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

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 comparis...

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Bibliographic Details
Published in:	Journal of AOAC International Vol. 81; no. 3; pp. 620 - 632
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:	Gaithersburg, MD AOAC International 01-05-1998
Subjects:	ALIMENT POUR NOURRISSON ALIMENTOS ALIMENTOS PARA NINOS ANALYTICAL METHODS Animals AZOTE Biological and medical sciences 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 Food industries FOODS Fundamental and applied biological sciences. Psychology 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 and cheese industries. Ice creams 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 Performance evaluation Dry process Multicenter study Dairy product Review Calcining Wet process Milk protein Analysis method Casein Mineralization Kjeldahl method Infant formula Whey protein Comparative study Quantitative analysis Milk
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Summary:	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
Bibliography:	Q04 1997091121 ObjectType-Article-1 SourceType-Scholarly Journals-1 ObjectType-Feature-2 content type line 23
ISSN:	1060-3271 1944-7922
DOI:	10.1093/jaoac/81.3.620