Lipid Oxidation (Rancidity)

Lipid oxidation is the free-radical chain reaction by which unsaturated marine lipids react with oxygen, producing peroxides that break down into aldehydes, ketones, and short-chain acids responsible for rancid off-flavors and toxic effects. It is accelerated by heat, light, trace metals (iron, copper), and by high levels of EPA and DHA, which are the most oxidation-prone lipids in aquafeed. Progression is tracked with peroxide value (PV, typically kept below 5-10 meq O2/kg feed in fresh products), anisidine value, and TOTOX (PV x 2 + p-anisidine, ideally under 25-30). Rancid feed depresses feed intake, damages gut epithelium and immune function, and in severe cases causes muscle myopathy and liver damage in the fish, while oxidized oils lose essential fatty acid value. Prevention combines low-temperature handling, nitrogen blanketing, metal chelators such as citric acid, and antioxidants: ethoxyquin historically, now increasingly natural tocopherol and rosemary extract blends, added at 100-400 ppm depending on oil unsaturation. Practical quality control means testing incoming fish oil (PV and anisidine at receipt), monitoring feed PV through shelf life, and storing finished feed cool, dark, and dry. A single hot warehouse summer can oxidize a premium marine feed into a growth-depressing liability.