Microbial spoilage of fish and fish products and its preservation

Introduction

• Fish is one of the most consumed seafood and it is a highly perishable food product.
• Fish and fish products are widely consumed as it is a good nutrition source 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 (Rigor mortis) will start within 12 h.
• The deterioration occurs very quickly due to the metabolic activity of microorganisms, endogenous enzymatic activity (autolysis), and the chemical oxidation of lipids.

Contamination source of fish

• Environmental factors.
• Equipment used such as catch boxes, bins, holds, dressing surfaces, decks, and cutlery handles.
• Water used for washing fish and cleaning the equipment.
• Method of harvesting, season handling, and processing.

Spoilage of fish 

• Fish contain important nutritional and digestive proteins, essential amino acids, lipid-soluble vitamins, micronutrients, and highly unsaturated fatty acids.
• It contains water (75–85%) and has a high water activity (0.98–0.99) which makes it prone to microbial growth.
• There are three modes of fish spoilage: Oxidation, Enzymatic and Microbial spoilage.

1. Oxidative spoilage

• Lipid oxidation is a major cause 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.
• In fish, lipid oxidation can occur enzymatically or non-enzymatically.
• Lipid oxidation promotes protein denaturation, modification of the protein, electrophoretic profiles, nutritional losses, and endogenous antioxidant systems losses.
• Lipid hydrolysis and oxidation cause “belly burst” in fish in which the enzymes and microorganisms of the digestive tract cause massive gas development.

2. Enzymatic spoilage

• After capture, biological and chemical changes take place in dead fish due to the action of various enzymes found in fish.
• The digestive enzymes cause extensive autolysis which results in fish muscle softening, rupture of the belly wall, and drain out of the blood.
• Several proteolytic enzymes are found in fish that contribute to degradation in fish muscle and fish products during storage and processing.
• Proteolysis is responsible for the degradation of proteins which leads to fish spoilage by microbial growth.
• The changes in fish caused due to the various enzymes found in fish are: 

3. Microbial growth

• 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, 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.
• At room temperature, Bacillus, Clostridium, Escherichia, Micrococcus, Proteus, Sarcina, and Serratia may predominate.
• 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, Bacillus, Clostridium, Corynebacterium, Brochothric thermosphacta, and Streptococcus are 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.

Defects observed on fish by microbial spoilage

• Bacterial growth causes slime layer, discoloration of gills and eyes (in whole fish), and loss of muscle texture (softened due to proteolysis).
• The volatile compounds from the putrefaction of proteins will result in the formation of different types of off-odor such as fishy (due to trimethylamine) and putrid odor.
• Bacteria such as Shewanella putrifaciens, Aeromonas spp., psychrotolerant Enterobacteriaceae, P.phosphoreum, and Vibrio spp. creates ammonia-like off-flavors and fishy off-flavor.
• Pseudomonas putrifaciens, fluorescent pseudomonads, and other spoilage bacteria increase rapidly and produce many proteolytic and hydrolytic enzymes.
• The greenish-yellow color on fish is caused by Pseudomonas fluorescens; yellow color by Micrococcus; red color by Bacillus, and Sarcina.
• The chocolate-brown color is caused by yeasts and molds; and a musty odor by Streptomyces.

Spoilage of fish products

1. Dried fish

• 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.

2. Smoked fish

• There are two types of smoking i.e. hot smoking 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.

3. Surmi

• 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.

Preservation of fish and fish products

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.

A. Heat treatment

1. Drying

• The principle of the drying preservation method is to remove water from the food and lower the water activity level.
• The method has proven to be effective in extending the shelf life of fish.
• There are three types of drying are used for fish preservation sun drying, vacuum drying, and freeze-drying.

2. Canning

• In this process, heat treatment is applied to fish in sealed containers made of tin plates, aluminum cans or glass, until the product has been fully sterilized.
• The heat treatment destroys all heat sensitive bacterial and spores, inactivate the enzymes, and cook the fish so that the product remains acceptable to the consumer after prolonged storage without refrigeration.

3. Smoking

Smoking is an ancient preservation technique, where fish is subjected to smoke, which enhances the sensory and nutritional characteristics of fish products. Common smoking methods are hot smoking, smoke roasting, and cold smoking.

• Hot smoking: In this method, the fish is hot smoked with mild addition of salt to inhibit bacterial growth. Hot smoking is done with temperatures ranges from 60°C to 93°C.
• Smoke roasting: In this method, the fish with curing is hot smoked at the temperature of about 300°C. Many spices are added to fish for flavor enhancement and also to inhibit bacterial growth.
• Cold smoking: In this method, the fish after being partially or fully cured is usually hung or placed on racks and allowed to smoke for days at an optimum temperatures ranges of 23-48 °C.

B. Chilling

• It is the simplest method to both preserve and process fish.
• The fresh fish is stored at a refrigeration temperature of 0ºC to 8ºC 
• Generally, fresh fish remains in good condition for a period of 5-7 days if kept at a refrigerated temperature of 4 ± 1°C.

C. Curing 

• Curing fish is an old-age technique for the preservation of fish and also gives a desired flavor to the fish.
• It preserves the fish by decreasing water activity and by increasing osmotic pressure that delays microbial growth.
• In this method, salt, sugar, nitrites, nitrates, seasonings or spices, and phosphates are added to fish.

D. Use of preservatives

Preservatives are substances that are capable of inhibiting or retarding the growth of microorganisms. Such preservatives used in food can be divided into three types:

• Natural preservatives
• Bio preservatives
• Chemical preservatives

Some preservatives used in fish and fish products are :

E. High-pressure treatments

• In this method, microorganisms are inactivated due to lethal structural and biochemical alterations caused to the cells as a result of high pressures applied.
• It is studied that the synergistic effect of high-pressure treatment at 200MPa has an inhibitory effect on Listeria monocyogenes in smoked salmon.

F. Ozonation

• O3 is a triatomic form of oxygen that has been gaining space in food processing due to its high sanitizing power and is also known as a highly reactive antimicrobial agent.
• In studies, it has shown that treatment of fish with 1.5ppm ozonated water for 15min reduces the microbial load by 88.25%.

G. Irradiation

• Irradiation in foods is described in three different ways:
  • Radurization (cold pasteurization) involves the use of low doses (<1kGy)
  • Radicidation involves the use of doses (2–8kGy).
  • Radappertization (cold sterilization) involves the use of a high dose (>10kGy).
• In studies, it has been shown that irradiation doses of 1.0 to 2.0 kGy were able to eliminate contaminant microorganisms in raw fish.
• Low dose irradiation has also proved to be effective for fish preservation when applied in combination with chilling or heating processes.

H. Packaging technologies

• Packaging technologies involving either removal of air or the replacement of air by certain gases (such as CO2, O2, N2, or a combination of all). 
• Such packages are often termed modified atmosphere packaging (MAP) and vacuum packaging (VP).
• The shelf-life of fish could be extended by using MAP containing CO2 which has been shown to retard the growth of spoilage and pathogenic bacteria.
• Air is removed under vacuum and the package is sealed. The products were kept under a lower O2 atmosphere, with less than 1% inhibiting the growth of aerobic spoilage microorganisms, particularly Pseudomonas spp. and Aeromonas spp.

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