It is not easy to measure the true protein content of fish products. There are a large number of proteins in fish tissues, and it is not possible to have a reasonably simple procedure to determine them as a group of constituents. What is normally given as the protein content of fish products – and of foods generally - is the 'crude protein' content. This is based on the fact that all proteins contain nitrogen and measurement of total nitrogen is an estimate of the amount of protein in a sample. The nitrogen content is commonly measured by the Kjehldahl method.