[目的] 评定凯氏定氮法测定巴氏杀菌乳中蛋白质含量的不确定度。[方法] 依据《GB 5009.5—2016 食品安全国家标准 食品中蛋白质的测定》凯氏定氮法测定巴氏杀菌乳中蛋白质含量。建立数学模型,识别试验过程中各不确定度来源,并对其进行量化,合成标准不确定度,计算扩展不确定度。[结果] 巴氏杀菌乳中蛋白质含量3.40 g/100 g时,其结果的扩展不确定度0.02 g/100 g（k=2）。[结论] 方法中测量不确定度的主要来源有3 个方面：测量重复性、滴定体积、盐酸标准溶液浓度,为蛋白质含量检测的质量控制提供科学依据。

[Objective] To evaluate the uncertainty of protein content in pasteurized milk by Kjeldahl method. [Method] According to GB 5009.5—2016 National Food Safety Standard-Determination of Protein in Food,the Kjeldahl method was used to determine the protein content in pasteurized milk.Through the establishment of a mathematical model,the sources of various uncertainties in the experimental process were identified and quantified,and the relative standard uncertainty and expanded uncertainty of protein content in pasteurized milk were calculated. [Result] When the content of protein in pasteurized milk was 3.40 g/100 g,the expanded uncertainty of the measurement results was 0.02 g/100 g（k=2）. [Conclusion] The main sources of measurement uncertainty in this analysis method included the measurement repeatability,titration volume,and concentration of hydrochloric acid standard titration solution,providing scientific basis for quality control of protein content determination.