Wiles P G, Gray I K, Kissling R C
New Zealand Dairy Research Institute, Palmerston North, New Zealand.
J AOAC Int. 1998 May-Jun;81(3):620-32.
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.
在11个实验室中，对凯氏定氮法和杜马斯（燃烧）法进行了比较，这些实验室分析了牛奶、脱脂奶粉、全脂奶粉、乳清蛋白浓缩物、婴儿配方奶粉、酪蛋白、酪蛋白酸盐、2种参考化合物（甘氨酸和乙二胺四乙酸）以及一种二级参考脱脂奶粉的样本。在适用的情况下，采用国际标准进行了比较。凯氏定氮法的总体均值为8.818 g N/100 g，杜马斯法的总体均值为8.810 g N/100 g。未发现两种方法之间存在可能在乳制品贸易中引起关注的一致偏差。对10多项相关试验的回顾表明，两种方法之间的偏差缺乏共识，这表明方法学差异和系统误差来源可能是造成这种情况的原因。对于氮含量>2 g N/100 g的样本，杜马斯法的相对重复性和再现性标准偏差分别始终约为0.35%和0.75%，而凯氏定氮法的相应值通常随氮含量下降且明显更大。杜马斯法的精密度特性可能是由于在本次试验以及大多数其他近期试验中Leco分析仪占主导地位，而非该方法的固有属性。乳制品的蛋白质测定方法需要进行审查和更新。杜马斯法需要获得食品法典委员会认可的检测方法地位。
