# Enhancing Traceability of Italian Almonds through IR Spectroscopy and Chemometric Classifiers
## 4 Discussion  
As outlined in Section 3.1, the outcome of SIMCA revealed a notable disparity in model accuracy, with the models trained on shell spectra outperforming those trained on kernel spectra. This indicates that shells present a more informative IR profile from the geographical origin point of view. This can be attributed to the composition, surface properties, and nature of the spectra collected on the shells. In fact, the spectral data collected on almond shells may exhibit greater consistency across samples due to the uniform nature of the shell's surface [28]. In contrast, kernel spectra may vary more due to factors like size, ripeness, and internal variations [29]. Furthermore, by collecting spectra on the kernel, the pressure exerted on the core can cause the oils to leak out, altering the collected spectrum. The outcome of PLS-DA confirms what was observed in SIMCA. Indeed, in both cases, the classification achieved on the shell spectra is better than that obtained by analyzing the spectra collected on the kernel. Furthermore, the results of PLS-DA are more accurate than those achieved using SIMCA. For this reason, the focus was placed on the PLS-DA model calculated on the shell spectra, and a Variable Importance in Projection (VIP) analysis was \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Pretreatment** & **LVs** & **\% CCR\({}_{\text{cv}}\)** & **Calabria** & **Apulia** & **Sicily** \\  & & **\% CCR\({}_{\text{pred}}\)** & **\% CCR\({}_{\text{pred}}\)** & **\% CCR\({}_{\text{pred}}\)** \\ \hline D1 & 12 & 96.2 & 92.5 & 90.0 & 85.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: PLS-DA on the spectra collected on the almonds’ kernels. Figure 3: PLS-DA: Projection of samples onto the two canonical variates (CVs). applied to understand the spectral variables that mostly contribute to the discrimination among the three different origins. Briefly, the VIP analysis allows for the estimation of VIP indices, which represent the contribution of each variable to the solution of the classification problem. By construction, VIP indices higher than 1 are considered to be associated with significant variables. In Figure 4, a graphical representation of the spectral variables contributing the most to the model is shown. In particular, the black lines represent the mean spectrum (offset to make them visible), whereas red, green, and blue dots highlight significant variables for Class Calabria, Apulia, and Sicily, respectively. From this plot, it is possible to obtain indications of the most significant features. The selected peaks found in the region between 3000 and 2700 cm\({}^{-1}\) are representative of asymmetric and symmetric vibrations (peaks at \(\sim\)2950 and \(\sim\)2850 cm\({}^{-1}\)) of alkene groups (=C-H). The peak at \(\sim\)2950 cm\({}^{-1}\) can be attributed to the stretching vibration of saturated aliphatic alkyl (C-H) groups present in celluloses and hemicelluloses, whereas the vibration at 2800 cm\({}^{-1}\) originates from the stretching of lignin [30]. The signal between 1740 and 1710 cm\({}^{-1}\) can be associated with the vibrations of acetyl groups and uronic ester groups in hemicelluloses or the linkage of carboxyl groups via esters of ferulic and p-coumaric acids of lignin, and to the stretching of -COOH groups of the ferulic and p-coumaric acids present in lignin. Furthermore, peaks in the range of 1850-1680 cm\({}^{-1}\) correspond to the ester carbonyl functional group (C=O) of triacylglycerlyceride esters [31; 32]. All the highlighted variables between 1600 and 1100 cm\({}^{-1}\) could be associated with vibrations of the aromatic rings in lignin or to the CH, CH2, OH, C-C, C-O, and CCO skeleton vibrations. Moreover, the absorptions around 1530 cm\({}^{-1}\) are ascribable to N\(-\)H in amide I and amide II of proteins [33]. The band at \(\sim\)1050 cm\({}^{-1}\) is attributable to the stretching of the C-O-C functional group due to the presence of polysaccharides associated with the hemicelluloses [34].