加权平均法表述菌落总数测定的数学模型与 测量不确定度的评定

摘 要 目的:建立食品菌落总数测定的通用数学模型和测量不确定度的评定方法。方法：按照加权平均的算法对菌落总数计算公式进行推导，建立了菌落总数计数方法的通用数学模型。以某橙汁饮料菌落总数的测定过程为例，从三个方面分析并量化了各不确度的分量，分别为：不同稀释度平板上菌落数的分布（以同一稀释度上两个平板计数结果的相对相差表示），样品移取和稀释过程（以各量具的容量允差及稀释过程对稀释倍数贡献的相对不确度表示），数字修约（以修约间隔引入的粗大误差表示）。按照均方根法则，将各计数平板的测量不确度进行加和，即为总测量不确度。结果：本例中该橙汁饮料菌落总数的扩展不确度U=0.2×104 CFU/mL, k =2。结论：建立了简便准确的食品菌落数测量不确定度的评定方法。

Establishment of mathematical model and evaluation of measurement uncertainty for Aerobic Plate Count by weighted average

Objective To establish the general mathematical model and the procedure of measurement uncertainty for the determination of the Aerobic Plate Count (APC) on food. Methods According to the weighted average method, the formula of the APC was derived, and a general mathematical model was established. To take the measurement process of an orange juice drink APC as an example, three uncertainty com-ponents were evaluated and expressed, including the distribution of colony forming units (CFUs) on selected plates at different dilutions (equal to the relative difference of CFUs at the same dilution plates), the sampling and diluting process (equal to the relative uncertainty of dilution factor from capacity allowable errors and dilution process cumulative errors), rounding off numerical values (equal to gross error of rounding interval). According to the root-mean-square principle, measurement uncertainty components of each plate were added to calculate the total measurement uncertainty. Results Expanded uncertainty for the APC is 0.2×104 CFU/mL, k=2. Conclusion A simple and accurate method for evaluating measurement uncertainty for the APC of food is established.