Spectral ranges of 370–860, 1020–1809 and 2800–2953 cm−1, including the most characteristic milk bands were selected for modelling purposes. The number of latent PLS variables were determined on the basis of RMSECV plots (Fig. S3 of supplementary material) and were set to 3, except for protein and carbohydrates in the case of spectra recorded with the resolution of 8 cm−1, when 4 PLS factors were used.
The detailed PLS modelling results are summarised in Table 1. Although a difference in the quality of models based on Raman spectra registered with different resolution is visible, it is not pronounced. Correlation coefficient for the obtained calibration curves varied in the range 0.930-0.994. The RSEP errors were generally lower for models based on Raman spectra registered with the resolution of 16 cm−1. These values varied in the 2.1–4.7% and 2.6–5.9% range for calibration and validation samples, respectively (Table 1).