Usually the quantity of bacteria in the body of a fish is small but they are often introduced through handling, cleaning, washing, etc. Most of these enzymes and bacteria are destroyed or rendered inactive in a concentrated solution of salt. As salt is being absorbed into the fish, the bacteria and the enzymes are still active, and their growth and activity continues until a certain level of salt concentration is reached. At that level, bacterial growth and activity are arrested and spoilage prevented. If the uptake of salt is slow, the fish can become spoiled before this level is reached.
Several factors affect the rate of salt absorption into the fish.
If a solution of pure sodium chloride is used, fish do not acquire the bitter taste commonly associated with salted fish, but they become soggy and yellow. These impurities give the fish a firmer white flesh that has proven to be more marketable. They also give the fish a sharp bitter taste characteristically associated with salted fish. If these impurities are present in sufficient quantities, the fish may spoil because the absorption of salt is inhibited. The thickness of the flesh of the fish can also slow the penetration of salt.