Supplemental Materials for "Influence of milk storage time on mid-infrared spectroscopy and its predictions for amino acid content".

Mid-infrared spectroscopy (MIRS) is increasingly used as a rapid and effective analytical method for the quantitative prediction of detailed milk composition, such as minerals, fatty acids, and amino acids (AA). These analyses require the transportation of samples to a certified laboratory. In this case, storage time may affect MIRS and its prediction results. This study aimed to determine the effect of milk storage time on MIRS and its predictions of AA content. 373 individual milk samples for the development of AA content prediction equations were collected from seven commercial dairy farms (dataset 1). 103 individual milk samples for the analysis of the effect of storage time were collected from two farms (dataset 2). Firstly, separate quantitative prediction models based on dataset 1 were developed for each AA using partial least squares regression; the accuracy of prediction was assessed using a cross-validation set. Secondly, repeatability and reproducibility of the predictions of AA were calculated using milk samples in dataset 2, whose spectra were measured once a day for seven consecutive days after sampling storing at 4 ℃, to assess the effect of storage time on the consistency of MIRS predicted AA content. Moderate to high prediction accuracy of AA was achieved, with RPD_CV and R_CV² being in the range from 1.59 (Gly) to 2.39 (Leu) and from 0.58 (Gly) to 0.79 (Leu), respectively. Results demonstrated that the absorbance of spectral points was affected by milk storage time, especially in the absorption areas associated with fat, protein, lactose, somatic cell count, urea, and acetone. However, the predictions of all the AA by MIRS were repeatable and reproducible across different milk storage time until 6 d after sampling except for Tyr, His, and Phe, with repeatability and reproducibility both greater than or close to 90%. In conclusion, it is 0–6 d after milk collection, stored at 4 °C with preserved-bronopol, that MIRS can provide relatively consistent and accurate predictions for AA content. However, a shorter storage time, such as within 3 d post-collection, is recommended when conditions permit.