Fish spoilage due to microbial growth

Raw fish is an infamous source of potential pathogenic parasites and bacteria due to the ability of bacteria to grow and thrive without destruction through the cooking process.

Bacteria establish themselves on the outer and inner surfaces of the live fish (gills, skin, gastro intestinal tract). The poikilotherm nature of fish allows bacteria with a broad temperature range to grow. When a fish dies, the bacteria already present in the fish attack the flesh and result in the formation of undesirable products.

Two of the most common types of microbes that can contaminate raw fish include Salmonella and Vibrio vulnificus.

Fish, mollusks, and crustaceans can acquire pathogens from various sources. All seafood can be susceptible to surface or tissue contamination originating from the marine environment.

Contamination in fish can be started from when it kept in ice due to low quality of ice or storage condition. At the time of transporting fish from fishing place to selling market, contamination may also occur if the temperature is not maintained.

The bacteria cause fish spoilage by the following means:
*Reducing of trimethylamine oxide into trimethylamine produces an offensive odor.
*Degradation of amino acid to primary amines: It can cause food poisoning.

Fish products with high salt contents may spoil due to growth of halophilic bacteria (salted fish) or growth of anaerobic bacteria and yeasts (barrel salted fish).
Fish spoilage due to microbial growth

Notes
A gill is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide.

An animal that regulates its internal body temperature throughout a wide range of temperatures, frequently in response to changes in the ambient temperature is known as poikilotherms.

Fish spoilage: Enzymatic autolysis

Fish spoilage results from three basic mechanisms:
*Enzymatic autolysis (reactions caused by the activities of the fish’s own enzymes)
*Oxidation (oxidation of unsaturated lipids)
*Microbial growth (metabolic activities of microorganisms)

Low temperature storage and chemical techniques for controlling water activity, enzymatic, oxidative, and microbial spoilage are the most common in the industry today.

Many different enzymes are present in living seafood species. They help build tissue, contract, and relax muscles, and digest food. Enzymatic spoilage is caused by the autolytic fish enzymes. Fishes are highly perishable than meat because of more rapid autolysis by fish enzymes, and favorable conditions for microbial growth due to less acid reactions.

Autolysis plays an important role in spoilage of fish. The enzymes involved in autolysis are many. However, phosphorylases, lipases, cathepsins and gut enzymes predominate.

The Rigor mortis is a physical effect on the muscle tissue of fish caused by chemical changes following the death. After the completion of rigor mortis, muscle stiffness gradually decreases accompanied by increase in pH, ending up in softening of muscle. This is followed by breakdown of proteins by enzymes. This process is called as autolysis.

Autolytic enzymes reduced textural quality during early stages of deterioration but did not produce the characteristic spoilage off-odors and off-flavors. Autolytic degradation can limit shelf-life and product quality even with relatively low levels of spoilage organisms. Enzymatic action also causes decomposition in the fish known as belly bursting. The belly bursting is caused by the action of digestive enzymes present in the gut of the fish.

In case of improper storage of whole fish, proteolysis is responsible for degradation of proteins and is followed by a process of solubilization. On the other hand, peptides and free amino acids can be produced as a result of autolysis of fish muscle proteins, which lead towards the spoilage of fish meat as an outcome of microbial growth and production of biogenic amines.
Fish spoilage: Enzymatic autolysis

Oxidative spoilage of fish

Fresh fish spoilage can be very rapid after it is caught. The spoilage process (Rigor mortis) will start within 12 h of their catch in the high ambient temperatures of the tropics.

Rigor mortis, the stiffening of the body, sets in and passes quickly in very active fish but slowly in inert fish. This period depends on the glycogen reserve of the fish.

Fish quality can be affected during storage at different temperatures by lipid oxidation through odors and lipid peroxide formation or by taste, texture, consistency, and nutritional value losses.

Lipid oxidation is a major cause of deterioration and spoilage for the pelagic fish species such as mackerel and herring with high oil/fat content stored fat in their flesh.

During the advanced stages of lipid oxidation, the breakdown of hydroperoxides generates low molecular-weight carbonyl and alcohol compounds that could lead to the changes in food quality, which affect the color, texture, flavor, and odor characteristics.

The oxidation of lipids involves a step-by-step mechanism of which free radicals are a part of. These stages are, by order; the initiation, propagation and termination steps.

Initiation involves the formation of lipid free radicals through catalysts such as heat, metal ions and irradiation. These free radicals which react with oxygen to form peroxyl radicals. This radical immediately reacts with lipids or other molecules at the site where it is generated.

During propagation, the peroxyl radicals reacting with other lipid molecules to form hydroperoxides and a new free radical. The hydroperoxides produced in relatively large amounts during propagation are tasteless, and it is therefore perhaps not surprising that the widely used "peroxide value" usually correlates rather poorly to sensorial properties.

Termination occurs when a buildup of these free radicals interacts to form nonradical products.

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.

In fish, lipid oxidation can occur enzymatically or non-enzymatically. The enzymatic hydrolysis of fats by lipases is termed lipolysis (fat deterioration). Lipid hydrolysis can occur with the action of enzymes. The majority of lipolysis in most stored fish originates from endogenous enzymes and microorganisms, mainly phospholipase and triacyl lipase.

Main lipolytic enzymes include triacyl lipase, phospholipase A2, and phospholipase B, and they can either be endogenous or derived from psychrotrophic microorganisms.

Non-enzymatic oxidation is caused by hematin compounds (hemoglobin, myoglobin and cytochrome) catalysis producing hydroperoxides. The peroxides are unstable and susceptible to hydrolysis, forming volatile compounds (like aldehydes, ketones, and alcohols), which causes off-flavors.

Other factors that determine lipid deterioration, including oxygen, pH, light, temperature, and water activity, are also important in oxidation.
Oxidative spoilage of fish

Fish spoilage

Fish is highly perishable due to high moisture content, availability of the nutrients for the growth of microorganisms and ambient temperature. Fish spoilage results from three basic mechanisms:
*Enzymatic autolysis (reactions caused by the activities of the fish’s own enzymes)
*Oxidation (oxidation of unsaturated lipids)
*Microbial growth (metabolic activities of microorganisms)

One-fourth of the world's fish supply and 30% of landed fish are lost through microbial activity alone.

Fresh fish spoilage can be very rapid after it is caught. The spoilage process (Rigor mortis) will start within 12 h of their catch in the high ambient temperatures of the tropics.

Spoilage of fish can be considered as any change that render the product unacceptable for human consumption. When fish dies, the biochemical changes responsible for anabolism stop. As an inevitable consequence of this process of cessation of anabolism, catabolism starts bringing spoilage to fish flesh. The digestive enzymes still being active, instead of acting on the food present inside the gut, begin to digest tissue components such as lipids, carbohydrates and proteins. This process is called “autolysis”.

One important action determining the onset of spoilage in freshly caught fish is rigor mortis, the stiffening of the body. Usually, this develops within 1-7 hours after death. Rigor mortis sets in and passes quickly in very active fish but slowly in inert fish.

Characteristics of spoiled fish
*The flesh is soft to touch. When the flesh is pressed it leaves a permanent indentation.
*The skin looks dull and has slime on the surface.
*The gills look brownish.
*It gives a bad smell.

With the ever-growing world population and the need to store and transport the food from one place to another where it is needed, food preservation becomes necessary in order to increase its shelf life and maintain its nutritional value, texture and flavor.
Fish spoilage