4种蛋白质沉淀剂测定乳粉中非蛋白氮含量的效果

Determination of non-protein nitrogen in milk powder by four protein precipitants

目的测定乳粉中非蛋白氮含量,比较4种蛋白质沉淀剂分离蛋白质和非蛋白含氮物质的效果。方法样品加水溶解后分别用15%三氯乙酸溶液、丙酮、20%乙酸铅溶液+3%草酸钾+7%磷酸氢二钠溶液、乙酸锌溶液和亚铁氰化钾溶液沉淀蛋白质,过滤后取滤液进行消化定氮,同时在样品中加标三聚氰胺,检测计算回收率。结果三氯乙酸沉淀法的非蛋白氮含量为0.2036%±0.0002%,丙酮沉淀法的非蛋白氮含量为0.0981%±0.0050%,乙酸铅沉淀法的非蛋白氮含量为0.1372%±0.0012%,乙酸锌沉淀法的非蛋白氮含量为0.3466%±0.0100%。当10 g样品中加标量高于2 mg低于20 mg三聚氰胺时,三氯乙酸沉淀法的加标回收率最好。结论三氯乙酸沉淀法结果准确度和稳定性较其他3种方法更好,但受到加标量的影响,当10g样品加标79.2 mg时,三氯乙酸沉淀法加标回收率低且相对标准偏差较大,此时乙酸铅沉淀法的加标回收率优于三氯乙酸沉淀法。

Objective To determine the content of non-protein nitrogen in milk powder and compare the effects of four protein precipitants in separating four protein and non-protein nitrogen. Methods After the sample was dissolved in water, the protein was precipitated with 15% trichloroacetic acid solution, acetone, 20% lead acetate solution + 3% potassium oxalate+7% disodium hydrogen phosphate solution, zinc acetate solution and potassium ferrocyanide solution, respectively. After filtration, filtrate was taken for digestion and determination of nitrogen. Meanwhile, standard melamine was added to the sample to detect and calculate the recovery rate. Results The non-protein nitrogen content separately gained by trichloroacetic acid precipitation method and acetone precipitation method were 0.2036%±0.0002% and 0.0981%±0.0050%. While dealing with lead acetate precipitation method and zinc acetate precipitation method, the figures were 0.1372%±0.0012% and 0.3466%±0.0100%. When the adding amount of melamine was more than 2 mg and less than 20 mg in 10 g sample, trichloroacetic acid precipitation method showed the best recovery rate. Conclusion The accuracy and stability of the trichloroacetic acid precipitation method was better than other three methods, while it would be affected by the spiked levels. When 79.2 mg melamine was added to 10 g sample, trichloroacetic acid precipitation method had a low recovery rate and large relative standard deviation. At this time, the recovery rate of lead acetate precipitation method was better than that of trichloroacetic acid precipitation method.