| Due to high nutritional value and delicious taste, milk and dairy products are well received by consumers. With the gradual increase of consumer groups and consumption of milk and dairy products, the quality and safety problems of milk and dairy products are becoming more and more important. However, in recent years, due to various reasons, major safety incidents of milk and dairy products that led to some harm to human health occurred frequently, which made people on the one hand on the safety of milk and dairy products panic and also urgent need to give more practical dairy quality control method to ensure its quality.The research had shown that the nonlinear chemical fingerprint method as a new kind of fingerprint technology has been successfully applied to the quality control of food. Facts show that this technology can solve the qualitative and quantitative control problems of the food quality. It also can effectively ensure the safety of food that is a kind of complex samples.In this paper, milk powder was used as the object of study by the nonlinear chemical fingerprint technology using two oscillation systems. Different brands of milk powders, different kinds of milk powders and mixed milk powders were detected. According to the information obtained in the test, different brands of milk powders, different kinds of milk powders and mixed milk were identificated and evaluated. The research contents of this paper are as follows:(1) In the condition optimization experiment of the nonlinear chemical fingerprint technology used for the detection of milk powders, H2SO4-CH2(COOH)2-NaBrO3-NH4Ce(SO4)2 (systems I) oscillating system and H2SO4-CH3COCH3-MnSO4-NaBrO3 (systems II) oscillating system were selected. In this study, induction time and maximum amplitude were referred to as detection indexes. Single factor test was designed to examine the effect of temperature, dosage of milk powder samples and main reagent concentrations on the oscillating systems, respectively. The optimal conditions of two systems for milk powders determination were obtained by response surface experimental design. The optimal reaction conditions of the systems I were determined by the experiments:the temperature was 50 and the dosage of milk powder sample was l.Og. The concentrations of NH4Ce(SO4)2, NaBrO3, H2SO4 and CH2(COOH)2 were 0.05mol/L (3mL),0.8 mol/L (5mL), l.Omol/L (25mL) and 1.0mol/L (10mL), respectively. The optimal reaction conditions of the systems II were determined by the experiments:the temperature was 50 and the dosage of milk powder sample was 1.0g. The concentrations of H2SO4, MnSO4, CH3COCH3 and NaBrO3 were 1.0mol/L (25mL),0.08mol/L (12mL),1.0 mol/L (15mL) and 0.8mol/L (5mL), respectively.(2) In this section, the systems I and systems II were used. The quantitative analysis on the addition of the dextrin in different brands of milk powders was performed. The nonlinear chemical fingerprint of milk powder with dextrin was obtained, and the least square method was used to build the mathematical model between the content of dextrin and the value of characteristic parameters of the fingerprint. In the systems I, there is a good linear relationship between inductive time of the nonlinear chemical fingerprints and dextrin content in milk powder when dextrin content in milk powder is in the range of 0-30%. The correlation coefficient is in the range of 0.9982-0.9998, and the recovery is 94.00%-108.75%. The relative standard deviations (RSD) are less than or equal to 1.96%, and the detection range is 0-0.30g/g. The detection limit for milk powder 1#,2# and 3# are 4.5,7.9 and 6.6?g/g, respectively. In the systems II, there is a good linear relationship between inductive time of the nonlinear chemical fingerprints and dextrin content in milk powder when dextrin content is in the range of 0-0.30g/g. The correlation coefficient is in the range of 0.9972-0.9991, and the recovery is 94.00%-104.89%. The relative standard deviations (RSD) are less than or equal to 1.17%, and the detection range is 0-0.30g/g. The detection limit for milk powder 1#,2# and 3# are 2.6,3.0 and 4.7?g/g, respectively. Through the comparison of the two systems, from the point of view of the characteristic of the nonlinear chemical fingerprint, the characteristic of the nonlinear chemical fingerprint was relatively obvious by the systems ?, and from the point of view of the detection time, the detection time for samples analysis was relatively short by the systems I. The nonlinear chemical fingerprint method of two systems has the advantages of simple operation, without sample pretreatment and low cost, and it is feasible for the determination of dextrin content in milk powder. The results show that this method can be used to detect urea in milk powder.(3) Using the systems I, milk powder with urea artificially added can be determined. The nonlinear chemical fingerprint of mixed milk powder was obtained, and the least square method was used to build the mathematical model between the content of urea and the value of characteristic parameters of the nonlinear chemical fingerprint. The results show that a good linear relationship is established between inductive time of the nonlinear chemical fingerprints and urea content in milk powder when urea content is in the range of 0-40mg/g. The correlation coefficient is in the range of 0.9987-0.9995, and the recovery is 95.48%-104.06%. The relative standard deviations (RSD) are less than or equal to 1.82%. The detection limit for milk powder 2#,6# and 7# was 0.17,0.33 and 0.45?g/g, respectively.(4) Using the systems I, dextrin and urea in milk powder were simultaneouslv determinated. The results find that the content of adulterant and the characteristic parameter value of the nonlinear chemical fingerprint of the corresponding milk powder have a good linear relationship when a kind of adulterant is added into milk powder. When dextrin and urea (dextrin and urea with mass ratio 1:1) are simultaneously added into milk powder, the induction time of characteristic parameters of the nonlinear chemical fingerprint of mixed milk powder and the content of dextrin and urea in milk powder have a good linear relationship. Based on the additivity property of the effects of dextrin and urea on inductive time of the nonlinear chemical fingerprint, milk powder sample with different mixing ratios of dextrin and urea is used, and the inductive time values of the nonlinear chemical fingerprints of the corresponding mixed milk powder sample are obtained under different temperatures. The system analysis model was established by the least squares method. The content of dextrin and urea in milk powder is calculated by this model. The results indicate that the nonlinear chemical fingerprint method combined with the classical least squares method is feasible for quantitative analysis and quality evaluation when dextrin and urea is simultaneously added into milk powder. Moreover, the method has high precision and accuracy. The detection limit for dextrin and urea in milk powder 2# is 8.0 and 9.8?g/g, respectively.(5) Using the systems ?, different kinds of milk powders, different brands of milk powders and the adulterated milk powders were detected and identificated. The average value of the information of the common model parameters of pure milk powder was referred to as the standard. The system similarity of the nonlinear chemical fingerprint between mixed milk powder and the corresponding pure milk powder was calculated by the system similarity model. According to the comparison of the nonlinear chemical fingerprint between the sample to be tested and normal milk powder samples, the sample to be tested was determined whether the sample was adulterated. The results indicate that the nonlinear chemical fingerprints for pure goats'milk powder, pure milk powder, and different brands of milk powders with starch, melamine or sodium nitrite can be identificated by the appearance characteristics of the corresponding milk powder fingerprints. According to the application of the nonlinear chemical fingerprint technology and the system similarity method, the system similarity of the same kind of pure milk powder is 0.9905, and the maximum system similarity of different kinds of milk powder is only 0.9379. The system similarity of the same brand of milk powder is higher, and the maximum system similarity of the same brand of milk powder with adulterant is only 0.8978. The different kinds of milk powders and milk powder with adulterant can be identificated by the nonlinear chemical fingerprint and the system similarity calculation of the corresponding milk powder. |
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