Thursday, June 17, 2021

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

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