Microbial spoilage of fish and fish products and its preservation

Fish is one of the most consumed seafood and it is a highly perishable food product. Fish and fish products are widely consumed as they are a good source of nutrition due to their high protein content, unsaturated fatty acids, especially omega-3 fatty acids.

The biological and chemical nature of fish leads to its deterioration after it is caught. The spoilage process starts within 12 hours. The deterioration occurs very quickly due to the metabolic activity of microorganisms, endogenous enzymatic activity (autolysis), and the chemical oxidation of lipids .

Fresh fish and shellfish are highly perishable products due to their biological composition. Under normal refrigerated storage conditions, the shelf life of these products is limited by enzymatic and microbiological spoilage. Therefore, particular care is required during harvesting and all along the supply chain in order to preserve nutritional attributes, to avoid contamination and loss and waste, and to deliver high quality fish products.

There are several mechanisms that make fish a highly perishable commodity. Some of them are:

• Fish live underwater where the temperature is low and some microorganisms are adapted to low temperatures. These microorganisms, called psychrotrophs, can survive and grow in refrigerated conditions and cause spoilage of fish.
• Fish contain a chemical called trimethylamine oxide (TMAO) that protects them from the protein-destabilizing effect of water pressure. When fish dies and its tissues get exposed to air, TMAO is degraded by bacteria into trimethylamine, which gives fish a strong, fishy odor.
• Fish have a high water content (75–85%) and a high water activity (0.98–0.99) which makes them prone to microbial growth. Fish also have a low acidity (pH > 6) which favors the growth of spoilage and pathogenic bacteria.
• Fish contain important nutritional and digestive proteins, essential amino acids, lipid-soluble vitamins, micronutrients, and highly unsaturated fatty acids. These compounds are susceptible to degradation by microorganisms and enzymes that result in undesirable changes in appearance, texture, flavor, and odor, reducing its quality.

In this article, we will discuss the three modes of fish spoilage: Oxidation, Enzymatic and Microbial spoilage. We will also describe the spoilage of different fish products and explain various preservation methods used to stop the spoilage of fish. We will conclude on the importance of preservation methods in extending the shelf life of fish and fish products.

Fish is a highly perishable food product because of its biological and chemical nature. The biological nature of fish refers to its structure, composition, and metabolism, while the chemical nature refers to its reactions with oxygen, enzymes, and microorganisms.

The structure of fish muscle is composed of muscle fibers that are surrounded by connective tissue and embedded in a matrix of water and soluble proteins. The muscle fibers contain contractile proteins (actin and myosin), sarcoplasmic proteins (myoglobin, enzymes, and vitamins), and lipids (phospholipids and triglycerides). The connective tissue consists of collagen and elastin, which provide strength and elasticity to the muscle. The water content of fish muscle ranges from 75% to 85%, depending on the species, season, and diet.

The composition of fish muscle varies widely among different species and even within the same species. The main components are water, protein, lipid, and ash (minerals). The protein content of fish muscle ranges from 15% to 25%, depending on the species and the lipid content. Fish proteins are rich in essential amino acids, especially lysine, methionine, and threonine. The lipid content of fish muscle ranges from less than 1% to more than 30%, depending on the species, season, diet, age, sex, and location. Fish lipids are rich in unsaturated fatty acids, especially omega-3 fatty acids (eicosapentaenoic acid and docosahexaenoic acid), which have beneficial effects on human health. The ash content of fish muscle ranges from 1% to 2%, depending on the species and the water content. Fish ash contains mainly sodium, potassium, calcium, magnesium, phosphorus, iron, zinc, copper, selenium, and iodine.

The metabolism of fish is influenced by several factors such as temperature, salinity, oxygen availability, food intake, stress, and hormonal regulation. After capture or death, the metabolism of fish does not stop immediately but continues at a reduced rate until the energy reserves are depleted. This leads to several biochemical changes that affect the quality and shelf life of fish. Some of these changes are:

• Rigor mortis: This is the stiffening of the muscle due to the formation of actomyosin cross-links as a result of ATP depletion. Rigor mortis starts within a few hours after death and lasts for several hours or days depending on the species and the storage conditions.
• Glycolysis: This is the breakdown of glycogen to lactic acid by anaerobic respiration. Glycolysis lowers the pH of the muscle from about 7.0 to about 6.0 or lower depending on the species and the glycogen content. A low pH inhibits microbial growth but also affects the texture and color of fish.
• Autolysis: This is the self-digestion of the muscle by endogenous enzymes that are released from lysosomes or sarcoplasmic reticulum. Autolysis causes proteolysis (breakdown of proteins), lipolysis (breakdown of lipids), and hydrolysis (breakdown of other compounds) that result in the formation of volatile compounds such as ammonia, amines, aldehydes, ketones, organic acids, alcohols etc. These compounds contribute to the flavor and odor of fish but also cause spoilage when they reach high levels.
• Oxidation: This is the reaction of oxygen with lipids or proteins that causes rancidity or browning. Oxidation can occur enzymatically or non-enzymatically. Enzymatic oxidation involves the action of lipases or lipoxygenases that hydrolyze or oxidize lipids respectively. Non-enzymatic oxidation involves the action of oxygen radicals or metal ions that attack unsaturated bonds in lipids or proteins respectively. Oxidation causes loss of nutritional value, color changes (yellowing or browning), off-flavors (rancid or metallic), and off-odors (fishy or stale).

The biological and chemical nature of fish makes it susceptible to deterioration after it is caught. Therefore, proper handling, processing, and preservation methods are required to maintain its quality and extend its shelf life.

Fish and fish products can be contaminated by various sources during their life cycle, from the aquatic environment where they live, to the harvesting, processing, transportation, and storage stages. Contamination can affect the quality, safety, and shelf life of fish and fish products, and pose health risks to consumers. Some of the common sources of contamination are:

• Environmental factors: Fish can be exposed to natural or anthropogenic pollutants in the water, such as metals, pesticides, industrial chemicals, microplastics, and radioactive substances. These contaminants can accumulate in the fish tissues, especially in fatty fish, and cause oxidative spoilage or toxic effects . Some examples of environmental contaminants are mercury, lead, PCBs, DDT, dieldrin, PFAS, and radionuclides .
• Microorganisms: Fish are naturally colonized by microorganisms on their skin, gills, and intestine. These microorganisms can cause spoilage or disease in fish if they grow excessively or produce toxins. Some microorganisms can also infect fish from external sources, such as water, ice, soil, equipment, or other animals. Some examples of microbial contaminants are bacteria (e.g., Vibrio, Pseudomonas, Shewanella, Aeromonas), fungi (e.g., Aspergillus, Penicillium), parasites (e.g., tapeworms, nematodes), and viruses (e.g., norovirus) .
• Enzymes: Fish contain various enzymes that are involved in their metabolism and digestion. After death, these enzymes can continue to act on the fish tissues and cause autolysis or hydrolysis. This can result in softening of the flesh, rupture of the belly wall, loss of blood, and formation of off-flavors.
• Handling and processing factors: Fish can be contaminated by physical, chemical, or biological agents during the handling and processing stages. These include improper cleaning, gutting, filleting, salting, drying, smoking, canning, freezing, or packaging of fish. Contamination can also occur due to cross-contamination from other foods or materials. Some examples of handling and processing contaminants are dirt, scales, bones, foreign objects (e.g., glass), residual chemicals (e.g., sanitizers), spoilage bacteria (e.g., lactic acid bacteria), or pathogens (e.g., Salmonella).

Contamination sources of fish can vary depending on the type of fish (e.g., freshwater or marine), the location of fishing (e.g., coastal or inland), the season of fishing (e.g., summer or winter), and the method of fishing (e.g., net or hook). Therefore, it is important to monitor and control the contamination sources of fish and fish products to ensure their quality and safety.

Fish spoilage is a process of deterioration in the quality of fish, which changes its appearance, odour and taste. The breakdown of biomolecules like proteins, amino acids and fats in the fish are the factors responsible for fish spoilage. Thus, a fish can be spoiled by either chemical or biological degradation. There are three modes of fish spoilage: oxidation, enzymatic and microbial spoilage.

Oxidation

Oxidation is a chemical reaction that involves the loss of electrons by a substance. In fish, oxidation mainly affects the lipids (fats and oils) that are present in the flesh, especially in fatty fish species. Oxidation can occur enzymatically or non-enzymatically. Enzymatic oxidation is catalyzed by enzymes such as lipases and lipoxygenases that are present in the fish tissue or microorganisms. Non-enzymatic oxidation is caused by exposure to oxygen, light, heat or metal ions.

Oxidation results in the formation of various compounds such as hydroperoxides, aldehydes, ketones, alcohols and acids that have unpleasant flavours and odours. Oxidation also causes the loss of nutritional value, colour and texture of fish. Oxidation can be prevented or delayed by reducing the exposure to oxygen, light, heat or metal ions, by adding antioxidants such as vitamin E or C, or by using modified atmosphere packaging (MAP) or vacuum packaging (VP).

Enzymatic

Enzymatic spoilage is caused by the action of enzymes that are present in the fish tissue or microorganisms. Enzymes are biological catalysts that speed up chemical reactions. After death, the enzymes in the fish continue to function and break down the proteins, carbohydrates and lipids in the flesh. This process is called autolysis.

Enzymatic spoilage results in the formation of various compounds such as ammonia, amines, biogenic amines, organic acids and peptides that have unpleasant flavours and odours. Enzymatic spoilage also causes the loss of texture, colour and nutritional value of fish. Enzymatic spoilage can be prevented or delayed by lowering the temperature, by adding inhibitors such as salt or acid, or by using heat treatment such as smoking or canning.

Microbial

Microbial spoilage is caused by the growth and metabolism of microorganisms such as bacteria, fungi and parasites on the fish surface or inside the flesh. Microorganisms are present on the skin, gills and intestines of living fish and can also be introduced from the environment, equipment or water during harvesting, handling or processing.

Microbial spoilage results in the formation of various compounds such as trimethylamine (TMA), total volatile base nitrogen (TVB-N), hydrogen sulfide (H2S), indole and skatole that have unpleasant flavours and odours. Microbial spoilage also causes the formation of slime, discoloration and softening of fish tissue. Microbial spoilage can be prevented or delayed by lowering the temperature, by reducing the water activity, by adding preservatives such as salt, sugar or acid, or by using irradiation or high-pressure treatments.

Oxidative spoilage is one of the major causes of deterioration and spoilage of fish that contain high oil/fat content stored fat in their flesh. Oxidation typically involves the reaction of oxygen with the double bonds of fatty acids, resulting in the formation of peroxides, aldehydes, ketones, and other volatile compounds. These compounds can affect the flavor, color, texture, and nutritional value of fish, as well as promote protein denaturation and modification.

Oxidation can occur enzymatically or non-enzymatically in fish. Enzymatic oxidation is catalyzed by enzymes such as lipoxygenase, lipase, and phospholipase, which are present in fish tissues or microorganisms. Non-enzymatic oxidation is initiated by heat, light, metal ions, or free radicals. The rate and extent of oxidation depend on several factors, such as the degree of unsaturation of fatty acids, the presence of antioxidants or pro-oxidants, the storage temperature and time, and the packaging conditions .

Oxidative spoilage can also cause "belly burst" in fish, which is a condition where the enzymes and microorganisms of the digestive tract cause massive gas development and rupture of the abdominal wall. This can lead to further exposure of fish flesh to oxygen and microbial contamination.

Oxidative spoilage can be prevented or reduced by various methods, such as:

• Removing or reducing oxygen exposure by using vacuum packaging, modified atmosphere packaging, or edible coatings .
• Adding natural or synthetic antioxidants, such as ascorbic acid, tocopherols, rosemary extract, butylated hydroxyanisole (BHA), or butylated hydroxytoluene (BHT) .
• Lowering the storage temperature to slow down the enzymatic and chemical reactions .
• Avoiding exposure to light, heat, or metal ions that can initiate or accelerate oxidation .
• Selecting fish species with low oil/fat content or removing the fatty parts before storage.

After capture, biological and chemical changes take place in dead fish due to the action of various enzymes found in fish. These enzymes are responsible for the breakdown of proteins, lipids, and carbohydrates in the fish muscle and other tissues, resulting in changes in texture, flavor, odor, and appearance of the fish. This process is called autolysis or enzymatic spoilage.

The main enzymes involved in enzymatic spoilage are:

• Proteases: These are enzymes that hydrolyze proteins into peptides and amino acids. Proteases are present in the digestive tract, muscle, skin, and blood of fish. Proteases cause softening of the fish muscle, rupture of the belly wall, and loss of water-holding capacity. Proteases also release free amino acids that can be used by bacteria or oxidized to produce off-flavors and off-odors.
• Lipases: These are enzymes that hydrolyze lipids into fatty acids and glycerol. Lipases are present in the digestive tract, liver, and adipose tissue of fish. Lipases cause rancidity of the fish oil, especially in fatty fish. Lipases also release free fatty acids that can be oxidized or used by bacteria to produce off-flavors and off-odors.
• Carbohydrases: These are enzymes that hydrolyze carbohydrates into sugars and alcohols. Carbohydrases are present in the digestive tract and liver of fish. Carbohydrases cause fermentation of the fish glycogen, especially in lean fish. Carbohydrases also release sugars and alcohols that can be used by bacteria or oxidized to produce off-flavors and off-odors.

The rate and extent of enzymatic spoilage depend on several factors, such as:

• Species: Different species have different amounts and types of enzymes in their tissues. For example, cod has more proteases than salmon, while salmon has more lipases than cod.
• Temperature: Higher temperatures accelerate the enzymatic reactions and increase the spoilage rate. For example, at 20°C, cod can spoil within 24 hours, while at 0°C, it can last for 10 days.
• Handling: Rough handling can damage the fish tissues and release more enzymes into the flesh. For example, stunning and killing by hypothermia (the fish is killed in iced water) gives a fast onset of rigor mortis and a high level of enzymatic spoilage.
• Storage: The storage conditions can affect the activity and stability of the enzymes. For example, freezing can inhibit or reduce the enzymatic reactions, while thawing can activate or increase them.

Enzymatic spoilage can be prevented or delayed by:

• Killing the fish quickly and gently to reduce stress and glycogen depletion.
• Removing the viscera (gutted) as soon as possible to eliminate the source of most enzymes.
• Chilling or freezing the fish as soon as possible to slow down or stop the enzymatic reactions.
• Applying curing methods such as salting, smoking, or drying to reduce the water activity and inhibit the enzyme activity.
• Adding natural or chemical preservatives such as spices, herbs, vinegar, citric acid, or sodium benzoate to inhibit or reduce the enzyme activity.

Enzymatic spoilage is one of the main causes of fish deterioration and quality loss. It affects not only the sensory attributes but also the nutritional value and safety of fish. Therefore, it is important to control the enzymatic spoilage by proper handling and preservation methods.

Fish flesh is composed of protein, fats, carbohydrates, water, and amino acid compounds such as trimethylamine oxide (TMAO), urea, taurine, creatine, free amino acids, and trace glucose, etc. The internal tissue of fish is generally considered sterile. Bacteria are present on the slime layer of the skin, gill surfaces, and the intestine. The microbial growth in fish is the main cause of fish spoilage and produces amines, biogenic amines, organic acids, alcohols, aldehydes, and ketones with unpleasant and off-flavors . The high water activity (0.98–0.99), low acidity (pH > 6) of fish result in the fast growth of microorganisms that leads to undesirable changes in appearance, texture, flavor, and odor, reducing its quality.

The microorganisms involved in fish spoilage refer to the SSOs (specific spoilage organisms) that result in the formation of numerous unwanted metabolites, which adds undesirable appearance, flavour, and odour to the fish. SSOs’ growth strongly depends upon the nutritional components of fish, moisture content, high temperature etc. The SSOs vary depending on the type of fish product and the storage conditions. For unpreserved fish, spoilage is caused by Gram-negative, fermentative bacteria (such as Vibrionaceae), whereas psychrotolerant Gram-negative bacteria (such as Pseudomonas spp. and Shewanella spp.) tend to spoil chilled fish . The fish spoilage is also caused by psychrotrophic, aerobic, or facultative anaerobic Gram-negative bacteria such as Pseudomonas, Moraxella, Acinetobacter, Shewanella putrifaciens, Vibrio, Flavobacterium, Photobacterium, and Aeromonas . Gram-positive bacteria such as Staphylococcus spp., Micrococcus spp., Bacillus spp., Clostridium spp., Corynebacterium spp., Brochothric thermosphacta and Streptococcus spp. are also found in fish . Lactic acid bacteria (LAB) can predominate in fish storage under vacuum or CO2 storage . Some parasites can also be transmitted by fish, including tapeworm (Diphyllobothrium latum), nematodes (Anisakis simplex and Capillaria philippinensis), and trematodes (Opisthorchis and Paragonimus).

Spoilage compounds are produced by microorganisms during the storage of fresh fish. The most common spoilage compounds are trimethylamine (TMA), dimethylamine (DMA), ammonia (NH3), hydrogen sulfide (H2S), indole and skatole. TMA is formed by the reduction of TMAO by bacterial enzymes and gives a characteristic fishy odor to spoiled fish. DMA is formed by the decarboxylation of amino acids or by the demethylation of TMAO by bacterial enzymes and gives a putrid odor to spoiled fish. NH3 is formed by the deamination of amino acids or by the hydrolysis of urea by bacterial enzymes and gives an ammonia-like odor to spoiled fish. H2S is formed by the reduction of sulfate or sulfur-containing amino acids by bacterial enzymes and gives a rotten egg odor to spoiled fish. Indole and skatole are formed by the degradation of tryptophan by bacterial enzymes and give a fecal odor to spoiled fish.

The microbial spoilage of fish can be detected by various methods such as sensory evaluation (appearance, odor, texture), chemical analysis (pH, total volatile base nitrogen (TVB-N), TMA-N), microbiological enumeration (total viable count (TVC), specific spoilage organisms count) or molecular techniques (PCR-DGGE) . These methods can help to assess the quality and safety of fish products and to determine their shelf life. However, each method has its advantages and limitations. For example, sensory evaluation is subjective and depends on the panelists` experience and training, chemical analysis is not specific and may not reflect the actual spoilage, microbiological enumeration is time-consuming and may not detect all the spoilage organisms, and molecular techniques are expensive and require specialized equipment and skills . Therefore, a combination of different methods may be more reliable and accurate to monitor the microbial spoilage of fish.

Microbial spoilage of fish and fish products is one of the major causes of quality deterioration and economic losses in the seafood industry. Microorganisms can cause various defects on fish, such as slime formation, discoloration, softening, off-odors, and off-flavors, depending on the type of fish, the storage conditions, and the microbial species involved.

Slime formation

Slime formation is a common defect observed on whole fish or fillets stored under refrigerated or iced conditions. Slime is a viscous layer of extracellular polymeric substances (EPS) produced by bacteria on the surface of fish. EPS can consist of polysaccharides, proteins, nucleic acids, and lipids, and can provide protection and adhesion for bacteria. Slime formation can affect the appearance, texture, and shelf life of fish products.

The main bacteria responsible for slime formation are Gram-negative psychrotrophic bacteria, such as Pseudomonas spp., Shewanella spp., Aeromonas spp., Moraxella spp., Acinetobacter spp., Flavobacterium spp., and Vibrio spp. These bacteria can utilize the nutrients present on the fish surface, such as proteins, amino acids, sugars, and trimethylamine oxide (TMAO), and produce various enzymes, such as proteases, lipases, amylases, and TMAO demethylase. These enzymes can degrade the fish tissue and release more substrates for bacterial growth.

Slime formation can be reduced by lowering the storage temperature, applying salt or acid treatments, using modified atmosphere packaging (MAP) or vacuum packaging (VP), or adding antimicrobial agents or natural preservatives.

Discoloration

Discoloration is another defect that can affect the visual quality and consumer acceptance of fish products. Discoloration can be caused by various factors, such as oxidation of pigments, enzymatic browning, microbial growth, or chemical contamination.

Microbial discoloration can occur on the skin, gills, or eyes of whole fish or on the flesh of fillets. Microbial discoloration can be caused by pigmented bacteria that produce colored metabolites or by non-pigmented bacteria that alter the natural pigments of fish.

Some examples of microbial discoloration are:

• Greenish-yellow discoloration caused by Pseudomonas fluorescens
• Yellow discoloration caused by Micrococcus spp.
• Red discoloration caused by Serratia marcescens
• Brown discoloration caused by Bacillus spp. or Sarcina spp.
• Black discoloration caused by Chromobacterium violaceum

Microbial discoloration can be prevented or delayed by using proper hygiene practices during handling and processing of fish, lowering the storage temperature, applying salt or acid treatments, using MAP or VP, or adding antimicrobial agents or natural preservatives.

Softening

Softening is a defect that affects the texture and firmness of fish flesh. Softening can be caused by enzymatic or microbial degradation of fish muscle proteins.

Enzymatic softening can occur due to the action of endogenous enzymes present in fish muscle or digestive tract after death. These enzymes include cathepsins, calpains, collagenases, elastases, and proteases. Enzymatic softening can result in muscle liquefaction, belly bursting, gaping (separation of muscle blocks), and drip loss.

Microbial softening can occur due to the action of exogenous enzymes produced by bacteria that colonize the fish flesh. These enzymes include proteases, collagenases, elastases, and gelatinases. Microbial softening can result in loss of water-holding capacity (WHC), reduced yield, and altered sensory properties.

The main bacteria involved in microbial softening are Gram-negative psychrotrophic bacteria, such as Pseudomonas spp., Shewanella spp., Aeromonas spp., Vibrio spp., Flavobacterium spp., and Photobacterium spp. These bacteria can hydrolyze various muscle proteins, such as myosin, actin, tropomyosin, troponin, titin, nebulin, and collagen.

Softening can be reduced by lowering the storage temperature, applying salt or acid treatments, using MAP or VP, or adding antimicrobial agents or natural preservatives.

Off-odors and off-flavors

Off-odors and off-flavors are defects that affect the aroma and taste of fish products. Off-odors and off-flavors can be caused by chemical oxidation of lipids, enzymatic degradation of proteins, or microbial metabolism of substrates.

Chemical oxidation of lipids can result in rancidity, which is characterized by unpleasant odors and flavors, such as fishy, metallic, cardboard, or painty. Chemical oxidation can be influenced by factors such as oxygen exposure, light exposure, temperature, metal ions, and pro-oxidants.

Enzymatic degradation of proteins can result in the formation of volatile compounds, such as ammonia, amines, sulfides, indoles, and skatole. These compounds can cause ammonia-like, fishy, putrid, or fecal odors and flavors. Enzymatic degradation can be influenced by factors such as pH, temperature, and enzyme activity.

Microbial metabolism of substrates can result in the formation of various volatile compounds, such as aldehydes, ketones, alcohols, acids, esters, and sulfur compounds. These compounds can cause sour, fruity, cheesy, buttery, or rotten odors and flavors. Microbial metabolism can be influenced by factors such as nutrient availability, temperature, water activity, pH, and oxygen.

The main bacteria involved in microbial spoilage of fish are Gram-negative psychrotrophic bacteria, such as Pseudomonas spp., Shewanella spp., Aeromonas spp., Vibrio spp., Flavobacterium spp., and Photobacterium spp. These bacteria can utilize various substrates present in fish flesh, such as TMAO, amino acids, sugars, and lipids, and produce various spoilage compounds, such as trimethylamine (TMA), dimethylamine (DMA), dimethyl sulfide (DMS), acetic acid, propionic acid, butyric acid, ethanol, acetone, and diacetyl.

Off-odors and off-flavors can be prevented or delayed by lowering the storage temperature, applying salt or acid treatments, using MAP or VP, or adding antimicrobial agents or natural preservatives.

Fish products are processed fish or fish parts that have been subjected to various treatments to enhance their sensory, nutritional, or functional properties. Some common fish products are dried fish, smoked fish, surimi, fish sauce, fish paste, fish balls, and fish oil. However, these products are also susceptible to spoilage by different microorganisms, enzymes, or chemical reactions. The type and extent of spoilage depend on the product characteristics, processing methods, and storage conditions.

Dried fish

Dried fish is a product prepared by removing water from fresh or salted fish by sun drying, vacuum drying, or freeze-drying. The water activity of fully dried or salted and dried fish is low so that bacteria cannot grow. However, fungal growth is a major problem and microbiological changes in fish also occur during its processing such as the salting and drying process. Aspergillus niger, Aspergillus flavus, and Penicillium spp are common molds found in dried fish. There is also a chemical change in this dried fish. Lipids compounds in fatty fish can undergo oxidation which leads to rancidity.

Smoked fish

Smoked fish is a product prepared by subjecting fresh or salted fish to smoke, which enhances the sensory and nutritional characteristics of fish products. Common smoking methods are hot smoking, smoke roasting, and cold smoking. Smoking will reduce gram-negative bacteria but gram-positive bacteria particularly micrococci and Corynebacterium are found in hot smoked while in cold-smoked fish Pseudomonas spoilage occurs. During smoking processes, there is a reduction of bacterial load due to phenolic compounds released in the smoked. These compounds include guaiacol, creosol, pyrogallol, which have a high phenol that acts against Salmonella Typhi and Staphylococcus aureus.

Surimi

Surimi is a product prepared from either deboned meat or fillet. It mainly consists of muscle protein fiber. The microflora on surimi consists of Moraxella, pseudomonads, and Corynebacterium. Surimi is usually pasteurized to reduce the microbial load and extend the shelf life. However, some psychrotrophic bacteria such as Lactobacillus spp., Leuconostoc spp., Carnobacterium spp., and Brochothrix thermosphacta can survive the pasteurization and cause spoilage during refrigerated storage. Spoilage of surimi is characterized by souring, slime formation, gas production, color change, and off-odor.

Fish sauce

Fish sauce is a product prepared by fermenting fish with salt for several months to years. The fermentation process involves the action of various microorganisms and enzymes that produce amino acids, peptides, organic acids, alcohols, aldehydes, ketones, and other volatile compounds that contribute to the flavor and aroma of fish sauce. The main microorganisms involved in fish sauce fermentation are halophilic bacteria such as Staphylococcus spp., Micrococcus spp., Bacillus spp., Vibrio spp., and Halobacterium spp.. However, some undesirable microorganisms such as molds and yeasts can also grow in fish sauce and cause spoilage. Spoilage of fish sauce is manifested by turbidity, sedimentation, discoloration, off-flavor, and loss of quality.

Fish paste

Fish paste is a product prepared by grinding fresh or salted fish into a fine paste with or without added ingredients such as starch, sugar, salt, spices, etc. Fish paste can be used as a raw material for making other products such as fish balls or cakes. The main spoilage microorganisms of fish paste are lactic acid bacteria (LAB) such as Lactobacillus spp., Leuconostoc spp., Pediococcus spp., etc.. These bacteria produce lactic acid and lower the pH of the product which inhibits the growth of other bacteria. However, some LAB can also produce undesirable compounds such as biogenic amines (e.g., histamine) which can cause food poisoning. Spoilage of fish paste is indicated by souring, slime formation, gas production, color change, and off-odor.

Fish balls

Fish balls are products prepared by shaping fish paste into balls and cooking them by boiling, frying, or steaming. Fish balls are popular in Asian cuisines and can be eaten as snacks or added to soups or noodles. The spoilage microorganisms of fish balls are similar to those of fish paste, mainly LAB. However, fish balls can also be contaminated by other bacteria such as Enterobacteriaceae, Staphylococcus spp., Bacillus spp., etc. during processing or handling. Spoilage of fish balls is characterized by souring, slime formation, gas production, color change, and off-odor.

Fish oil

Fish oil is a product extracted from the liver or flesh of fish. It is rich in omega-3 fatty acids and has various health benefits. However, fish oil is also prone to oxidation which leads to rancidity and loss of nutritional value. The oxidation of fish oil is influenced by factors such as temperature, light, oxygen, moisture, metal ions, etc.. Oxidation of fish oil produces peroxides, aldehydes, ketones, and other volatile compounds that cause off-flavor and off-odor. Spoilage of fish oil is detected by measuring the peroxide value, anisidine value, thiobarbituric acid value, or sensory evaluation.

The spoilage mechanisms can be carried out by microbial growth, enzymatic activities, or chemical reactions. Thus, preservation methods are used to stop the various spoilage of fish. Some of the common preservation methods are:

• Heat treatment: This method involves applying high temperatures to fish to destroy microorganisms and enzymes, and to cook the fish. Examples of heat treatment are drying, canning, and smoking. Drying removes water from the fish and lowers the water activity level, making it less prone to microbial growth. Canning sterilizes the fish in sealed containers by applying heat until all heat-sensitive bacteria and spores are killed. Smoking preserves fish by drying, by deposition of creosote ingredients, and by heat penetration. Smoking also enhances the sensory and nutritional characteristics of fish products.
• Chilling: This is the simplest method to both preserve and process fish. The fresh fish is stored at a refrigeration temperature of 0ºC to 8ºC, which slows down the metabolic activity of microorganisms and enzymes. Chilling can extend the shelf life of fresh fish for up to a week depending on the fish quality and the refrigerator temperature.
• Curing: This is an ancient technique for preserving fish and also giving it a desired flavor. It involves adding salt, sugar, nitrites, nitrates, seasonings or spices, and phosphates to fish. Curing preserves the fish by decreasing water activity and by increasing osmotic pressure that delays microbial growth. Curing also inhibits lipid oxidation and enhances color development in fish products.
• Use of preservatives: Preservatives are substances that are capable of inhibiting or retarding the growth of microorganisms. Preservatives used in food can be divided into three types: natural preservatives (such as salt, sugar, vinegar, spices, etc.), bio preservatives (such as bacteriocins, lysozyme, nisin, etc.), and chemical preservatives (such as benzoates, sorbates, sulfites, etc.). Some preservatives used in fish and fish products are butyl hydroquinone (TBHQ), sodium tripolyphosphate (STPP), sodium nitrite (NaNO2), etc.
• High-pressure treatments: This method involves applying high pressures to fish to inactivate microorganisms and enzymes without affecting the nutritional and sensory qualities of the fish. High-pressure treatments can range from 100 to 800 MPa depending on the type and quality of the fish product. High-pressure treatments can extend the shelf life of fresh fish for up to 21 days at refrigeration temperatures.
• Ozonation: Ozone is a triatomic form of oxygen that has a high sanitizing power and is a highly reactive antimicrobial agent. Ozonation involves treating fish with ozonated water or gas to reduce the microbial load and spoilage compounds. Ozonation can also improve the color and texture of fish products.
• Irradiation: Irradiation involves exposing fish to ionizing radiation such as gamma rays, X-rays, or electron beams to destroy microorganisms and enzymes, and to reduce spoilage compounds. Irradiation can also improve the safety and shelf life of fish products by eliminating pathogens such as Salmonella or Vibrio. Irradiation doses for fish preservation can vary from 1 to 10 kGy depending on the type and quality of the fish product.
• Packaging technologies: Packaging technologies involve either removing air or replacing air by certain gases (such as CO2, O2, N2, or a combination of all) in the package that contains the fish product. Such packages are often termed modified atmosphere packaging (MAP) or vacuum packaging (VP). Packaging technologies can inhibit the growth of aerobic spoilage microorganisms, reduce lipid oxidation, prevent dehydration, and maintain color and texture of fish products.

These preservation methods are important for extending the shelf life of fish and fish products and for ensuring their safety and quality for human consumption.

Fish and fish products are highly nutritious and delicious foods that are widely consumed around the world. However, fish are also highly perishable and prone to spoilage by various biological, chemical and physical factors. Therefore, preservation methods are essential to extend the shelf life of fish and fish products and to maintain their quality and safety.

There are many preservation methods that can be applied to fish and fish products, depending on the type of fish, the desired product characteristics, the available facilities and the market demand. Some of the most common preservation methods are:

• Freezing: This method involves lowering the temperature of fish to below 0°C, which inhibits the growth of microorganisms and slows down the enzymatic and oxidative reactions that cause spoilage. Freezing can preserve fish for several months or even years, depending on the type of freezer and packaging used.
• Canning: This method involves heating fish in sealed containers to destroy all microorganisms and spores that can cause spoilage or foodborne illness. Canning can preserve fish for several years at room temperature, without the need for refrigeration.
• Smoking: This method involves exposing fish to smoke, which reduces the moisture content, adds flavor and color, and inhibits microbial growth. Smoking can preserve fish for several days or weeks, depending on the type and degree of smoking.
• Pickling: This method involves immersing fish in a solution of salt, vinegar, spices and other ingredients, which lowers the pH, increases the osmotic pressure and adds flavor. Pickling can preserve fish for several weeks or months, depending on the type and concentration of the pickling solution.
• Curing: This method involves applying salt, sugar, nitrites, nitrates or other chemicals to fish, which reduces the water activity, inhibits microbial growth and enhances flavor. Curing can preserve fish for several days or weeks, depending on the type and amount of curing agent used.

In addition to these traditional methods, there are also some modern techniques that can be used to preserve fish and fish products, such as:

• High-pressure processing: This method involves applying high pressure (up to 600 MPa) to fish, which causes lethal damage to microorganisms without affecting the sensory and nutritional qualities of fish. High-pressure processing can extend the shelf life of fresh or cooked fish by several days or weeks.
• Ozonation: This method involves treating fish with ozone gas (O3), which is a powerful oxidizing agent that kills microorganisms and degrades organic pollutants. Ozonation can improve the microbiological quality and safety of fresh or processed fish.
• Irradiation: This method involves exposing fish to ionizing radiation (such as gamma rays or X-rays), which damages the DNA of microorganisms and inactivates enzymes that cause spoilage. Irradiation can increase the shelf life of fresh or processed fish by several days or weeks.
• Modified atmosphere packaging: This method involves packing fish in a gas mixture (such as CO2, O2 or N2) that modifies the atmospheric composition inside the package. Modified atmosphere packaging can slow down microbial growth, enzymatic activity and oxidative reactions that cause spoilage. Modified atmosphere packaging can extend the shelf life of fresh or processed fish by several days or weeks.

All these preservation methods have their advantages and disadvantages, depending on the type of fish, the desired product characteristics, the available facilities and the market demand. Therefore, it is important to select the most appropriate method for each situation and to follow good hygienic practices throughout the processing chain. By doing so, we can ensure that we enjoy high-quality and safe fish and fish products for longer periods of time.

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