3.1.1. Infrared Spectra

Mid-infrared (IR) spectroscopy is the most widely used spectroscopic technique in meat and meat products as it provides fundamental information about protein and lipid structure through well-resolved characteristic infrared bands. In recent years, mid-infrared Fourier transform (FT-IR) spectrometers with attenuated total reflection (ATR) stand out from among the different mid-infrared (IR) spectroscopy techniques employed to analyze the structure of meat and meat products.

The typical protein bands in the mid-infrared (IR) spectra are amide I (1700–1600 cm<sup>−</sup>1), amide II (1560–1510 cm<sup>−</sup>1) (Figure 2) and amide III (1300–1200 cm<sup>−</sup>1) which provide information on protein secondary structure ( α-helix, β-sheet, turn, unordered). Modifications in the intensity and/or frequency of these infrared bands is indicative of protein structural modifications [29,30]. The amide I band, high intensity (Figure 2) due to carbonyl stretching vibration with a slight influence from C-N stretching and N-H bending vibrations, is the one most frequently employed to analyze the secondary structure of proteins. Proteins with α-helical conformation show robust amide I bands between 1657 and 1650 cm<sup>−</sup>1, whereas bands between 1640 and 1612 cm<sup>−</sup><sup>1</sup> are usually related with β-sheets.

**Figure 2.** Typical FT-IR spectra from cooked sausage (type frankfurter) in the 1700–1500 cm<sup>−</sup>1.

The mid-IR spectra of lipids encompass various bands in the 3000–1700 cm<sup>−</sup><sup>1</sup> region and overlapping bands in the 1500–700 cm<sup>−</sup><sup>1</sup> region. It is worth noting that in the elaboration of meat products with health-enhanced lipid content, infrared studies of the region between 3000–2800 cm<sup>−</sup><sup>1</sup> are characterized by two strong bands at 2925 and 2854 cm<sup>−</sup><sup>1</sup> (Figure 3) caused respectively from the asymmetric (<sup>ν</sup>asCH2) and symmetric (<sup>ν</sup>sCH2) stretching vibrations of the acyl CH2 groups of lipids [31–33]. Modifications in lipid chain

order/disorder causing from protein–lipid interactions can alter the half-bandwidth of these bands [34].

**Figure 3.** Representative FT-IR spectra in the 3000–2820 cm<sup>−</sup><sup>1</sup> region from cooked meat products (type frankfurter).
