A data grouping was observed from the scatter plot of the PC scores (Figure 4a), but the data for samples adulterated at low concentration appear to overlap more. This confirmed our assumption that the different spectral attributes among the samples were associated with the concentration of the starch in the onion powder. Furthermore, we assessed the potential of PCA to distinguish among the low starch concentration samples (1−10%), but the data were overly scattered and overlapping (data not shown). The first three PC loadings account for nearly 99% of the total variance in the samples. PC loadings are correlation coefficients between the PC scores and the original variables, which measure the importance of each variable in accounting for the variability in the PC. It is possible to interpret the first few PCs in terms of overall effect of a contrast between groups of variables based on the structures of PC loadings. The PC loading plot (Figure 4b) shows the highest loading for PC1 and PC3 at ∼1960 nm, related to the
starch. PC1 and PC2 show other important bands between 1435 and 1450 nm, corresponding to the first O−H stretching overtone, also assigned to the starch.