Bacteria is the main cause of seafood spoilage. Millions of bacteria are present in the surface slime, gills, and gut of living seafood species. When seafood species die, they produce fishy odours and flavours, and discolourations. This is caused by bacteria. They invade the flesh through the gills, along blood vessels, and directly through the skin and belly cavity lining in the flesh. If food poisoning bacteria are present, they can multiply and cause illness when the seafood is eaten.
Many different enzymes are present in living seafood species. They help build tissue, contract and relax muscles, and digest food. When seafood species die, enzymes continue to work and start to breakdown the flesh. This causes the flesh to soften and lowering the quality. Enzymes also produce more food for bacteria to feed on, increasing rate of spoilage.
Oxygen in the air can attack unsaturated oils or, seafood causing rancidity, off-odors and off-flavors. This is especially important in fatty fish such as salmon and mackerel.
High temperatures speed spoilage and low temperatures slow spoilage. When temperature is increased from 32ºF to 40ºF, it will double the rate of spoilage and cuts the shelf life in half.
Sanitation is also important. Contamination of seafood by bacteria from dirty ice, containers and surfaces can increase the number of bacteria on seafood and speed spoilage. Contamination with food poisoning bacteria can cause illness when the seafood is eaten. Keeping seafood handling and storage equipment clean reduces bacterial contamination and slows spoilage.