Raman spectroscopy as a probe of protein structure in food systems

Raman spectroscopy can be a useful tool to probe protein structure in solid and liquid food systems. Bands in the Raman spectrum arising from amide I, amide III and skeletal stretching modes of peptides and proteins are useful for characterizing backbone conformation, including the estimation of secondary structure fractions. Bands attributed to various stretching or bending vibrational modes of functional groups of amino-acid residues can be used to monitor the environment around these side-chains. In particular, valuable information may be obtained on SS or SH groups of cystinyl or cysteinyl residues, CH groups of aliphatic residues, and aromatic rings of tryptophanyl, tyrosinyl and phenylalanyl residues. One important parameter distinguishing Raman spectroscopy from many other spectroscopic methods is its applicability to systems containing high concentrations of proteins, which is critical for the investigation of structural changes during processes such as coagulum or gel formation. Thus, changes in both intramolecular and intermolecular interactions can be studied. In this chapter, examples are presented on the application of Raman spectroscopy to investigate protein structure as a function of processing, such as heating, drying, salt addition or homogenization with lipids, which may be important to correlate with protein functionality in food systems.