As a source of nutrients, fish contain protein, lipids, minerals, and vitamins. The proteins, as a whole, supply the essential amino acids in proportions to give a high biological value to the protein and to enable fish protein to be used as a supplement to protein sources of lower biological value. Discovery of the toxicity and carcinogenicity of N-nitrosodimethylamine caused concern regarding the possibility that nitrates and nitrites, used in curing salt, could react with amines present to form nitroso compounds. In one investigation 18 samples of smoked and five of canned fish were analyzed after cooking with and without sodium nitrite. Frozen storage of fishery products prevents microbial spoilage and retards most chemical changes, although some reactions may be accelerated. Denaturation of proteins, oxidation and hydrolysis of lipids, protein-lipid interactions, and protein-carbohydrate interactions can all take place during frozen storage.