# Mechanistic Insights into Fish Spoilage and Integrated Preservation Technologies
## 2 Mechanisms of Spoilage
### Lipid Oxidation  
Lipid oxidation represents a significant factor contributing to the deterioration and spoilage of pelagic fish species, such as mackerel and herring, which are characterized by their elevated oil and fat content within their flesh tissues [19; 25]. This biochemical process has profound implications for the flavor, color, texture, and nutritional quality of stored fish, rendering it a critical concern within the fish industry [7]. Lipid oxidation in fish can occur via multiple pathways, including photo-oxidation, thermal oxidation, enzymatic oxidation, and auto-oxidation, with auto-oxidation being the predominant mechanism. First, auto-oxidation is a spontaneous, free-radical chain reaction initiated when molecular oxygen abstracts hydrogen atoms from unsaturated fatty acids, generating lipid radicals and hydroperoxides. Second, photo-oxidation occurs when ultraviolet or visible light activates endogenous sensitizers (chlorophyll, riboflavin, heme proteins), producing singlet oxygen that reacts directly with lipids to form peroxides. Third, enzymatic oxidation, catalyzed by lipoxygenases and other endogenous oxidases, selectively converts polyunsaturated fatty acids into lipid hydroperoxides under mild conditions. Finally, thermal oxidation is driven by elevated temperatures during processing or storage, which accelerate both radical initiation and propagation steps, exacerbating peroxide breakdown and formation of secondary oxidation products. Together, these pathways compromise fish quality by generating volatile off-odors, off-flavors, and nutritional losses [26]. As shown in Figure 3, the process of lipid oxidation is characterized by a three-stage free-radical mechanism: initiation, propagation, and termination [27; 28; 29; 30]. The initiation phase is instigated by the abstraction of hydrogen atoms adjacent to double bonds in fatty acids, facilitated by catalysts such as heat, metal ions (Fe\({}^{2+}\), Cu\({}^{2+}\)), and irradiation. The initiation phase involves hydrogen abstraction from methylene groups adjacent to double bonds in unsaturated fatty acids, forming lipid radicals. This leads to the generation of lipid free radicals, which subsequently react with oxygen to produce peroxyl radicals [31]. In the propagation phase, lipid radicals react rapidly with molecular oxygen (O\({}_{2}\)), forming lipid peroxyl radicals. These radicals abstract hydrogen from other lipids, creating lipid hydroperoxides and new radicals, propagating a cyclic chain reaction. The hydroperoxides are inherently unstable and decompose rapidly into secondary oxidation products, including malondialdehydedehyde, 4-hydroxynonenal, hydrocarbons, and volatile organic acids [32; 33]. These secondary products are primarily responsible for the off-odors, undesirable flavors, and textural alterations commonly associated with spoiled fish. Secondary oxidation products--especially aldehydes (e.g., malondialdehydedehyde, hexanal,4-hydroxynonenal) and ketones (e.g., 2-pentanone, 2-heptanone)--exert profound effects on fish quality. Aldehydes are highly volatile and soluble in water; therefore, even at low concentrations, they dominate off-odors (green, grassy, metallic) and off-flavors (bitter, cardboard-like), drastically reducing consumer acceptance. 4-hydroxynonenal and malondialdehydedehyde also form covalent adducts with muscle proteins, causing protein cross-linking, discoloration, and deterioration of texture and water-holding capacity. Ketones contribute sweet or fruity notes at first but quickly turn rancid, masking the fish's natural flavor. Beyond sensory defects, these reactive carbonyls degrade essential omega-3 fatty acids and fat-soluble vitamins (A, D, E), lowering nutritional value and raising potential safety concerns due to their cytotoxicity and genotoxicity when ingested over time [17; 34]. The termination phase transpires when the radicals combine to form stable, non-radical products, including dimers and polymers, ultimately slowing oxidation.