The amount of protein in a sample of fish muscle that can be brought into solution under specified conditions of solvent and procedure. It is used especially as an index of damage to fish muscle structure and proteins incurred by frozen storage. Its value decreases with frozen storage at rates that depend on storage temperature; the lower the temperature, the slower the rate. Some 20-25% of the proteins in fish muscle can be solubilised by simple extraction of minced fish muscle with water. Proteolysis The breaking down of proteins to their consitituent amino acids by enzymes. Proteins can be broken down to amino acids by the action of acids and alkalis, but this process is better referred to as hydrolysis. Organs in the gut of fish, for example, the pyloric caeca, excrete proteolytic enzymes, (proteases), which assist in the digestion of food, and muscle tissue contains proteases called cathepsins.