Linda Owen

A comparison of the Kjeldahl and Dumas methods for the determination of protein in foods, using data from a proficiency testing scheme.

Both the Kjeldahl and the Dumas methods for the determination of protein in foodstuffs are currently in use, but the empirical nitrogen factors used to convert the determined nitrogen content to protein content are based on the Kjeldahl method alone. Non-equivalence between the two methods could therefore result in some laboratories reporting an incorrect protein content. We report here a study using data accumulated over several years in the results of a proficiency testing scheme. On average the Dumas method provided results that were relatively higher by about 1.4% than the Kjeldahl method, but the difference between the methods depended on the type of foodstuff. The methodology of looking for bias between analytical methods is critically discussed.

A review of analytical methods for lead, cadmium, mercury, arsenic and tin determination used in proficiency testing

The methods used by laboratories participating in FAPAS® proficiency testing covering trace metals have been reviewed. Techniques that tend to give the highest percentage of satisfactory z-scores are identified. Comparisons are made of digestion and extraction techniques, use of atomic absorption spectrophotometry (AA: graphite furnace and flame) and inductively coupled plasma techniques (ICP-OES and ICP-MS). It does not follow that one technique is more accurate than the others, because some individual laboratories consistently achieve satisfactory results using any of the five main digestion techniques and any of the major detection techniques. The results do, however, give an indication of which techniques are easier to perform correctly and with a high standard of accuracy. There are also other factors that may influence results such as instrument set-up, background correction and data analysis.

Use of the stable isotope 106Cd for studying dietary cadmium absorption in humans

Hydroponically grown wheat was intrinsically labelled with the stable isotope 106cadmium (106Cd) and the flour made into a porridge. The abundance of the isotope in the porridge was approximately 30 times the natural abundance, but the total level of Cd in the porridge was 0.03 mg/kg fresh weight, which was the same as expected in a normal diet. Cadmium measurements were made using inductively coupled plasma-mass spectrometry (ICP-MS). The porridge was eaten at breakfast by adult and infant volunteers. Bulked faecal collections were analysed for unabsorbed Cd. Initial results suggest that the apparent absorption of Cd may be higher than 5% as commonly quoted, but longer faecal collection times may be necessary to confirm this.

Testing for bias between the Kjeldahl and Dumas methods for the determination of nitrogen in meat mixtures, by using data from a designed interlaboratory experiment

Bias between the Dumas and the Kjeldahl methods for the determination of protein nitrogen in food was studied by conducting an interlaboratory study involving 40 laboratories and 20 different test materials. Biases were found to be small and statistically significant only for the chicken test materials, where a bias of 0.020±0.004% m/m was detected.