Measures for consumer protection against food adulteration and misleading labeling are integrated into EU legislation, including methods for detection of misleading practices. Verification of meat content is available for marine products but not for freshwater fish because of the lack of standard nitrogen factors. The aim of this study was to establish nitrogen factors for European pike-perch Sander lucioperca (Linnaeus, 1758), northern pike Esox lucius Linnaeus, 1758, and sheatfish Silurus glanis Linnaeus, 1758. The study involved analysis of 808 fillet samples obtained in spring (March–April) and autumn (October–November) harvest seasons, 2018–2019, from seven Czech Republic fish rearing facilities. Samples with and without skin were analyzed for nitrogen content, dry matter, protein, ash, and fat according to established ISO methods. The recommended nitrogen factor for European pike-perch with the skin is 3.28 ± 0.09 and without the skin is 3.21 ± 0.09; for northern pike with the skin is 3.18 ± 0.09 and without skin is 3.15 ± 0.09; and for sheatfish with skin is 2.73 ± 0.13 and without skin is 2.75 ± 0.12. The established nitrogen factors will enable analysis of meat content to ensure that consumers are purchasing correctly described and labeled fish products.

Consumer protection against food adulteration and misleading labelling is integrated into EU legislation, but accurate analysis of the meat content of farmed freshwater fish products is not possible because of the lack of established nitrogen factors for farmed common carp. The aim of this study was to determine nitrogen factors for farmed common carp Cyprinus carpio. Seven-hundred samples collected in 2018–2019 in three harvest seasons (March/April, Jun/July, and October/November) at seven locations in the Czech Republic were analysed for nitrogen, dry matter, protein, ash, and fat content according to standard ISO methods. The recommended nitrogen factor for fat-free common carp fillet with skin is 3.04 ± 0.13 and, for fillet without skin, 2.95 ± 0.12. Availability of nitrogen factors for common carp can help ensure that consumers are purchasing correctly labelled products.

Measures for consumer protection against food adulteration and misleading labelling are integrated into EU legislation, including methods for detecting misleading practices. Verification of the meat content is available for marine products, but not for salmonid fish due to the lack of standard nitrogen factors. This study aimed to establish nitrogen factors for rainbow trout (Oncorhynchus mykiss) and brook trout (Salvelinus fontinalis). The study analysed 340 fish from Czech fisheries obtained in the summer of 2018–2020. According to the established ISO methods, fillet samples with and without skin were analysed for their nitrogen content (protein), dry matter, ash, and fat. The recommended nitrogen factor for rainbow trout fillets with and without the skin is 3.07 ± 0.12 and 3.06 ± 0.14, respectively, and the nitrogen factor for fat-free rainbow trout fillets with and without the skin is 3.33 ± 0.15 and 3.29 ± 0.15, respectively. The recommended nitrogen factor for brook trout fillets with and without the skin is 3.16 ± 0.10 and 3.12 ± 0.09, respectively, and the nitrogen factor for fat-free brook trout fillets with and without the skin is 3.42 ± 0.13 and 3.36 ± 0.12, respectively. The established nitrogen factors will enable the analysis of the meat content to ensure that consumers purchase correctly described and labelled fish products.