Journal of the Association of Public Analysts (Online) 2011 39 44-78 P Colwell et al -44- Nitrogen Factors for Atlantic Salmon, Salmo salar, farmed in Scotland and in Norway and for the derived ingredient, “Salmon Frame Mince”, in Fish Products. Peter Colwell (1)(2) , Stephen L. R. Ellison (2) , Michael J. Walker (2) , Selvarani Elahi (2) , Duncan Thorburn Burns (3) and Kirstin Gray (2) . 1 To whom correspondence should be addressed, e-mail address: pete.colwell@lgcgroup.com 2 The Laboratory of the Government Chemist, LGC, Queens Road, Teddington, Middlesex, TW11 0LY, United Kingdom. 3 School of Chemistry and Chemical Engineering, The Queen’s University of Belfast, Belfast, BT9 5AG, Northern Ireland, UK Summary Pre-packed named fish products are required to be labelled with a declaration of the amount of named fish present as a percentage of the final weight of the product, (quantitative ingredient declaration, QUID). The basis of independent analytical assessment of QUID claims for fish is comparison of the fish-specific nitrogen content of a particular food sample with the known species nitrogen content, the “nitrogen factor”. The only extant species nitrogen content datum for Atlantic salmon, Salmo salar, is that reported in 1973 and it has been claimed that this nitrogen factor is not representative of modern farmed Atlantic salmon. In addition, there is no published nitrogen factor for the commercial ingredient derived by machine recovery in minced form of the residual salmon flesh remaining on the salmon “frames" after removal of the fillets. Pursuant to his statutory duties, the Government Chemist instigated at the request of both trade and enforcement interests, a review of the appropriate nitrogen factors for farmed Atlantic salmon and salmon frame mince. We present data confirming that the 1973 nitrogen factor is not appropriate for farmed Atlantic salmon and suggest that the estimation of salmon content should be based on nitrogen factors expressed on a fat-free basis. Dumas nitrogen factors on a fat-free basis for various cuts of Norwegian and Scottish farmed salmon are given (Table 7) and a general Dumas factor, on a fat-free basis of 3.80 (3.75 Kjeldahl) is suggested. For salmon frame mince a general factor, on a fat-free basis, of 2.85 (2.81 Kjeldahl) is suggested. Introduction UK and European food law underpins the concept of informed consumer choice in the purchase of food 1,2,3 . The European Food Labelling Directive 4 includes the requirement for a Quantitative Ingredients Declaration (QUID) which means that most pre-packed fish products are required to be labelled with a declaration of the amount of fish present as the percentage of the final weight of the product. If a specific named fish is in the name of the food, then the percentage of the named fish in the product has to be declared. The basis of independent analytical assessment of QUID claims for fish (and meat) is comparison of the fish-specific-nitrogen content of a particular food with the known species nitrogen content. For readers unfamiliar with the calculation an outline is shown in Appendix 1; standard works give a fuller treatment 5,6 . Typically, the food nitrogen is determined using a “standard method” whose performance characteristics are well documented. However, difficulties can arise in the absence of an appropriately validated database against which interpretation of the sample data can be made. When interpretations have to be able to withstand challenges in a court of law, it is essential that the reference databases have proven validity. The difficulties of enforcement officers who are charged with verifying product label declarations have recently been discussed by Hargin 7 . The Kjeldahl process was introduced in 1883 8 for the determination of nitrogen in food and since then has been used as a marker to measure the protein content of food samples. The method is well characterised 9,10,11 and