阅读说明：本技术 一种婴幼儿奶粉剩余保质期的预测方法 (Method for predicting residual shelf life of infant milk powder ) 是由 薛伟锋 刘梦遥 曹文军 刘明 于 2020-11-04 设计创作，主要内容包括：一种婴幼儿奶粉剩余保质期的预测方法,包括如下步骤：步骤1、将被检测奶粉放入恒温恒湿箱内；步骤2、在不同的时间下随机取出不同温度下的被检测奶粉进行相应指标的检测；步骤3、将步骤2获得的参数结果带入公式GSI_j＝1-∑α_iV_(ij)(1)中,计算GSI_j实验值；步骤4、根据零级动力学模型公式[GSI]_t＝[GSI]_0-k_θt(3)获得零级动力学模型拟合相关系数；步骤5、根据一级动力学模型公式[GSI]’_(t’)＝[GSI]’_0exp(-k’_θt’)(4),获得一级动力学模型拟合相关系数；步骤6、选择拟合系数之和大的动力学模型用于奶粉剩余保质期预测；步骤7、按照公式或公式计算GSI_j实验值与模型预测值相对误差P(％)；步骤8、将公式(3)或公式(4)代入阿伦尼乌斯方程k＝k_0e~(-Ea/RT),计算得到不同温度下奶粉的剩余保质期t。(A method for predicting the residual shelf life of infant milk powder comprises the following steps: step 1, putting detected milk powder into a constant temperature and humidity box; step 2, randomly taking out the detected milk powder at different temperatures at different times to detect corresponding indexes; step 3, substituting the parameter result obtained in the step 2 into a formula GSI j ＝1‑∑α i V ij (1) In calculating GSI j Experimental values; step 4, according to a zero-order kinetic model formula [ GSI ]] t ＝[GSI] 0 ‑k θ t (3) obtaining a zero-order kinetic model fitting correlation coefficient; step 5, according to a primary dynamic model formula (GSI)]' t' ＝[GSI]' 0 exp(‑k' θ t') (4), obtaining a first-order dynamic model fitting correlation coefficient; step 6, selecting a dynamic model with large fitting coefficient sum for predicting the residual shelf life of the milk powder; step 7, according to the formula Or formula Computing GSI j Relative error P (%) between the experimental value and the model predicted value; step 8, substituting formula (3) or formula (4) into arrhenius equation k ═ k 0 e ‑Ea/RT And calculating to obtain the residual shelf life t of the milk powder at different temperatures.)

1. A method for predicting the residual shelf life of infant milk powder is characterized by comprising the following steps:

step 1, placing detected milk powder into a constant temperature and humidity box, and setting the humidity and temperature in the constant temperature and humidity box through a control panel of the constant temperature and humidity box;

step 2, randomly taking out the detected milk powder at different temperatures at different times to detect corresponding indexes, wherein the corresponding indexes are the measurement of reflectivity, the measurement of vitamin C, the measurement of hydroxymethylfurfural and the measurement of water activity, and the method comprises the following specific steps:

(1) measurement of reflectance: weighing 10-20 g of detected milk powder, and measuring by using a reflectivity measuring instrument to obtain the reflectivity;

(2) determination of vitamin C: accurately weighing 1-4 g of milk powder, adding a zinc sulfate solution, adding water to a constant volume, shaking uniformly, standing for 10-20 min, and then filtering to obtain a filtrate for later use; putting 25-30 mL of filtrate into a volumetric flask I, performing constant volume treatment by using an oxalic acid solution, adding activated carbon, shaking, filtering, removing primary filtrate, putting 10-20 mL of filtrate into a volumetric flask II, adding a thiourea solution, and uniformly mixing; adding 3-5 mL of equal amount of mixed solution into a plurality of test tubes, selecting one test tube as a blank, adding 1-2 mL of 2% 2, 4-dinitrophenylhydrazine solution into the other test tubes respectively, then putting all the test tubes into a water bath kettle at 37 +/-0.5 ℃ for heat preservation for 3h, taking out all the test tubes, putting the other test tubes except the blank test tube into ice water, adding 1-2 mL of 2% 2, 4-dinitrophenylhydrazine solution when the blank tube is cooled to room temperature, placing the test tubes in the ice water after standing at room temperature for 10-15 min, adding 3-5 mL of 85% sulfuric acid solution into each test tube when all the test tubes are put into the ice water, dropwise adding the test tubes for 1min or more, shaking the test tubes while dropwise adding the test tubes, taking out the ice water, placing the test tubes at room temperature for 20-30 min, carrying out color comparison, using a 1cm or 3cm cuvette, adjusting the zero point of the blank solution, and measuring the light absorption value at, calculating the content of vitamin C;

(3) determination of hydroxymethylfurfural: restoring milk powder according to the ratio of 5: 1-10: 1, transferring 5-10 mL of restored milk into a volumetric flask III by using a liquid transfer gun, adding an oxalic acid solution, and uniformly mixing; then placing the mixture into a boiling water bath at the temperature of 100 ℃, standing for a period of time, taking out the mixture, and cooling the mixture to room temperature by using cold water; adding a trifluoroacetic acid solution, uniformly mixing, standing for a period of time, filtering, transferring 3-5 mL of filtrate into a test tube by using a liquid transfer gun, adding a thiobarbituric acid solution, placing the test tube in a water bath at 40-50 ℃ for 30-40 min, taking out, and cooling to room temperature; replacing milk powder with water, repeating the above process to obtain a blank solution, adjusting zero point with the blank solution with a 1cm or 3cm cuvette, measuring light absorption value at 443nm wavelength, and calculating hydroxymethylfurfural content;

(4) determination of water activity: weighing 0.5-1 g of detected milk powder, and measuring by using a water activity meter to obtain water activity;

the final critical value of the reflectivity, vitamin C, hydroxymethylfurfural and water activity is L_i(ii) a The weight coefficient of the reflectivity, the vitamin C, the hydroxymethyl furfural and the water activity is alpha_i(ii) a The measured values of reflectance, vitamin C, hydroxymethylfurfural and water activity are C_ij；

Step 3, C obtained in step 2_ijAnd L_iSubstituting into formula (2), calculating to obtain the change rate of reflectivity, the change rate of vitamin C, the change rate of hydroxymethylfurfural, and the change rate of water activity V_ijWill V_ijAnd the weight coefficients alpha of the four indexes_iSubstituting into formula (1), multiple indexes representing milk powder quality can be converted into an overall index GSI_jThe value is defined as GSI_jThe experimental values were as follows,

GSI_j＝1-∑α_iV_ij (1)

wherein j is the storage time; sigma is the sum of corresponding values of i-1-n (n is the number of key evaluation indexes for selecting the quality of the milk powder by the model); v_ijThe rate of change of index i on day j; alpha is alpha_iWeight coefficient, sigma alpha, of importance of index i_i1 is ═ 1; v in formula (1)_ijCan be obtained by calculation of formula (2), wherein C_ijMeasured value on day j as index i; c_i0Is the initial value of index i on day 0; l is_iIs the end-of-shelf-life threshold for index i;

step 4, according to the zero-order kinetic model formula (3), the storage time t is used as an abscissa, and GSI_jTaking the experimental value as a vertical coordinate, and performing linear fitting to obtain a fitting correlation coefficient of a zero-order kinetic model;

[GSI]_t＝[GSI]₀-k_θt (3)

wherein [ GSI]_tCalculating a value for the comprehensive quality at the time t; [ GSI]₀Is an initial value of the comprehensive quality; k is a radical of_θIs a rate constant; t is the storage time;

step 5, according to a first-level dynamic model formula (4), after logarithmic functions ln are taken at two sides of the formula at the same time, storage time t' is taken as an abscissa, and lnGSI_jTaking the experimental value as a vertical coordinate, and performing linear fitting to obtain a fitting correlation coefficient of a primary dynamic model;

[GSI]'_t'＝[GSI]'₀exp(-k'_θt') (4)

wherein [ GSI]'_t'Calculating a value of the comprehensive quality at the time t'; [ GSI]'₀Is an initial value of the comprehensive quality; k'_θIs a rate constant; t' is the storage time;

step 6, repeating the step 4 and the step 5, respectively calculating fitting correlation coefficients of the zero-order kinetic model and the first-order kinetic model at different temperatures, then respectively calculating the sum of the fitting coefficients of the zero-order kinetic model and the first-order kinetic model, and selecting the kinetic model with the large sum of the fitting coefficients for predicting the residual shelf life of the milk powder;

step 7, calculating GSI according to formula (5) or formula (6)_jRelative error P (%) between experimental value and model predicted value, and comparing GSI at different temperatures_jExperimental and model predictive values for validating GSI_jReliability of shelf life prediction models; when the sum of the fitting coefficients of the zero-order kinetic model is larger than the sum of the fitting coefficients of the first-order kinetic model, selecting the zero-order kinetic model, and calculating according to a formula (5):

when the sum of the fitting coefficients of the zero-order kinetic model is smaller than the sum of the fitting coefficients of the first-order kinetic model, selecting the first-order kinetic model, and calculating according to a formula (6);

step 8, applying Arrhenius equation k ═ k₀e^-Ea/RTWherein k is a reaction rate constant; k is a radical of₀Is a pre-exponential factor; e_aActivation energy (kJ/mol); r is a molar gas constant of 8.3144J/(mol.K); t is the thermodynamic temperature (K);

substituting the formula (3) into an Arrhenius equation to obtain a quality guarantee period formula (7) simulated by a zero-order kinetic equation,

initial time [ GSI]₀When milk powder quality reaches the end of shelf life, [ GSI [ ] 1]_tCalculating the residual shelf life t of the milk powder at different temperatures as 0;

substituting the formula (4) into an Arrhenius equation to obtain a shelf life formula (8) simulated by a first-order kinetic equation,

initial time [ GSI]'₀When milk powder quality reaches the end of shelf life, [ GSI [ ] 1]'_t'The remaining shelf life t of the milk powder at different temperatures was calculated from 0.

Technical Field

The invention belongs to the technical field of milk powder detection, and particularly relates to a method for predicting the residual quality guarantee period of infant milk powder.

Background

The shelf life of the food is the longest time that the product can achieve physical, chemical and microbiological characteristics under specific environmental conditions and can be stored on the basis of ensuring the safety of nutrients and conversion products thereof. The shelf life of the food is of great significance to the production, storage, transportation and sale of the product, and more importantly, the shelf life of the food is directly related to the life safety of consumers. Therefore, it is important to obtain an accurate and reliable shelf life of the food.

The safety of infant milk powder is related to the healthy growth of new life, so that the quality requirement of the milk powder is extremely high when consumers choose the infant milk powder. Milk powder contains various nutrients, the physical and biochemical reactions which occur in the milk powder are very complex, and the understanding of all the occurring reaction mechanisms and the interaction thereof is extremely difficult. If the key indexes for representing the food quality are considered to replace the whole reaction change and simultaneously combined with a corresponding mathematical analysis method, the complexity is simple, and the prediction research of the shelf life of the milk powder is easier. At present, no method for acquiring the shelf life of infant milk powder exists in national standard and industrial standard. In the prior art, the quality guarantee period is obtained mainly depending on whether researchers meet the requirements on single physicochemical and microbial indexes in the milk powder or not, and a quality guarantee period is given according to experience. The result judged by the method has strong subjectivity. Moreover, only based on a certain key evaluation index, a single evaluation index in a complex milk powder system is considered in an isolated and one-sided manner, and the prediction method has certain one-sided property and cannot strictly and comprehensively monitor the quality and the quality guarantee period of the milk powder.

The infant milk powder is a powdery product prepared by the production process of sterilization, concentration, drying and the like, and the shelf life of the infant milk powder at normal temperature is generally 18-24 months under the condition of no damage to packaging. The disassembled milk powder is rich in various nutrient substances such as protein, fat and the like, so that the milk powder is easy to change under the action of illumination, temperature, humidity, chlorine and the like, such as flavor change, browning and moisture absorption, change of nutrient components and microorganisms and the like, and the shelf life is shortened. In this case, the shelf life of the milk powder that has been disassembled cannot be evaluated again using the shelf life results on the milk powder label.

Milk powder contains various nutrients, the physical and biochemical reactions which occur in the milk powder are very complex, and the understanding of all the occurring reaction mechanisms and the interaction thereof is extremely difficult. The prior art only keeps studying whether the single physicochemical and microbial indexes of the milk powder meet the requirements or not, gives a rough quality guarantee period according to experience, has strong subjectivity and certain one-sidedness, and cannot strictly and comprehensively monitor the quality and the quality guarantee period of the milk powder. The reason is that no suitable method is found, which can effectively link the independent indexes representing the shelf life of the milk powder and integrate the independent indexes into a comprehensive evaluation index to achieve the purpose of predicting the residual shelf life of the milk powder. The quality guarantee period of the disassembled and assembled milk powder is called as the residual quality guarantee period. Therefore, the method for predicting the residual shelf life of the infant milk powder is provided, and has important significance.

Disclosure of Invention

In order to achieve the purpose, the invention adopts the following technical scheme:

a method for predicting the residual shelf life of infant milk powder comprises the following steps:

step 1, placing detected milk powder into a constant temperature and humidity box, and setting the humidity and temperature in the constant temperature and humidity box through a control panel of the constant temperature and humidity box;

step 2, randomly taking out the detected milk powder at different temperatures at different times to detect corresponding indexes, wherein the corresponding indexes are the measurement of reflectivity, the measurement of vitamin C, the measurement of hydroxymethylfurfural and the measurement of water activity, and the method comprises the following specific steps:

(1) measurement of reflectance: weighing 10-20 g of detected milk powder, and measuring by using a reflectivity measuring instrument to obtain the reflectivity;

(2) determination of vitamin C: accurately weighing 1-4 g of milk powder, adding a zinc sulfate solution, adding water to a constant volume, shaking uniformly, standing for 10-20 min, and then filtering to obtain a filtrate for later use; putting 25-30 mL of filtrate into a volumetric flask I, performing constant volume treatment by using an oxalic acid solution, adding activated carbon, shaking, filtering, removing primary filtrate, putting 10-20 mL of filtrate into a volumetric flask II, adding a thiourea solution, and uniformly mixing; adding 3-5 mL of equal amount of mixed solution into a plurality of test tubes, selecting one test tube as a blank, adding 1-2 mL of 2% 2, 4-dinitrophenylhydrazine solution into the other test tubes respectively, then putting all the test tubes into a water bath kettle at 37 +/-0.5 ℃ for heat preservation for 3h, taking out all the test tubes, putting the other test tubes except the blank test tube into ice water, adding 1-2 mL of 2% 2, 4-dinitrophenylhydrazine solution when the blank tube is cooled to room temperature, placing the test tubes in the ice water after standing at room temperature for 10-15 min, adding 3-5 mL of 85% sulfuric acid solution into each test tube when all the test tubes are put into the ice water, dropwise adding the test tubes for 1min or more, shaking the test tubes while dropwise adding the test tubes, taking out the ice water, placing the test tubes at room temperature for 20-30 min, carrying out color comparison, using a 1cm or 3cm cuvette, adjusting the zero point of the blank solution, and measuring the light absorption value at, calculating the content of vitamin C;

(3) determination of hydroxymethylfurfural: restoring milk powder according to the ratio of 5: 1-10: 1, transferring 5-10 mL of restored milk into a volumetric flask III by using a liquid transfer gun, adding an oxalic acid solution, and uniformly mixing; then placing the mixture into a boiling water bath at the temperature of 100 ℃, standing for a period of time, taking out the mixture, and cooling the mixture to room temperature by using cold water; adding a trifluoroacetic acid solution, uniformly mixing, standing for a period of time, filtering, transferring 3-5 mL of filtrate into a test tube by using a liquid transfer gun, adding a thiobarbituric acid solution, placing the test tube in a water bath at 40-50 ℃ for 30-40 min, taking out, and cooling to room temperature; replacing milk powder with water, repeating the above process to obtain a blank solution, adjusting zero point with the blank solution with a 1cm or 3cm cuvette, measuring light absorption value at 443nm wavelength, and calculating hydroxymethylfurfural content;

(4) determination of water activity: weighing 0.5-1 g of detected milk powder, and measuring by using a water activity meter to obtain water activity;

the final critical value of the reflectivity, vitamin C, hydroxymethylfurfural and water activity is L_i(ii) a The weight coefficient of the reflectivity, the vitamin C, the hydroxymethyl furfural and the water activity is alpha_i(ii) a The measured values of reflectance, vitamin C, hydroxymethylfurfural and water activity are C_ij；

Step 3, C obtained in step 2_ijAnd L_iSubstituting into formula (2), calculating to obtain the change rate of reflectivity, the change rate of vitamin C, the change rate of hydroxymethylfurfural, and the change rate of water activity V_ijWill V_ijAnd the weight coefficients alpha of the four indexes_iSubstituting into formula (1), multiple indexes representing milk powder quality can be converted into an overall index GSI_jThe value is defined as GSI_jThe experimental values were as follows,

GSI_j＝1-∑α_iV_ij (1)

wherein j is the storage time; sigma is the sum of corresponding values of i-1-n (n is the number of key evaluation indexes for selecting the quality of the milk powder by the model); v_ijThe rate of change of index i on day j; alpha is alpha_iWeight coefficient, sigma alpha, of importance of index i_i1 is ═ 1; v in formula (1)_ijCan be obtained by calculation of formula (2), wherein C_ijMeasured value on day j as index i; c_i0Is the initial value of index i on day 0; l is_iIs the end-of-shelf-life threshold for index i;

step 4, according to the zero-order kinetic model formula (3), the storage time t is used as an abscissa, and GSI_jTaking the experimental value as a vertical coordinate, and performing linear fitting to obtain a fitting correlation coefficient of a zero-order kinetic model;

[GSI]_t＝[GSI]₀-k_θt (3)

wherein [ GSI]_tCalculating a value for the comprehensive quality at the time t; [ GSI]₀Is an initial value of the comprehensive quality; k is a radical of_θIs a rate constant; t is the storage time;

step 5, according to a first-level dynamic model formula (4), after logarithmic functions ln are taken at two sides of the formula at the same time, storage time t' is taken as an abscissa, and lnGSI_jTaking the experimental value as a vertical coordinate, and performing linear fitting to obtain a fitting correlation coefficient of a primary dynamic model;

[GSI]'_t'＝[GSI]'₀exp(-k'_θt') (4)

wherein [ GSI]'_t'Calculating a value of the comprehensive quality at the time t'; [ GSI]'₀Is an initial value of the comprehensive quality; k'_θIs a rate constant; t' is the storage time;

step 6, repeating the step 4 and the step 5, respectively calculating fitting correlation coefficients of the zero-order kinetic model and the first-order kinetic model at different temperatures, then respectively calculating the sum of the fitting coefficients of the zero-order kinetic model and the first-order kinetic model, and selecting the kinetic model with the large sum of the fitting coefficients for predicting the residual shelf life of the milk powder;

step 7, calculating GSI according to formula (5) or formula (6)_jRelative error P (%) between experimental value and model predicted value, and comparing GSI at different temperatures_jExperimental and model predictive values for validating GSI_jReliability of shelf life prediction models; when the sum of the fitting coefficients of the zero-order kinetic model is larger than the sum of the fitting coefficients of the first-order kinetic model, selecting the zero-order kinetic model, and calculating according to a formula (5):

when the sum of the fitting coefficients of the zero-order kinetic model is smaller than the sum of the fitting coefficients of the first-order kinetic model, selecting the first-order kinetic model, and calculating according to a formula (6);

step 8, applying Arrhenius equation k ═ k₀e^-Ea/RTWherein k is a reaction rate constant; k is a radical of₀Is a pre-exponential factor; e_aActivation energy (kJ/mol); r is a molar gas constant of 8.3144J/(mol.K); t is the thermodynamic temperature (K);

substituting the formula (3) into an Arrhenius equation to obtain a quality guarantee period formula (7) simulated by a zero-order kinetic equation,

initial time [ GSI]₀When milk powder quality reaches the end of shelf life, [ GSI [ ] 1]_tCalculating the residual shelf life t of the milk powder at different temperatures as 0;

substituting the formula (4) into an Arrhenius equation to obtain a shelf life formula (8) simulated by a first-order kinetic equation,

initial time [ GSI]'₀When milk powder quality reaches the end of shelf life, [ GSI [ ] 1]'_t'The remaining shelf life t of the milk powder at different temperatures was calculated from 0.

The invention has the beneficial effects that:

the infant milk powder is safe and related to the healthy growth of new life, and the quality requirement of the milk powder is extremely high when the milk powder is selected and purchased by consumers. However, the frequent occurrence of a series of milk powder safety incidents has created a huge shadow on the quality safety of infant milk powder. Under the background, the government of China ensures the quality safety of the infant milk powder of China through ways of making/revising policies, standards and the like. As an important parameter for evaluating the safety of the infant milk powder, the evaluation of the shelf life is worthy of focusing attention. However, no standard method suitable for evaluating the shelf life of infant milk powder is found at home at present. Compared with the method that milk powder processing enterprises only consider a single evaluation index in a complex system in an isolated and one-sided manner based on a certain key evaluation index, and a prediction method established based on the index has certain one-sided property and cannot strictly and comprehensively monitor the quality and the quality guarantee period of milk powder, the quality guarantee period prediction method established by the invention combines the evaluation of the physicochemical indexes of the milk powder, effectively reduces the consumption time of quality guarantee period experiments through accelerated experiments, and reduces the time for normally determining the quality guarantee period of the milk powder from 18-24 months to 30 days. Moreover, the shelf life of the milk powder is mostly predicted under the condition that the milk powder is packaged completely at present, in the actual life, the shelf life given on the label can not be used any more after the milk powder is disassembled and assembled, and at the moment, the residual shelf life of the milk powder is closer to the actual requirement. Based on the consideration, the reflectivity, the vitamin C, the hydroxymethylfurfural and the water activity are selected as evaluation indexes, and the important reaction process in the milk powder is considered more comprehensively, so that the result of the embodiment is more credible than a single-index prediction result, and an effective method is provided for predicting the residual shelf life of the milk powder.

Drawings

FIG. 1 is a graph showing the quality evaluation indexes of milk powder according to the embodiment of the present invention along with the storage time and storage temperature;

FIG. 2 shows the difference (C) between the measured value and the initial value of the milk powder to be tested according to the embodiment of the present invention_ij-C_i0) Graph of changes with storage time and storage temperature;

FIG. 3 shows the change rate V of the milk powder tested in the embodiment of the present invention_ijAs a function of storage temperature and between storages;

FIG. 4 GSI of milk powder tested at 20 deg.C, 30 deg.C, 40 deg.C and 50 deg.C according to the present invention_jGraph of experimental values versus storage time;

FIG. 5 is a fitting graph of zero-order kinetic model of milk powder to be tested according to the embodiment of the invention;

FIG. 6 is a first-order kinetic model fitting graph of the milk powder to be tested according to the embodiment of the present invention;

FIG. 7 is a graph of the variation of the fitting correlation coefficient and rate constant of the zero-order kinetic model and the first-order kinetic model of the milk powder to be detected in the embodiment of the invention;

FIG. 8 shows GSI at storage temperatures of 20 deg.C, 30 deg.C, 40 deg.C and 50 deg.C in examples of the present invention_jRelative error maps of experimental values and predicted values;

FIG. 9 shows the storage temperature and the GSI of the milk powder to be tested according to the embodiment of the present invention_jA plot of decay rate constants;

Detailed Description

The present invention will be described in further detail with reference to the accompanying drawings and examples.

A method for predicting the residual shelf life of infant milk powder comprises the following steps:

step 1, selecting infant formula milk powder of 6-12 months of age and 2 paragraphs, and detecting the original packaging specification of the milk powder: 900 g/can; putting the detected milk powder into a constant temperature and humidity box, and setting the humidity and the temperature in the constant temperature and humidity box through a control panel of the constant temperature and humidity box;

step 2, exposing the detected milk powder to four temperature levels of 20 ℃, 30 ℃, 40 ℃ and 50 ℃ under the condition of 30% relative humidity; on day 0, three groups of milk powder to be detected at the storage temperature of 20 ℃ are extracted for detection of corresponding indexes, wherein the corresponding indexes comprise reflectivity measurement, vitamin C measurement, hydroxymethylfurfural measurement and water activity measurement, and the method comprises the following steps:

(1) measurement of reflectance: weighing 10g of detected milk powder, and measuring by using a reflectivity measuring instrument to obtain the reflectivity;

(2) determination of vitamin C: accurately weighing 4g of milk powder, adding 5mL of 0.42mol/L zinc sulfate solution, adding water to a constant volume of 100mL, shaking uniformly, standing for 15min, filtering, keeping the filtrate for later use, putting 25mL of filtrate into a 100mL volumetric flask I, adding 1% oxalic acid solution to a constant volume, adding 2g of activated carbon, shaking for 1min, filtering, discarding the primary filtrate, putting 10mL of filtrate into a 25mL volumetric flask II, adding 10mL of 2% thiourea solution, mixing, taking 3 test tubes, adding 4mL of the equal mixed solution into three test tubes, taking one test tube as a blank, adding 1-2 mL of 2% 2, 4-dinitrophenylhydrazine solution into the rest test tubes, putting all the test tubes into a 37 +/-0.5 ℃ water bath, keeping the temperature for 3h, taking out all the test tubes, putting the rest test tubes except the blank test tubes into ice water, and cooling the blank tubes to room temperature, adding 1-2 mL of 2% 2, 4-dinitrophenylhydrazine solution, placing the mixture at room temperature for 10-15 min, then placing the mixture into ice water, adding 3-5 mL of 85% sulfuric acid solution into each test tube after all the test tubes are placed into the ice water, dropwise adding the sulfuric acid solution for more than or equal to 1min, shaking the test tubes while dropwise adding, taking the test tubes out of the ice water, placing the test tubes at room temperature for 20-30 min, carrying out color comparison, adjusting the zero point with a 1cm or 3cm cuvette by using a blank solution, measuring the light absorption value at the wavelength of 500nm, and calculating the corresponding vitamin C content;

(3) determination of hydroxymethylfurfural: restoring milk powder according to the proportion of 10:1, transferring 10mL of restored milk into a 50mL volumetric flask III by using a liquid transfer gun, adding 5mL of 1.89% oxalic acid solution, and uniformly mixing; then placing in a boiling water bath at 100 ℃ for 1h, taking out, and cooling to room temperature by using cold water; adding 5.0mL of 40% trifluoroacetic acid solution, uniformly mixing, standing for 3min, filtering, transferring 4.0mL of filtrate into a 10mL test tube by using a liquid transfer gun, adding 1.0mL of 0.05mol/L thiobarbituric acid solution, placing the test tube in a water bath at 40 ℃ for 35min, taking out, and cooling to room temperature; replacing milk powder with water, repeating the above process to obtain a blank solution, adjusting zero point with the blank solution in a 1cm or 3cm cuvette, and measuring light absorption value at 443nm wavelength;

(4) determination of water activity: weighing 1g of detected milk powder, and measuring by using a water activity meter;

(5) repeating the steps (1) to (4) to respectively obtain the reflectivity, vitamin C, hydroxymethylfurfural and water activity of the detected milk powder at 20 ℃ for 5, 10, 15, 20, 25 and 30 days; then repeating the steps (1) to (4) to obtain the reflectivity, the vitamin C, the hydroxymethylfurfural and the water activity of the detected milk powder at 30 ℃ and 40 ℃ for 0, 5, 10, 15, 20, 25 and 30 days and at 50 ℃ for 0, 5, 10, 15 and 20 days;

the changes of the reflectivity, vitamin C, hydroxymethylfurfural and water activity of the detected milk powder at 20 ℃, 30 ℃, 40 ℃ and 50 ℃ along with the storage time are shown in figure 1, the reflectivity, the vitamin C and the water activity are gradually reduced along with the increase of the storage time, but the content of the hydroxymethylfurfural is gradually increased; with the rise of the temperature, the changes of the four evaluation indexes are accelerated;

when the critical values of the reflectivity, the vitamin C and the water activity reach half of the day 0, the failure point is reached, when the critical value of the hydroxymethylfurfural reaches 40 mu mol/L, the failure point is reached, the quality guarantee period end point critical values of the reflectivity, the vitamin C, the hydroxymethylfurfural and the water activity of the detected milk powder are respectively 44%, 30mg/100g, 40 mu mol/L and 0.186, the critical values are determined as standard indexes, and when the four indexes measured by the method are the same as the standard indexes, the failure point is reached; the four evaluation indexes are generally of equal importance, so the weight coefficient alpha of the four evaluation indexes on the quality of the detected milk powder_iAre all 0.25;

step 3, converting four indexes representing the quality of the milk powder into an overall index GSI_jThe value is defined as GSI_jExperimental values; the reflectivity of the detected milk powder is C_1jVitamin C is C_2jHydroxymethyl furfural is C_3jAnd a water activity of C_4jAt 20 ℃ on day 0, the measured values C of the four indices are measured_1j＝88、C_2j＝60.0、C_3j22.0 and C_4j0.372 and its critical threshold is L₁＝44；L₂＝30；L₃40 and L₄The formula (2) is substituted for each of 0.186, and the corresponding rate of change V is calculated and obtained_1j＝0、V_2j＝0、V_3j0 and V_4jWhen the value is equal to 0, then V is added_1j、V_2j、V_3jAnd V_4jAnd respective weight coefficients alpha₁、α₂、α₃、α₄Substituting into formula (1) to obtain GSI₀As shown in FIGS. 2 and 3, the GSI at 20 ℃ was calculated by repeating this procedure₅、GSI₁₀、GSI₁₅、GSI₂₀、GSI₂₅And GSI₃₀GSI at 30 ℃ and 40 ℃₀、GSI₅、GSI₁₀、GSI₁₅、GSI₂₀、GSI₂₅And GSI₃₀To therebyAnd GSI at 50 ℃₀、GSI₅、GSI₁₀、GSI₁₅、GSI₂₀(ii) a The milk powder GSI to be detected at different temperatures can be obtained_jExperimental values as a function of time, as shown in fig. 4; GSI of milk powder during storage_jThe value was gradually decreased, and the rate of decrease was increased with the increase in temperature, whereby it was found that the deterioration of the quality of the powdered milk could be delayed by decreasing the storage temperature,

GSI_j＝1-∑α_iV_ij (1)

wherein j is the storage time; sigma is the sum of corresponding values of i-1-n (n is the number of key evaluation indexes for selecting the quality of the milk powder by the model); v_ijThe rate of change of index i on day j; alpha is alpha_iWeight coefficient, sigma alpha, of importance of index i_i1 is ═ 1; v in formula (1)_ijCan be obtained by calculation of formula (2), wherein C_ijMeasured value on day j as index i; c_i0Is the initial value of index i on day 0; l is_iIs the end-of-shelf-life threshold for index i;

step 4, according to the zero-order kinetic model formula (3), the storage time t is used as an abscissa, and GSI_jTaking the experimental value as a vertical coordinate, and performing linear fitting to obtain a fitting correlation coefficient of a zero-order kinetic model;

[GSI]_t＝[GSI]₀-k_θt (3)

wherein [ GSI]_tCalculating a value for the comprehensive quality at the time t; [ GSI]₀Is an initial value of the comprehensive quality; k is a radical of_θIs a rate constant; t is the storage time;

step 5, according to a first-level dynamic model formula (4), after logarithmic functions ln are taken at two sides of the formula at the same time, storage time t' is taken as an abscissa, and lnGSI_jTaking the experimental value as a vertical coordinate, and performing linear fitting to obtain a fitting correlation coefficient of a primary dynamic model;

[GSI]'_t'＝[GSI]'₀exp(-k'_θt') (4)

wherein [ GSI]'_t'Calculating a value of the comprehensive quality at the time t'; [ GSI]'₀Is an initial value of the comprehensive quality; k'_θIs a rate constant; t' is the storage time;

step 6, repeating the step 4 and the step 5, and respectively calculating fitting correlation coefficients R of the zero-order kinetic model and the first-order kinetic model at 20 ℃, 30 ℃, 40 ℃ and 50 DEG²As shown in fig. 5, 6 and 7, respectively determining the sum of fitting correlation coefficients of the zero-order kinetic model to be 3.9803 and the sum of fitting correlation coefficients of the first-order kinetic model to be 3.9538, and comparing the sums of fitting correlation coefficients of the zero-order kinetic model and the first-order kinetic model, it can be known that the zero-order kinetic model reflects the quality change of the milk powder more accurately than the first-order kinetic model, so that the shelf life of the detected milk powder is predicted by using the zero-order kinetic model;

step 7, calculating GSI according to the formula (5)_jComparing the relative error P (%) between the experimental value and the model predicted value, and comparing the GSI at 20 deg.C, 30 deg.C, 40 deg.C, and 50 deg.C_jExperimental and model predictive values for validating GSI_jThe reliability of the shelf-life prediction model, as shown in fig. 8; step 6 shows that the sum of the fitting coefficients of the zero-order kinetic model is larger than the sum of the fitting coefficients of the first-order kinetic model, the zero-order kinetic model is selected, and calculation is carried out according to the formula (5):

in the experimental period, GSI_jThe relative error P (%) absolute value of the experimental value and the model predicted value is below 3%, which shows that the GSI of the milk powder to be detected is the milk powder GSI of the invention_jThe change dynamics model and the shelf life prediction model are reliable and effective;

step 8, applying Arrhenius equation k ═ k₀e^-Ea/RTK is a reaction rate constant; k is a radical of₀Is a pre-exponential factor; e_aActivation energy (kJ/mol); r is a molar gas constant of 8.3144J/(mol.K); t isA thermodynamic temperature (K);

substituting the zero-order kinetic model formula (3) into an Arrhenius equation to obtain a quality guarantee period formula (7) simulated by the zero-order kinetic equation,

the velocity-temperature relationship described by the arrhenius equation is shown in fig. 9, the fitting correlation coefficient is 0.9589, it is shown that the velocity-temperature relationship model can be established by the arrhenius equation, and the absolute value 4.6366 of the slope of the fitting straight line represents E_a10.878 th power of/R, e represents k₀Calculating to obtain E_aAnd k₀Respectively 38.55kJ/mol and 52997, and the formula (7) is substituted to obtain the formula for predicting the shelf life of the milk powder:

initial time [ GSI]₀When milk powder quality reaches the end of shelf life, [ GSI [ ] 1]_tThe remaining shelf lives of the milk powder at 20 ℃, 30 ℃, 40 ℃ and 50 ℃ are respectively 140.7, 83.5, 51.2 and 32.4 days, the service shelf lives obtained through experiments are respectively 140, 85, 50 and 30 days, the absolute value of the relative error is less than 10%, and the prediction result is reliable.