2.1.1. Analytical methods 60% of the total samples were randomly selected as the cali- bration samples, and the remaining samples were selected as the prediction samples(external validation samples which were not used for calibration), the statistic glucose concentrations and p values of the samples are shown in the Table 1. RBFNN was employed to develop the models for quantitative analysis of the glucose and the pH of fermentation broth with NIRs using Matlab R2010a MathWorks, Inc., USA). The degree of approximation(Da) was used as the criterion for optimizing the developed models. The definition of Da is given by Eq. (I