Forty-one
samples of skim milk powder (SMP) and nonfat dry milk
(NFDM) from 8 suppliers, 13 production sites, and 3 processing temperatures
were analyzed by NIR diffuse reflectance spectrometry over a period
of 3 days. NIR reflectance spectra (1700–2500 nm) were converted
to pseudoabsorbance and examined using (a) analysis of variance-principal
component analysis (ANOVA-PCA), (b) pooled-ANOVA based on data submatrices,
and (c) partial least-squares regression (PLSR) coupled with pooled-ANOVA.
ANOVA-PCA score plots showed clear separation of the samples with
respect to milk class (SMP or NFDM), day of analysis, production site,
processing temperature, and individual samples. Pooled-ANOVA provided
statistical levels of significance for the separation of the averages,
some of which were many orders of magnitude below 10–3. PLSR showed that the correlation with Certificate of Analysis (COA)
concentrations varied from a weak coefficient of determination (R2) of 0.32 for moisture to moderate R2 values of 0.61 for fat and 0.78 for protein
for this multinational study. In this study, pooled-ANOVA was applied
for the first time to PLS modeling and demonstrated that even though
the calibration models may not be precise, the contribution of the
protein peaks in the NIR spectra accounted for the largest proportion
of the variation despite the inherent imprecision of the COA values.