Selenium-enriched yeast possesses the unique ability of transforming chemical selenium, such as sodium selenite, into a biologically active form, which mitigates its toxic effects on the human body. The transformation product of this process, selenomethionine, can be safely and effectively absorbed and utilized by the human body; hence, it has been spiked into a selenium-enriched supplement. This study employs two distinct measurement strategies to determine the selenomethionine content in two candidate reference materials, a selenium-enriched yeast powder and supplement, using both organic and inorganic mass spectrometry. The concentrations of selenomethionine in the selenium-enriched yeast were determined using HPLC-ICP-MS and HPLC- ESI-MS/MS, with mass fractions measured at 718 mg SeMet kg⁻¹ and 715 mg SeMet kg⁻¹, respectively. Notably, both methods yielded consistent results for the selenium supplement, with a selenomethionine mass fraction of 59 mg SeMet kg⁻¹. Ultimately, the certified values of these candidate reference materials were determined as 716 mg kg⁻¹ and 59 mg SeMet kg⁻¹ with expanded uncertainties of 36 mg SeMet kg⁻¹ (k = 2) and 5 mg SeMet kg⁻¹ (k = 2), respectively. The development of these candidate reference materials serves as a valuable reference for diverse methods aiming to determine the value of organic selenium speciation in complex food substrates.

A necessary condition for obtaining high-quality food products that satisfy human physiological needs for essential nutrients and energy is continuous monitoring of the macronutrient content. The correctness of measurements in the process of such monitoring is verified by analyzing, in parallel with working samples reference materials of the composition of food products and food raw materials. The certified values of these reference materials shall be traceable to measurement standards and/or measurement results of national metrology institutes. For mutual recognition of the calibration and measurement capabilities of national metrology institutes, they must obtain equivalent results in key comparisons. The possibility of organizing key comparisons is assessed during pilot comparisons. The article presents the results of pilot comparisons in the field of measuring the nutritional value of milk powder COOMET 880/RU-а/23 with the participation of national metrology institutes of the Russian Federation, Republic of Belarus, Kyrgyz Republic and China. Samples of skim and whole milk powder were selected as the objects of comparison. The measured characteristics are mass fraction of moisture, nitrogen, protein, fat, ash, carbohydrates (lactose). Each participant in the comparison used measurement procedures, measurement instruments and equipment that ensure the highest accuracy. When assessing the uncertainty of the values of the specified measured characteristics, the participants took into account the standard deviation of the results of parallel determinations, uncertainties from the quantities included in the measurement equations, and methodological factors. The comparisons demonstrated the consistency of most of the measurement results presented by the participants. Some deviating measurement results of operationally determined quantities (mass fraction of ash, mass fraction of moisture) are due to the empirical nature of the methods used, i.e. the dependence of the measurement results on the applied drying or ashing conditions. For rational values (mass fraction of nitrogen), the discrepancies are probably due to incomplete extraction of the component from the test sample, as well as the lack of established traceability of the sample used to standardize the titrant. The results of the comparisons will be used in preparing proposals for conducting key comparisons for the purpose of mutual recognition of national standards and calibration and measurement certificates in the field of measuring the nutritional value of milk powder.

To ensure the reliability of measurements of the content of proteins, fats, moisture and ash in food products and food raw materials, it is necessary to use reference materials similar in composition to the analyzed samples and traceable to measurement standards and / or measurement results of national metrology institutes. The equivalence of measurement results from national metrological institutes should be confirmed in key comparisons. The results of the COOMET 881/RU-a/23 pilot comparisons in the field of measurements of the nutritional value of soy flour conducted in 2024 are presented as a preparatory stage before organizing key comparisons in this field. The national metrology institutes of Russia, the Kyrgyz Republic and China took part in the comparisons. Samples of defatted and semi-defatted soy flour composition with preliminarily studied homogeneity and stability were used as samples for comparisons. The measured characteristics were mass fractions of moisture, nitrogen, protein, crude fat and ash. When carrying out measurements, the participants used measurement methods implemented on standard measurement equipment, as well as primary reference measurement procedures. When assessing the uncertainty, the participants took into account the uncertainty components caused by the following: discrepancy between the results of parallel determinations; rounding of measurement results; direct measurements of quantities included in measurement equations; fulfillment of conditions for achieving constant mass after drying, extraction and ashing; deviation of measurement conditions from optimal values. In comparisons, consistent measurement results for mass fraction of moisture, nitrogen, protein and crude fat with expanded uncertainties (at the coverage rate 2) 0.06–0.12 %, 0.034–0.060 %, 0.19–0.40 %, 0.04–0.08 % respectively were obtained. It is two to eight times higher than the accuracy of standard measurement procedures (methods). The observed discrepant measurement result of the ash mass fraction obtained by one participant in the comparisons is due to the empirical nature of the measured value. In the case of measuring the values of empirical quantities, consistency of results can be achieved by strictly establishing the conditions for performing measurements and the permissible limits of their variation. The comparison results are planned to be used in the preparation of proposals for conducting key comparisons in the field of measuring the nutritional value of soy flour.