## 3 Results and Discussion
### FTIR-ATR Analysis  
The collected absorbance spectra of all AUS samples studied are represented in Figure 5. The spectra profile are in accordance with those previously reported for wine spirits and grape marc spirits [29; 45; 46; 47; 48; 49]. Since the representative absorbance spectra are very well explained in these works, only a summary is shown in the present one. Briefly, the AUS IR region has important information from 3000 to 2900 cm\({}^{-1}\) [O-H stretching, C-H stretching of CH\({}_{3}\) and CH\({}_{2}\)], and from 1500 to 860 cm\({}^{-1}\) (C-C absorption bands, C-O vibrations, C-OH bending deformation, C-H bond stretching and C=O and C=C groups) [49; 50; 51; 52; 53].  
Figure 4: Principal component analysis of AUS aged with oak wood aged during three and six months for: (**A**)—PC1 vs. PC2; (**B**)—PC1 vs. PC3. LT—aged in light toast oak wood; MT—aged medium toast oak wood; MPT—aged with medium plus toast oak wood; Va—Vanilla; W—woody; Ca—caramel; Tot—toasted; DF—dried fruits; Co—coffee; Sw—sweet; Sm—smooth; Bu—burning; Bo—body; FE—flavour evolution; FC—flavour complexity; RA—retronasal aroma; O.A.—overall aroma; O.F.—overall flavour; O.Q.—overall quality.  
Figure 6A shows the score plot of two principal components made with FTIR-ATR spectra for AUS aged according to the wood toasting levels and the ageing time. The first two principal components accounted for 94% of the total variance, which is mainly explained by the toasting levels. In the first ANOVA performed (Table 1), the toasting level also had a significant effect on the physicochemical characteristics and on some volatile compounds. This effect was already noticed for wine spirit aged at an industrial scale by other authors [54,55]. They reported an increasing trend of some volatile and low molecular weight compounds over the ageing time. However, it is important to emphasize that the present study was made intentionally in a short period to preserve the aromatic profile of the strawberry tree's fruits in the beverage.  
The separation between MT and MPT is also notified but only by the PC2, which explains 5% of the total variation.  
The most relevant region to explain the observed variation was between 1300 cm\({}^{-1}\) and 1500 cm\({}^{-1}\), and from 2850 cm\({}^{-1}\) to 3000 cm\({}^{-1}\). [PERSON] (2001) [53] reported that the  
Figure 5: FTIR-ATR absorbance spectra of all AUS samples.  
Figure 6: Score plot of two principal components from spectral data acquired with FTIR-ATR of AUS, concerning the toasting level of the wood used in the ageing process for three and 6 months. (**A**)—Score plot, (**B**)—loading plot.  
peaks at 1085 cm\({}^{-1}\) and 1043 cm\({}^{-1}\) are important for ethanol and methanol quantification, corresponding to the C-O stretch absorption bands. Despite the high effect of the toasting level, it was also possible to identify the effect of the ageing time, which confirmed the observed differences and the possible applicability of FTIR-ATR to distinguish these kind of samples.  
These results showed that FTIR-ATR is a useful technique to discriminate AUS aged with wood according to the toasting level, and also to the ageing time. The applicability of the FTIR-ATR technique was also validated to perform calibration models of some volatile compounds for this alcoholic beverage [46; 47; 56; 57; 58; 56].