4种蛋白质沉淀剂测定乳粉中非蛋白氮含量的效果

目的 通过测定乳粉中非蛋白氮含量,比较四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 %乙酸铅溶液和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,同时用三聚氰胺做加标回收,过滤后取滤液消化定氮。结果 三氯乙酸沉淀法计算非蛋白氮含量为0.2036%±0.0002%,相对标准偏差在四种方法中最小,为0.1023%,加标回收率为96.07±2.29%；丙酮沉淀法计算非蛋白氮含量为0.0981 %±0.0050 %,加标回收率在四种方法中最低,为50.56 %±7.90 %；乙酸铅沉淀法计算非蛋白氮含量为0.1372 %±0.0012 %,加标量为79.2mg三聚氰胺/10g样品时,乙酸铅沉淀法加标回收率比三氯乙酸沉淀法更稳定,为96.57%±1.07%；乙酸锌沉淀法计算非蛋白氮含量为0.3466 %±0.0100 %,加标回收率为86.81 %±2.87 %。结论 三氯乙酸沉淀法结果准确度和稳定性较其他三种方法更好,但受到加标量的影响。

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

Objective To determine the content of non-protein nitrogen in milk powder and compare the effects of four protein precipitants in separating protein and non-protein nitrogen. Methods Dissolve sample in water and make the protein be precipitated respectively with 15% trichloroacetic acid solution, acetone, 20% lead acetate solution and 3% potassium oxalate-7% disodium hydrogen phosphate solution, zinc acetate solution and potassium ferrocyanide solution. Meanwhile, melamine is added for spike recovery. The filtrate is taken to digest and determine nitrogen content. Results The non-protein nitrogen content gained by the trichloroacetic acid precipitation method was 0.2036% ± 0.0002%, with the smallest relative standard deviation, which was 0.1023%, among the four methods, and the recovery was 96.07 ± 2.29%; the non-protein nitrogen content applied with the acetone precipitation method was 0.0981% ± 0.0050%, accompanied with the lowest recovery, which is 50.56% ± 7.90%; the non-protein nitrogen content determined by the lead acetate precipitation method was 0.1372% ± 0.0012%. When the spike level was 79.2 mg melamine / 10g sample, the recovery of lead acetate precipitation method is more stable than that of trichloroacetic acid precipitation method, which could sustain 96.57% ± 1.07%; the non-protein nitrogen content acquired by zinc acetate precipitation method was 0.3466% ± 0.0100%, and the recovery was 86.81% ± 2.87%. 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.