On the problem of adding urea to fish meal

At the end of this post, zangon edited the national standard GB/T19164-2003 for fishmeal at 14:10 from 2009-11-25. Why is the content of urea no more than 0.7%? The detection method is that under the condition of ethanol and acid, urea reacts with p-dimethylaminobenzaldehyde (DMAB) to form a yellow substance, which has the maximum absorption at 420nm wavelength, and the absorbance has a linear relationship with the concentration of urea. The content of urea in the sample is calculated by checking the standard curve. Why is it not required by national standards that it cannot be detected? Is there any other substance in fishmeal that may be mistaken for urea by this method? Very confused, please experts to solve the problem! According to Li Xiaochuan and Wang Lianzhu, drafters of GB/T19164-2003 "Fish meal" standard, urea exists in trace amounts in fish and shellfish tissues, and together with trimethylamine oxide play a role in regulating osmotic pressure. Therefore, trace urea can also be detected in normal fishmeal. In order to better supervise the behavior of adding urea in fishmeal, protect high-quality fishmeal and crack down on inferior fishmeal, we have stipulated the limit of urea in fishmeal in this revision. Through our test data of a large number of fishmeal samples, it is concluded that the urea content of high-grade products is ≤ 0.3%, and the urea content of primary, secondary and tertiary products is ≤ 0.7%. In addition, according to the protein metabolism in animal biochemical theory, there is a urea cycle in fish, that is, arginase arginine + H 2O-ornithine + urea. It can be seen that the source of urea in fish is the metabolism of amino acids. As can be seen from the above, because there is a small amount of urea in the tissues of fish, GB/T 19164-2003 "fishmeal" sets a limit on urea. -- work out your nutritional indicators so that it is convenient for everyone to comment. -- Yes, teachers, please impart your knowledge to us. Thank you very much.