## 2 Analytical Strategies for Authentication and Quality Control
### Spectroscopic Methods
#### 2.1.1 Infrared Spectroscopy  
IR showed good potential in food authentication as a fast, low-cost analytical technique with minimal sample preparation, demonstrating the capacity to identify different compounds or classes of compounds [12; 13]. IR theory utilizes the concept that different functional groups display different absorption frequencies. In this way, it is possible to identify compound categories present in foodstuffs, such as proteins, carbohydrates, lipids, vitamins and minerals [14]. The IR region of the electromagnetic spectrum spans from 14,000 to 50 cm\({}^{-1}\) and is divided into three areas: near-IR (14,000-4000 cm\({}^{-1}\)), mid-IR (4000-400 cm\({}^{-1}\)) and far-IR (400-50 cm\({}^{-1}\)) spectroscopies [15]. In particular, near-IR (NIR) and mid-IR (MIR) spectroscopies have proven to be efficient and cost-effective compared to reference analysis methods. A study on the quality control of a group of 166 grape-derived spirits was carried out to predict some legally relevant parameters, namely the alcohol strength and contents of methanol, acetaldehyde and fused alcohols, by applying Fourier transform infrared (FTIR) spectroscopy with attenuated total reflectance (ATR) in the MIR region and partial least square (PLS) chemometric regression models [16]. The comparison of the results with those obtained with official methods based on gas chromatography coupled with flame ionization detection (GC-FID) and densitimetry [17; 18] showed satisfactory correlation coefficients for methanol (r\({}^{2}\) = 99.4%), alcoholic strength (r\({}^{2}\) = 97.2%), acetaldehyde (r\({}^{2}\) = 98.2%) and fused alcohols (r\({}^{2}\) from 97.4% to 94.1%). The same dataset allowed the determination of the ratio of performance to deviation of the method, obtaining values of 12.8 (methanol), 6.0 (alcoholic strength), 7.5 (acetaldehyde) and 6.2-4.1 (fused alcohols); a value of 2.5 is the minimum threshold for a reliable calibration [19]. These results demonstrated the good accuracy of the proposed method for quick and easy sample screening. Another study showed the potential of NIR spectroscopy to predict the aging time of brandies. [20]. The authors applied NIR spectroscopy combined with principal component analysis (PCA), PLS and linear discriminant analysis (LDA) to discriminate the samples. Ethanol, esters, acids, methanol and higher alcohols were also determined (with official methods) to obtain reference values to correlate with NIR spectra. The study showed that the most significant regions of the NIR spectra (linked to phenolic and aromatic compounds) were in the ranges from 904 nm to 935 nm and from 1400 nm to 1699 nm. The same authors found that the PCA based on the phenolic composition and on alcohols with a higher molecular weight allowed the best discrimination between brandies of different ages. Furthermore, the same authors developed some PLS calibration models for each parameter evaluated, i.e., total phenolic content, total higher alcohol content and total phenolic plus higher alcohol content, and an LDA classification model based on individual phenols with a classification rate of 100%. Other authors [21] studied the capacity of NIR to discriminate between wine spirits aged in wooden barrels or in stainless-steel tanks with wooden staves made from different types of wood (Limousin oak and Portuguese chestnut). They determined the alcoholic strength and the main volatile compounds in wine spirits aged for 6, 12 and 18 months by electronic densimetry and by GC-FID. The same samples were analyzed by NIR spectroscopy coupled to PCA. The results showed that the aging technology had more influence than the type of wood on the features of the samples and that the most significant spectral regions for differentiating the samples were around 6859 cm\({}^{-1}\) and from 5200 cm\({}^{-1}\) to 4200 cm\({}^{-1}\). Another study [22] applied MIR and NIR spectroscopies combined with four different chemometric methods to discriminate between Gr and other spirits and between authentic and adulterated Gr.