VIBRATIONAL SPECTROSCOPY

Vibrational spectroscopy consists of two approaches, infrared (IR) absorption and Raman scattering, and provides structural and chemical information about molecules based on their vibrational transitions. Both approaches are fast, low-cost, highly specific for target molecules, robust, and easy to use. Furthermore, no or a minimum sample preparation is required and chemical constituents can be determined qualitatively and quantitatively down to very low concentrations, with these characteristics making vibrational spectroscopy very suitable for food contamination detection. Furthermore, the advantage over other analytical techniques is the fact that the measurements are nondestructive, reagentless, and may be applied directly to food surfaces. In addition, the instruments can be transferred to portable devices, allowing immediate analysis of the food products and onsite evaluations throughout the whole food-processing chain.

In the last few years, the number of studies aimed at the use of vibrational spectroscopy for food safety applications increased markedly and different applications have been reviewed recently for FT-IR (Huang et al., 2008; Jimare Benito et al., 2008; Woodcock et al., 2008) and Raman spectroscopy (Craig et al., 2013; Yang and Ying, 2011), as well as for different food classes, such as milk (Cattaneo and Holroyd, 2013), fish (Cheng et al., 2013), and water (Lopez-Roldan et al., 2013).

IR spectroscopy has widely been studied for bacterial identification purposes and in lesser extents for the detection of chemical contaminants, such as melamine and pesticides. Raman spectroscopy gives complementary information to IR spectroscopy and has gained increasing attention in the last decades for both foodborne pathogen and chemical contaminants detection. Although IR spectroscopy is cheaper and easier in instrumentation and handling, Raman spectroscopy has several advantages over conventional IR spectroscopy, such as less interference of water and more detectable features over the same wave-number range (Lu et al., 2011). Nevertheless, Raman signals are weak and the spectra are highly interfered with by noise signals and fluorescence background, making it difficult to obtain good results for low concentrations of contaminants. New advances in nanotechnology have made it possible to enhance Raman signals by surface-enhanced Raman spectroscopy (SERS). Raman spectroscopy is combined with nanomaterials, such as gold or silver nanosphere colloids, solid surface gold-coated nanosubstrates, bimetallic nanosubstrates, or spherical magnetic-core gold-shell nanoparticles (Fan et al., 2014).

Fingerprints obtained by vibrational spectroscopy are affected by various factors and other nontargeted food components may significantly interfere with the obtained signals. To overcome these challenges and permit its use in routine analysis of food control laboratories, advanced data preprocessing and statistical analyses are required. The use of chemometrics for the interpretation of high-dimensional spectral fingerprints have made complex analyses possible.

 
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