Lynch J M, Barbano D M, Fleming J R
Cornell University, Department of Food Science, Northeast Dairy Foods Research Center, Ithaca, NY 14853, USA.
J AOAC Int. 1998 Jul-Aug;81(4):763-74.
The classic method for determination of milk casein is based on precipitation of casein at pH 4.6. Precipitated milk casein is removed by filtration and the nitrogen content of either the precipitate (direct casein method) or filtrate (noncasein nitrogen; NCN) is determined by Kjeldahl analysis. For the indirect casein method, milk total nitrogen (TN; Method 991.20) is also determined and casein is calculated as TN minus NCN. Ten laboratories tested 9 pairs of blind duplicate raw milk materials with a casein range of 2.42-3.05% by both the direct and indirect casein methods. Statistical performance expressed in protein equivalents (nitrogen x 6.38) with invalid and outlier data removed was as follows: NCN method (wt%), mean = 0.762, sr = 0.010, sR = 0.016, repeatability relative standard deviation (RSDr) = 1.287%, reproducibility relative standard deviation (RSDr) = 2.146%; indirect casein method (wt%), mean = 2.585, repeatability = 0.015, reproducibility = 0.022, RSDr = 0.560%, RSDR = 0.841; direct casein method (wt%), mean = 2.575, sr = 0.015, sR = 0.025, RSDr = 0.597%, RSDR = 0.988%. Method performance was acceptable and comparable to similar Kjeldahl methods for determining nitrogen content of milk (Methods 991.20, 991.21, 991.22, 991.23). The direct casein, indirect casein, and noncasein nitrogen methods have been adopted by AOAC INTERNATIONAL.
测定牛奶中酪蛋白的经典方法是基于在pH 4.6条件下使酪蛋白沉淀。通过过滤除去沉淀的牛奶酪蛋白，然后采用凯氏定氮法测定沉淀物（直接酪蛋白法）或滤液（非酪蛋白氮；NCN）中的氮含量。对于间接酪蛋白法，还需测定牛奶总氮（TN；方法991.20），酪蛋白含量通过TN减去NCN来计算。十个实验室采用直接和间接酪蛋白法对9对酪蛋白含量在2.42 - 3.05%范围内的盲样重复生乳原料进行了检测。去除无效和异常数据后，以蛋白质当量（氮×6.38）表示的统计性能如下：NCN法（重量%），均值 = 0.762，sr = 0.010，sR = 0.016，重复性相对标准偏差（RSDr） = 1.287%，再现性相对标准偏差（RSDR） = 2.146%；间接酪蛋白法（重量%），均值 = 2.585，重复性 = 0.015，再现性 = 0.022，RSDr = 0.560%，RSDR = 0.841；直接酪蛋白法（重量%），均值 = 2.575，sr = 0.015，sR = 0.025，RSDr = 0.597%，RSDR = 0.988%。方法性能良好，与用于测定牛奶氮含量的类似凯氏定氮法（方法991.20、991.21、991.22、991.23）相当。直接酪蛋白法、间接酪蛋白法和非酪蛋白氮法已被美国官方分析化学家协会（AOAC INTERNATIONAL）采用。
