Proteins and other nitrogenous compounds

The nitrogenous compounds found in seafood muscle mainly comprise proteins (80 to 90 %) and non-protein nitrogenous substances.

Proteins are macromolecules of high molecular mass, mainly composed of carbon, hydrogen, oxygen and nitrogen, albeit they may also contain sulphur, phosphorous and, even in minor portions, iron, copper and magnesium. The percentage of protein is relatively stable when compared with water and lipids. Generally, seafood present a protein content between 9 and 21 % and the proteins in fish muscle tissue can be divided into the following three groups: structural myofibrillar proteins (actin, myosin, tropomyosin and actomyosin) making up 70-80 % of all protein fraction; sarcoplasmic proteins (myoglobin, globulin and enzymes) representing 25 to 30 % of the total proteins; connective tissue proteins (collagen) which constitute approximately 3 % of the protein in bony fish and about 10 % in cartilaginous fish.

The non-protein nitrogenous substances encompass low molecular weight compounds soluble in water, among which it can be outlined the volatile bases (ammonia, methylamine, dimethylamine and trimethylamine), trimethylamine oxide, creatine, free amino acids, nucleotides, purine bases and urea in the case of cartilaginous fish. This fraction makes up 9 to 18 % of the total nitrogen in bony fish. All these compounds have an influence on the sensory characteristics and play an important role in the seafood spoilage.