## 4 Vibrational Spectroscopic Techniques for Non-Invasive Food Authentication
### FTIR Spectroscopy for Non-Invasive Food Authentication  
FTIR spectroscopy, like NIR spectroscopy, is highly versatile and can be applied to solids, liquids, and gases using various measurement techniques, including ATR, diffuse reflectance, high-throughput transmission (HTT), and transmission cell methods. Among the various FTIR sampling techniques, ATR is the predominant method employed in food authenticity and quality assessments, accounting for approximately 72% of reported applications. This technique relies on guiding infrared radiation through a high-refractive-index crystal, where the beam undergoes multiple internal reflections at the interface with the sample [84]. A key advantage of ATR-FTIR spectroscopy is its ability to perform both qualitative and quantitative analyses with minimal or no sample preparation. Additionally, several accessories, such as single-bounce ATR (SBATR), micro-ATR (mATR), and horizontal ATR (HATR), offer varying internal reflection geometries, making ATR effective for characterizing materials that absorb infrared light strongly. FTIR transmission spectroscopy, whether through HTT or transmission cell methods, also finds extensive use in the food sector. For instance, HTT is employed to identify fungi responsible for food spoilage [85] and to conduct metabolic fingerprinting of legume silage [86]. Comparative studies of FTIR sampling methods have demonstrated the advantages of ATR for analyzing microbial spoilage in milk [87] and screening beer quality using HATR, mATR, and HTT (Figure 3) [88]. These studies highlighted that ATR provides superior accuracy and reliability in assessing food quality and authenticity. This increased precision is likely due to the multi-bounce crystal in ATR, which allows the infrared light to reflect multiple times within the sample, enhancing absorbance. While HTT also yielded satisfactory results, it is particularly useful for rapidly analyzing a large number of samples. Transmission mode is preferred for liquids, as the infrared radiation passes through the entire sample, providing a higher signal-to-noise ratio. Moreover, the automation of the filling and cleaning process in transmission measurements streamlines sample preparation, improving efficiency compared to ATR. FTIR spectroscopy, along with the aforementioned sampling techniques, has shown considerable promise in detecting honey authenticity and adulteration. It has been successfully used to identify adulteration in honey with various substances, such as corn syrup, beet sugar, cane sugar [89], and sugar solutions [90]. ATR-FTIR with PCA-LDA detected syrup adulterants at 0.5% \(w/w\)[89]. Additionally, numerous studies have utilized FTIR spectroscopy to determine the sugar content in fruit juices [91]. The application of FTIR spectroscopy for the authentication of red wines based on grape variety achieved an impressive 95% classification accuracy [92]. Moreover, Greek red wines were successfully distinguished by grape variety through the comparison of FTIR spectra from samples with existing reference spectra in a database, where HAT-FTIR achieved 95% varietal classification [93]. FTIR has also proven to be a simple and cost-effective method for quality control and authenticity verification in beer [81]. Other notable uses of FTIR in food quality assessments include detecting adulteration in chocolate products [94]; monitoring meat spoilage through total viable bacterial counts, sensory quality, and pH [95]; and identifying added sugar and citric acid in fruit jams [96]. Additionally, FTIR has been utilized as an alternative method for characterizing and identifying microorganisms when coupled with a light microscope [97]. This approach has facilitated the discrimination and classification of enterococci [98] and the identification of bacteria in complex matrices [97]. FTIR spectroscopy combined with chemometrics for the detection, quantification, and classification of bacteria, including foodborne pathogens, indicates its promise as a powerful tool in microbiological analysis.