Routine analysis of proteins by Kjeldahl and Dumas methods: review and interlaboratory study using dairy products

• Published in: Journal of AOAC International Vol. 81; no. 3; p. 620
• Main Authors: Wiles, P.G. (New Zealand Dairy Research Institute, Palmerston North, New Zealand.), Gray, I.K, Kissling, R.C
• Format: Journal Article
• Language: English
• Published: England 01.05.1998
• Subjects: ALIMENT POUR NOURRISSON; ALIMENTOS; ALIMENTOS PARA NINOS; ANALYTICAL METHODS; Animals; AZOTE; CASEIN; CASEINA; CASEINATE; CASEINATES; CASEINATOS; CASEINE; Caseins - analysis; COMPARISONS; COMPOSICION APROXIMADA; COMPOSITION GLOBALE; CONCENTRADOS DE PROTEINA; CONCENTRE DE PROTEINE; CONTENIDO DE NITROGENO; CONTENIDO PROTEICO; CONTROLE DE PERFORMANCE; CRUDE PROTEIN; Dairy Products - analysis; DIET; DIETA; DRIED MILK; DRIED SKIM MILK; ENSAYO; EVALUACION; EVALUATION; FOOD ANALYSIS; Food Analysis - methods; FOOD COMPOSITION; FOODS; Infant Food - analysis; INFANT FOODS; INFANT FORMULAS; KJELDAHL METHOD; LACTOSERUM; LACTOSUERO; LAIT; LAIT DESHYDRATE; LAIT ECREME EN POUDRE; LECHE; LECHE DESCREMADA EN POLVO; LECHE EN POLVO; MILK; Milk - chemistry; Milk Proteins - analysis; NITROGEN; NITROGEN CONTENT; NITROGENO; PERFORMANCE TESTING; PRODUIT ALIMENTAIRE; PROTEIN CONCENTRATES; PROTEIN CONTENT; PROTEINA BRUTA; PROTEINE BRUTE; PROXIMATE COMPOSITION; PRUEBAS DE RENDIMIENTO; Reference Standards; REGIME ALIMENTAIRE; TECHNIQUE ANALYTIQUE; TECNICAS ANALITICAS; TENEUR EN AZOTE; TENEUR EN PROTEINES; TEST PROCEDURE; TESTAGE; TESTING; WHEY; Whey Proteins
• DOI: 10.1093/jaoac/81.3.620
• ISSN: 1060-3271

The Kjeldahl and Dumas (combustion) methods were compared in 11 laboratories analyzing samples of milk, skim milk powder, whole milk powder, whey protein concentrate, infant formula, casein, caseinate, 2 reference compounds (glycine and EDTA), and a secondary reference skim milk powder. The comparison was conducted by using international standards where applicable. Overall means were 8.818 g N/100 g by the Kjeldahl method and 8.810 g N/100 g by the Dumas method. No evidence was found for a consistent bias between methods that may be of concern in the trading of dairy produce. A review of more than 10 related trials revealed a lack of consensus in the bias between the 2 methods, suggesting that differences in methodology and sources of systematic error may be contributors. For samples containing 2 g N/100 g, the Dumas relative repeatability and reproducibility standard deviations were consistently about 0.35 and 0.75%, respectively, whereas the corresponding Kjeldahl values declined generally with N content and were significantly larger. The Dumas precision characteristics may be due to the dominance of Leco analyzers in this trials, and in most other recent trials, rather than an inherent method attribute. Protein determination methods for dairy products need to be reviewed and updated. The Dumas method needs Codex Alimentarius status as a recognized test method