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A Rapid Titration Method for the Determination of

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    RESEARCH ARTICLESA Rapid Titration Method for the Determination of Protein in Milk PowderJun Hao*, Liping He, Ruibin Zhang, Jun ShenAbstract: The milk powder was dissolved and acidated. The resulted solution was titrated against the 0.20% sodium dodecyl sulphate (SDS). At the end point the content of conical flask suddenly separated into curdy white coagulum. The time for determining every sample is only 4 minutes. The relative standard deviation was 0.86% (n=6). The results determined by this method were coincidence with those of Kjieldahls method for 8 different samples (p>0.05). Key words: milk powder; protein; titration; sodium dodecyl sulphate Recently reported low-quality milk powder found in the market presents a threat to public health and it has drawn great attention of the government. The major feature of such milk powder is the much lower protein content than normal. Therefore it is essential to establish a fast and simple method for quantification of protein contents for the purpose to identify the low-quality milk powder in a high throughput mode. The conventional Kjieldahls method has several steps including digestion, distillation and titration, which are tedious and time-consuming. Here we report a novel method for rapid titration of total protein content in milk powder. This method is based on the fact that when SDS is slowly added into the milk, the soluble protein will precipitate at a certain point and the amount of SDS added at that end point is proportional to the protein content.MATERIALS AND METHODS1. Reagents and instruments 0.20% SDS solution was prepared by dissolving 2.00g of SDS powder dehydrated in a desiccator for 24h in 1L-distilled water. If the room temperature is lower than 25, 0.10% SDS solution should be prepared to prevent SDS precipitation. 0.1mol/L HCl Reagent and instruments of Kjieldahls method [1] for calibration2. Experimental methods 2.50g of milk powder sample was dissolved in warmed distilled water (37~42). After the solution cooled down to room temperature, an aliquot of 25.0ml was transferred to a 200ml flask and 1.0ml of 0.1mol/L HCl was added to the solution. Then SDS solution was added slowly to the solution until white precipitation was observed. At the end point the content of conical flask suddenly separated into curdy white coagulum. The end point can be easily judged. m: the amount of milk powder in the solution titrated K: a calibration factor. Equivalent aliquots from a sample were titrated by our method and Kjieldahls method and were determined by averaging results of six experiments. K can be determined by following equation: The K value would have minor variations for different SDS. But for the same SDS, only one calibration is necessary. In our experiment we used SDS purchased from Tianjin chemicals Inc. (AC grade) and the K was determined as 7.8510-3.RESULTS AND DISCUSSION1. Determination of K value Protein content in a milk sample was titrated by our method and Kjieldahls method respectively in six independent experiments and K values are shown in table 1.Table 1. The calibration for K value Number of sample123456 A*0.1830.1890.1830.1890.1840.1870.186 B**23.623.823.523.723.524.023.7K=7.757.947.797.977.837.797.85 A*: The content of protein in sample by Kjieldahls method (g/g). B**: The titer of SDS solution for every gram sample by this method (ml/g)2. Precision Same samples were determined by the two methods, the relative standard deviation was 0.86% (n=6).3. Reliability 8 different samples were determined by the two methods, results are shown in table 2.Table 2. The contents of protein in power milk by two methods (%)Number of sample12345678This method22.522.018.215.516.413.210.28.30Kjieldahls method21.921.818.615.316.613.110.38.21 The data was analyzed by SSPS software and no significant deviation was found between the two methods (p>0.05). In conclusion, our method is fast, simple and highly repeatable for quantification of protein content in milk powder. REFERENCES1. Fulin Liu, Xingjun Dai, edited. Food physical and chemical analysis. Beijing: Light Industry Press. 1987, 20-24. (Edited by Yingqi Zhao, Tiantian Xie and Lian Hu)* Corresponding to Jun Hao, professor of Department of Sanitary Chemistry, Tianjin Medical University; Address: Box 135, Tianjin Medical University, No. 22 Qixiangtai Road, Tianjin, Postcode: 300070; Tel: 022-23542640; E-mail: haojun@tijmu.edu.cn
 
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