Relationships between conformation of β-lactoglobulin in solution and gel states as revealed by attenuated total reflection Fourier transform infrared spectroscopy

Attenuated total reflection Fourier transform infrared spectroscopy (ATR FT-IR) has been used to compare the structure of β-lactoglobulin, the major component of whey proteins, in solution and in its functional gel state. To induce variation in the conformation of β-lactoglobulin under a set of gelling conditions, the effect of heating temperature, pH, and high pressure homogenization on the conformation sensitive amide I band in the infrared spectra of both solutions and gels has been investigated. The results showed that gelification process has a pronounced effect upon β-lactoglobulin secondary structure, leading to the formation of intermolecular hydrogen-bonding β-sheet structure as evidenced by the appearance of a strong band at 1614 cm⁻¹ at the expense of other regular structures. These results confirm that this structure may be essential for the formation of a gel network as it was previously shown for other globular proteins. However, this study reveals, for the first time, that there is a close relationship between conformation of β-lactoglobulin in solution and its capacity to form a gel. Indeed, it is shown that conditions which promote predominance of intermolecular β-sheet in solution such as pH 4, prevent the formation of gel in conditions used by increasing thermal stability of β-lactoglobulin. On the basis of these findings, it is suggested that by controlling the extent of intermolecular β-structure of the protein in solution, it is possible to modify the ability of protein to form a gel and as a consequence to control the properties of gels.