-acid foods with a sluggish rate of heat penetration, making them challenging to heat-process.
• In addition, several types of seafood become significantly softer or even disintegrate when sterilising in a can is tried.
• Depending on the substance being canned and the size and form of the container, the method varies. In general, the heat methods for vegetables are more harsh than those used for meats, although there are a few exceptions.
• The FAO code of practise for canning reduces the health risks associated with seafood in a can.

3. Use of Low Temperatures 

• As previously stated, rigour mortis delays the onset of autolysis, softening, and off-flavor production, as well as uncontrolled microbial growth, until after the fish or other marine animal has died, at which point autolysis, softening, and off-flavor production commence, and microbial growth becomes uncontrolled.
• Oysters in their shells, for instance, will not degrade as long as they are alive, and freezing oysters in their shells prolongs their lifespan.
• By transporting them in tanks to the New York market, carp captured from midwestern lakes were maintained alive and hence in good condition.
• “Feedy” fish, i.e., those packed with food, appear to decay more rapidly than regular fish.

a. Chilling 

• Due to the fact that fish flesh autolyzes and lipids get oxidised at temperatures over freezing — swiftly in the summer and more slowly as the temperature approaches freezing — preservation by lowering temperatures is, at best, short.
• When fish or other seafood is retrieved at a considerable distance from the receiving plate, the need for chilling on the boat varies on the type of fish, whether or not it is prepared on the boat, and the ambient temperature.
• In general, little fish are more perishable than large ones, and while dressed fish autolyze more slowly than whole fish, they are more susceptible to bacterial spoilage.
• When ambient temperatures are warm and transportation distances are considerable, it is important to cool fish and associated foods aboard the fishing boat using crushed ice or mechanical refrigeration to prevent autolysis and microbial development until the items are sold or processed for prolonged preservation.
• Subsequently, we shall address the addition of preservatives to the ice used to cool fish.
• In most cases, the period allowed for storing fish or other seafood in ice or cooling storage will not be very lengthy.
• Generally speaking, chilling storage on land is only practical when retail markets are nearby and turnover is high.
• Otherwise, another technique of preservation, such as freezing, salting, drying, smoking, canning, or combinations thereof, is utilised.

b. Freezing 

• The majority of contemporary techniques for freezing foods were first developed for freezing fish. Historically, ice with additional salt was utilised.
• With the introduction of mechanical refrigeration, fish were rapidly frozen and “glazed,” or coated with a covering of ice on the outside.
• Whole fish, particularly bigger ones, are typically quick-frozen in air or salt brine. Quick freezing is used to wrapped fillets or steaks, while smaller entire fish can also be frozen in this manner.
• As with meats, fish that has been rapidly frozen may thaw to resemble its original state more than fish that has been frozen slowly. The fats of frozen fish are vulnerable to hydrolysis and oxidation during storage.
• Possibly as a result of more hydrolysis, fatty fish decay faster than lean fish. Raw shrimp with their heads removed are frozen and glazed, and some cooked shrimp are also frozen.
• Scallops, clams, oysters, spiny lobster tails, and cooked crab and lobster flesh are also kept by freezing.
• The majority of these items are wrapped prior to freezing. As with meats, freezing kills some but not all germs, and if time allowed, growth will occur after thawing.
• Most psychrotrophic bacteria, such as Pseudomonas, Acinetobacter, Moraxella, Alcaligenes, and Flavobacterium species, survive freezing and are ready to develop upon thawing in fish.
• Clostridium botulinum type E spores may withstand freezing and storage and may germinate and generate toxin at temperatures above 3.3 degrees Celsius.
• Few enterococci, coliforms, or staphylococci are present in frozen uncooked fish. In the processing facility, cutting, breading, and battering might boost the population of these microorganisms. Precooking only decreases coliforms to a significant degree.

4. Use of Irradiation 

• The preservation of fish by UV light has been attempted but never implemented.
• Experiments suggest that gamma or cathode irradiation of certain species of fish may be effective.

5. Preservation by Drying 

• Dry-salting fish or immersing it in brine is a way of drying because it removes or binds moisture.
• Fish oil oxidation is not inhibited and may result in degradation. In the United States, fish is salted to a lesser level than in the rest of the world, yet salting is still commonly practised worldwide.
• The preparation of salt cod involves a combination of salting and air drying. Afterwards, the flesh is taken off the bones and skin.
• The sun-drying of fish, either little fish or flesh strips, is not often practised in the United States. The drying of the fish contributes to the preserving effects of smoking.

6. Use of Preservatives 

• The salting or marination of fish with dry salt or in brine is useful not only due to the drying impact described in the previous section, but also due to the chemical preservation effect of sodium chloride.
• In a significant number of nations, this technique is employed. Important are the chemical and bacteriological properties of the salt, as contaminants such as calcium and magnesium salts may impede the sodium chloride’s penetration, and the introduction of halophilic or salt-tolerant bacteria may discolour the fish.
• Due to the high perishability of fish, researchers have experimented with a variety of chemicals as preservatives, either applied directly to the fish or integrated into the ice used to chill them.

a. Preservatives Used on Fish 

• In the intense search for chemical preservatives that might be applied directly or as dips to round fish or fillets, a huge variety of compounds, ranging from those that would be approved by most control bodies to those whose usage would be problematic, have been tested. An appropriate preservative, sodium chloride, has been discussed.
• It is possible to dry-salt fish such that it contains 4 to 5 percent salt. The salt contributes halo-philes that may stain the fish (such as Serratia salinaria’s crimson pigment).
• Typically, Micrococcus species grow on the fish, while Flavobacterium, Alcaligenes, Pseudomonas, and others decline.
• Fish may be cured “mildly,” or with light salting, or in strong brine or with solid salt, and smoking may follow. As preservatives, benzoic acid and benzoates have achieved only mediocre success.
• In some countries, sodium and potassium nitrites and nitrates are approved to extend the shelf life of food.
• It has been discovered that sorbic acid delays the deterioration of smoked or salted fish. Boric acid has been utilised in Europe to increase preservation quality, however its use is prohibited in the United States.
• Formaldehyde, hypochlorites, hydrogen peroxide, sulphur dioxide, undeylenic acid, capric acid, poxybenzoic acid, and chloroform are more substances for which success claims have been made but whose use is contraindicated.
• Antibiotics have also been tested experimentally, typically as a dip or frozen. Chlortetracycline and oxytetracycline appeared to be the most effective of those examined, and their usage is now authorised.
• The effectiveness of chloramphenicol is average, but penicillin, streptomycin, and subtilin are poor or ineffective. It has been discovered that storing fish in an atmosphere with elevated amounts of carbon dioxide extends the shelf life.
• When a product begins to decay, the regular spoilage flora is replaced by lactobacilli and others, and the product becomes “sour.”
• Fish can be pickled by salting or acidifying with vinegar, wine, or sour cream. Herring is salted, seasoned, acidified, and flavoured in various ways.
• Various combinations of these procedures, along with an airtight container, are used to preserve the fish; however, some items also require refrigeration.
• Historically, fish was smoked heavily for preservation purposes, but now that canning, chilling, and freezing are available to extend shelf life, much of the smoking of fish is done mainly for flavour and is therefore light.
• The smoke treatment and additional preservation procedures combined with it vary based on the type of fish and the intended length of storage.
• Fish intended for smoking are typically gutted and decapitated, but they may also be in the round, split, or sliced into pieces.
• Commonly, mild or heavy salting precedes smoking and not only imparts flavour to the fish, but also enhances its keeping quality by reducing its moisture content.
• Air currents can facilitate drying. Fish may be smoked at relatively low temperatures (26.7 to 37.8 C) or at high temperatures ranging from 63 to 92 C, resulting in partial cooking.

Microbiology of Fish Brines 

• Depending on the proportion of salt, the temperature of the brine, the type and degree of contamination from the fish injected, and the period of use, the number of bacteria in fish-curing brines ranges from 10,000 to 10 million per millilitre.
• Salt concentrations are typically between 18 percent and saturation, but they may be lower after the introduction of fish.
• The greater the brine’s temperature, the more salt is required to avoid its deterioration. Fish typically introduce Pseudomonas, Acinetobacter, Moraxella, Alcaligenes, and Flavobacterium species; ice, which brings these genera plus Corynebacterium and cocci; and mechanically introduced sources, such as dust, which introduce cocci.
• Due to the addition of organisms from subsequent fish lots and the growth of salt-tolerant bacteria such as micrococci*, the number of organisms in the brine increases with ongoing use.
• As brine ages, corynebacteria in low-salt brines and micrococci* in high-salt brines proliferate and the brine’s total number decreases.

b. Preservatives Incorporated in Ice 

• Prior to freezing, a chemical preservative is added to water to produce so-called germicidal ices.
• These ices are eutectic when the added chemical is dispersed equally throughout, such as with sodium chloride, or noneutectic when distribution is not uniform, such as with sodium benzoate.
• For usage on fish, non-eutectic ice is coarsely crushed so that the chemical is evenly distributed. A large number of chemicals, including hypochlorites, chloramines, benzoic acid and benzoates, colloidal silver, hydrogen peroxide, ozone, sodium nitrite, sulfonamides, antibiotics, propionates, and levulinic acid, have been tested with varying degrees of success by a number of researchers in an effort to find the optimal chemical to be incorporated into ice for icing fish.
• The governments of the United States, Canada, and other nations currently approve the addition of up to 7 ppm of tetracyclines to ice used by fishermen to preserve fish on trawlers and during shipping.
• The goal of applying preservative chemicals directly on fish, or as dips or germicidal ices, is to kill or inhibit microorganisms on the fish’s surfaces, where they are initially most abundant and active.

c. Antioxidants 

• The fats and oils of many species of fish, particularly the fattier ones, such as herring, mackerel, mullet, and salmon, have a high proportion of unsaturated fatty acids and are therefore susceptible to oxidative changes that result in rancidity and often unpleasant colour changes.
• Antioxidants can be applied as dips, coatings, glazes, or gases to prevent these unwanted alterations.
• The storage of nordihydroguaiaretic acid, ethyl gallate, ascorbic acid, and other chemicals in carbon dioxide has produced positive results.

7. Smoking

Smoking is an old method of preservation in which fish is exposed to smoke, which improves the flavour and nutritional value of fish products. Hot smoking, smoke roasting, and cold smoking are common types of smoking.

• Hot smoking: In this process, the fish is smoked at a high temperature with a small amount of salt to limit bacterial growth. During hot smoking, temperatures vary from 60 to 93 degrees Celsius.
• Smoke roasting: In this procedure, fish that has been cured is smoked at a temperature of approximately 300 degrees Celsius. Numerous spices are added to fish to enhance its flavour and reduce bacterial growth.
• Cold smoking: Following partial or complete curing, the fish is typically hung or placed on racks and smoked at optimal temperatures between 23 and 48 °C for many days.

8. Canning

• In this method, fish in sealed containers composed of tin plates, aluminium cans, or glass is subjected to heat treatment until the product is completely sterilised.
• The heat treatment eliminates any heat-sensitive bacteria and spores, inactivates the enzymes, and cooks the fish so that the product can be sold without refrigeration for an extended period.

9. Drying

• The principle of drying as a method of food preservation is to remove water from the food and reduce its water activity level.
• The technique has proven effective at extending the fish’s shelf life.
• Sun-drying, vacuum-drying, and freeze-drying are the three forms of drying employed for fish preservation.

10. Curing 

• Curing fish is an age-old technique for preserving fish that also imparts the proper flavour.
• It maintains the fish by decreasing water activity and boosting osmotic pressure, which slows the growth of microorganisms.
• In this approach, fish is treated with salt, sugar, nitrites, nitrates, seasonings or spices, and phosphates.

11. Ozonation

• O3 is a triatomic form of oxygen that is gaining popularity in food processing due to its great cleaning capacity and is also a highly reactive antibacterial agent.
• According to research, treating fish with 1.5 ppm ozonated water for 15 minutes reduces microbial burden by 88.25%.

12. High-pressure treatments

• In this technique, bacteria are rendered inactive as a result of structural and metabolic abnormalities caused by high pressures.
• The inhibitory effect of the synergistic effect of high-pressure treatment at 200MPa on Listeria monocyogenes in smoked salmon is investigated.

13. Packaging technologies

• Packaging technologies involving either the elimination of air or the substitution of air with specific gases (such as CO2, O2, N2, or a combination of all).
• These packages are frequently referred to as modified atmosphere packaging (MAP) or vacuum packing (VP).
• Using MAP containing CO2, which has been demonstrated to inhibit the growth of spoilage and pathogenic bacteria, could extend the shelf life of fish.
• Under vacuum, air is evacuated and the package is sealed. The items were stored in a reduced O2 environment, with less than 1% preventing the growth of aerobic spoiling microbes, including Pseudomonas spp. and Aeromonas spp.

Spoilage of Fish and Other Seafoods

• As with meat, fish and other seafood can become rancid due to autolysis, oxidation, bacterial activity, or most typically, a combination of these processes.
• However, the majority of fish flesh is considered more perishable than meat due to the faster autolysis by fish enzymes and the less acidic reaction of fish tissue, which promotes microbial development.
• Additionally, certain unsaturated fish oils appear to be more sensitive to oxidative degradation than the majority of animal fats.
• The experts concur that bacterial deterioration of fish does not commence until after rigour mortis, when the meat fibres release their fluids.
• Therefore, the longer the fish can be stored, the longer this process is delayed or prolonged. Rigor mortis is accelerated by a struggling fish, a shortage of oxygen, and a high temperature, whereas it is slowed by a low pH and proper cooling.
• Not only because of its effect on rigour mortis, but also because of its effect on the growth of germs, the pH of fish flesh has a significant impact on its perishability.
• The lower the fish flesh’s pH, the slower bacterial breakdown will be in general. The conversion of muscle glycogen to lactic acid causes a decrease in the pH of the fish meat.

Factors Influencing Kind and Rate of Spoilage

The kind and rate of spoilage of fish vary with a number of factors: 

1. The kind offish

• The perishability of the many species of fish varies significantly. Thus, some flat fish decompose more quickly than round fish because they undergo rigour mortis more rapidly. However, a flat fish such as the halibut has a lower pH (5.5) in its flesh, allowing it to last longer.
• Due to the oxidation of the unsaturated lipids in their oils, certain fatty fish degrade quickly. Trimethylamine oxide-rich fish quickly produce considerable levels of “stale fish” trimethylamine.

2. The condition of the fish when caught

• Possibly due to the depletion of glycogen and, as a result, a lesser decrease in the pH of the flesh, fish that are fatigued as a result of struggle, oxygen deprivation, and extensive handling perish more quickly than those caught with less difficulty.
• “Feedy” fish, i.e., those who are full of food when captured, are more perishable than those with an empty intestine. 

3. The kind and extent of contamination of the fish flesh with bacteria

• These may originate from mud, water, humans, and the outer slime and intestinal content of the fish, and are thought to infiltrate the gills of the fish, from whence they move through the circulatory system and invade the flesh, or penetrate the digestive tract and enter the body cavity.
• Even then, growth is most likely confined, although the byproducts of bacterial breakdown reach the flesh very quickly by diffusion. In general, the rate of spoiling is proportional to the amount of germs on the fish.
• This contamination may occur in the net (dirt), the fishing boat, the docks, and eventually the plants.
• The flesh of fish in the round, i.e., not gutted, is not contaminated with intestinal organisms; however, it may become odorous due to the decomposition of food in the stomach and the diffusion of decomposition products into the flesh.
• This process is accelerated by the digestive enzymes attacking and perforating the intestinal wall, abdominal wall, and viscera, which have a high autolysis rate.
• Gutting the fish on the boat spreads intestinal and surface-slime bacteria throughout the flesh, but thorough washing will eliminate the majority of organisms and appropriate chilling will restrict the growth of any remaining organisms.
• Any damage to skin or mucous membranes will compromise the product’s quality.

4. Temperature

• The most frequent approach for avoiding or delaying bacterial development and, consequently, spoiling until the fish is consumed or otherwise processed is chilling.
• 0 to -1 C should be cooled as quickly as possible, and this temperature should be maintained.
• Obviously, the shorter the fish’s shelf life, the higher the temperature. Immediate and quick freezing of the fish is more effective for its preservation than other methods. 

5. Use of an antibiotic ice or dip. 

Bacteria Causing Spoilage

• The bacteria most frequently involved in the rotting of fish are a natural component of the exterior slime and intestinal contents of fish.
• The primary types of spoilage-causing bacteria vary depending on the temperatures at which the fish are stored, however at the cooling temperatures often employed, Pseudomonas species are most likely to predominate, followed by Acinetobacter, Moraxella, and Flavobacterium species.
• Bacteria of the genera Micrococcus and Bacillus appear less frequently and only at higher temperatures.
• Other taxa, such as Esherichia, Proteus, Serratia, Sarcina, and Clostridium, have been implicated with fish rotting, according to published reports. The majority of species would only thrive at average air temperatures and would likely perform poorly in frigid conditions.
• Normally, the number of pseudomonads increases on cold fish during holding, while the number of achromobacters* decreases and flavobacteria increases temporarily before decreasing.
• First, the bacteria develop on the surface, then they invade the flesh. Fish have a high nonprotein nitrogen content, and the autolytic alterations generated by their enzymes boost the supply of nitrogenous meals (such as amino acids and amines) and glucose for bacterial development.
• Trimethylamine, ammonia, amines (e.g., putrescine and cadaverine), lower fatty acids, aldehydes, and eventually hydrogen and other sulphides, mercaptans, and indole are produced from these molecules by bacteria, which are symptomatic of putrefaction.
• The musty or muddy aroma and flavour of fish has been attributed to the formation of Streptomyces species in the mud at the bottom of the body of water and the fish’s subsequent absorption of the flavour.
• As previously mentioned, discolorations of the fish flesh can occur during spoilage; yellow to greenish-yellow colours caused by Pseudomonas fluorescens, yellow micrococci, and others; red or pink colours from growth of Sarcina, Micrococcus, or Bacillus species or by moulds or yeasts; and a chocolate-brown colour by an aspergillus yeast.
• Fish parasitized by pathogens may develop discolorations or lesions.

Spoilage of Special Kinds of Fish and Seafoods 

• The majority of the prior discussion focused on the deterioration of chilled-preserved fish.
• Salt fish are damaged by salt-tolerant or halophilic bacteria of the genera Serratia, Micrococcus, Bacillus, Alcaligenes, Pseudomonas, and others, which frequently cause red discolorations.
• Molds are the primary organisms that cause deterioration in smoked fish. There should be no spoiling issues with fish that has been marinated (sour pickled), unless the acid content is low enough to support the formation of lactic acid bacteria or the presence of air permits mould growth.
• Similarly, frozen fish should not show any bacterial concerns after freezing, however their quality will rely on what happened to the fish before freezing.
• Despite the use of nitrite and other preservatives, Japanese fish sausage is susceptible to sourness due to volatile acid generation by bacilli or putrefaction.
• In general, mussels are susceptible to the same types of microbial deterioration as fish. However, Acinetobacter, Moraxella, and Vibrio are mostly responsible for spoiling in chilled shrimp, although a short increase in pseudomonads and a drop in Flavobacterium may occur.
• Micrococcus*, as well as Bacillus. At low temperatures, Pseudomonas, Acinetobacter, and Moraxella degrade crabmeat, but at higher temperatures, Proteus is mostly responsible.
• Bacillus, Pseudomonas, Alcaligenes, and Flavobacterium have been implicated in the rotting of raw lobsters. Crabs and oysters may harbour Vibrio species, such as V. parahaemolyticus.
• The concentrations of these substances vary with seasonal temperature fluctuations. Oysters remain in good condition as long as they are kept alive in the shell at a cold temperature, but they degrade quickly after they are dead, such as when they are shucked.
• The sort of deterioration of shucked oysters is dependent on their storage temperature.
• In addition to being rich in protein, oysters also contain sugars derived from the hydrolysis of glycogen.
• At temperatures close to freezing, the most significant spoilage bacteria are Pseudomonas, Acinetobacter, and Moraxella species, however Flavobacterium and Micrococcus* species may also develop.
• The deterioration is referred to as “souring,” although the predominant alterations are proteolytic. At higher temperatures, sourness may come from coliform bacteria, streptococci, lactobacilli, and yeast fermenting carbohydrates to generate acids and a sour odour.
• Possible early growth of Serratia, Pseudomonas, Proteus, and Clostridium. Oysters that are pink are the result of a rare type of deterioration caused by asporogenous yeast.

FAQ

References

• Ghaly, Abdel & Dave, Deepika & S, Budge & Brooks, M.. (2010). Fish Spoilage Mechanisms and Preservation Techniques: Review. American Journal of Applied Sciences. 7. 
• Austin, B. (2006). The bacterial microflora of fish, revised. TheScientificWorldJournal, Vol. 6, pp. 931–945. https://doi.org/10.1100/tsw.2006.181
• Contamination, M. (2016). Spoilage of Fish and Other Seafoods. Food Microbiology: Principles into Practice, 301–306. https://doi.org/10.1002/9781119237860.ch18
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• William C. Frazier; (1995) Food Microbiology, Fourth Edition.pdf.
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