In laboratory practice, there are many protein quantification methods, and all of them have their own advantages and disadvantages. The most common and widely used method for the protein analysis in food products, including fish, is the Kjeldahl method. However, the current standards for measurement methods for the determination of the protein content in fish food products do not provide for the use of devices that meet the modern level of technical development, and also do not contain metrological indicators that guarantee the reliability of the results obtained. The aim of the study was to substantiate the method for measuring the protein mass fraction in fish food products by the Kjeldahl method on an automatic analyzer and to establish metrological parameters. The assessment of the quality indicators of the Kjeldahl measuring method was carried out using a Kjeltec System 2300 Nitrogen Analyzer (Foss Analytical AB, Sweden) in the form of a characteristic of the measurement error and its components, which will provide results with the required accuracy.

The paper  examines the  problem of falsification of edible  fish products, which concerns producers and consumers worldwide. Food products are most often falsified by imparting them some of the most typical features, for example appearance, upon  the overall  worsening or losing the other most  significant properties of nutritional value, including safety. Market analysis shows that there is a tendency of replacing natural roe with imitated roe by its selling  in metal and glass jars with indication of the address of the Far Eastern producer. It has been established that the protein  content is not higher than one percent upon  imitated roe production using  modern technologies. To establish product authenticity, a methodology for measuring the protein mass fraction in grained salmonid roe by the Kjeldahl method was developed. The metrological characteristic of the developed methodology in three concentration ranges is presented. The used values  can serve for identification of claimed properties with the  specified  indicators of accuracy, trueness, repeatability and reproducibility. In addition to the methodology, product organoleptic indicators are proposed, including assessment of appearance, consistency, taste, odor, as well as specific features of the cooking test  that can serve as distinctive features and are taken into consideration in identification of grained salmonid roe.

Comparative studies of the content of total nitrogen of volatile bases (ALO) in fish raw materials, as an objective indicator of quality, were carried out by direct distillation of the sample and by distillation of acid extract. It was found that the method based on direct distillation of the sample results in 25-30% higher values than the method of acid extract distillation due to alkaline hydrolysis of the sample protein. A comparison of the reproducibility of the two methods using the Fisher statistical criterion showed the heterogeneity of the variances and the impossibility of attributing them to a single general population. For the gradation of the quality of fish raw materials, the method of distilling acid extract is recommended. The objects of three different categories are studied, grouped according to the proposed three degrees of quality, based on the maximum permissible levels of nitrogen content of volatile bases. The reliability of the results is shown and a conclusion is made about the use of the ALO content as an indicator characterizing both the initial state of fish raw materials and the dynamics of the storage process and the establishment of the moment of spoilage. A method for measuring the mass fraction of total nitrogen of volatile bases has been developed and certified (certificate of certification No. 0168/ROSS RU.0001.310430/2020).