Application of an experimental design to the detection of foreign substances in milk

NIRA was used in order to study interactions between temperature (T) and soluble substances and to detect solutes added to milk. A two-level factorial design was planned to estimate the effects of the main factors and of the most interesting interactions on spectral data. Water (W), NaCl (N), reconstituted skim milk powder (P) and W with N were alternately added to milk. The plan was repeated on eight different milks. The interactions studied were W × N, W × T, N × T, P × T, W × N × T. The values assigned to the experimental design model were 1 and —1 for active and non-active factors, respectively. This model and the corresponding analysis of variance were assessed independently at each wavelength, making it possible to plot results versus wavelengths as a spectrum. Thus, absorbance (predicted variable) was considered as the response to experimental design variations (predictors). Principal Component Analysis (PCA) was applied to the overall data collection to identify the main variation factors. The abi...