Fish Oil and Oxidation

When I was younger we were dosed daily with cod liver oil.
It was a horrible but bearable experience.

Fish oils started getting plenty of positive attention about a decade ago. So I thought I’d jump back on that bandwagon and bought some Cod Liver Oil.
I couldn’t get a teaspoon full down – I must have softened with age!

Big capsules aren’t for me after getting a boiled sweet stuck in my throat at a young age.
Not to be discouraged I bought some high-quality fish oil as a liquid which was fine.
But boy, even in the fridge did it go rancid quickly.

Which got me wondering why you can ‘hang meat’ to improve it but fish goes off very quickly?

So here is why:
1) The bacteria are different. Land animals live in warm environments, so their bacteria struggle to grow when the meat is chilled. Fish live in cold water, so their bacteria thrive even at fridge temperatures.

2) Fish muscle enzymes are more active. Fish have enzymes that keep working at low temperatures, breaking down the flesh rapidly even on ice.

3) The fat and water balance. Fish flesh is softer, holds more water (good for bacteria), and has unsaturated fats that go rancid faster. Red meat is denser and drier, so it resists spoilage.

4) The structure and acidity. Fish muscle fibres are short and delicate, while red meat has tougher fibres and becomes slightly acidic after death — both factors that slow bacteria.

5) Hanging meat is controlled aging. We hang beef or game in cool, dry conditions to let enzymes within the muscle tenderize it and create flavour. Fish muscle breaks down too quickly to benefit from this process.

Why does fish smell bad when decaying?
When fats and oils are exposed to oxygen, light, or heat, they start to oxidise — the chemical bonds in the fat molecules break down. That produces small, volatile compounds (like aldehydes, ketones, and acids), which give off that sharp, sour, or “off” smell and taste we call rancid.

Fish goes rancid fast because its fats are highly unsaturated (lots of double bonds), which makes them very reactive with oxygen. Saturated fats (like those in beef or butter) are more stable, so they resist rancidity much longer.