Effect of phospholipid molecular structure on its interaction with whey proteins in aqueous solution

Because most dairy processes require heat treatment to ensure microbiological safety during storage, dairy industry has invested a lot of effort to acquire knowledge related to all the changes that may take place during heating. More precisely, measures have to be taken to avoid undesired changes that can badly affect the quality of the final product. Whey protein denaturation is one of the major changes that take place upon sterilization. The purpose of the present study was to investigate and compare the degree and type of interaction that occurs between whey protein isolate and different kinds of dialkyl phospholipids, as a function of their molecular structure, upon heating at different temperatures. Circular Dichroism (CD) experiments were performed to assess the extent of secondary structural changes of β-lactoglobulin upon incubation in the presence of both anionic and zwitterionic phospholipids. The far-UV spectra of the former revealed a significant change in ellipticities in the region between 208 and 222 nm, which represents the α-helical content of peptides and proteins upon incubation in the presence of anionic phospholipids at temperatures above their phase transition temperature. On the other hand, these changes were not evident upon incubation with phospholipids in the gel state or when applying zwitterionic phospholipids. Nuclear Magnetic Resonance (NMR) has been used to investigate the whey protein–surfactant interaction into more detail. The chemical shift of the surfactant methyl groups toward a more hydrophobic region in the NMR spectra revealed a hydrophobic whey protein–phospholipid interaction. This finding was further supported by Saturation Transfer Difference-NMR (STD-NMR) measurements. The NMR experiments also revealed that the lack of protein structural reorganization upon incubation in the presence of zwitterionic (lyso)-phospholipids could not be ascribed to a lack of interaction, but was caused by the fact that the interaction did not affect the protein's structure. Overall, this work indicated that phospholipids can modify the secondary structure of whey proteins (i.e. when using anionic phospholipids in the liquid crystalline state) due to hydrophobic interactions. In addition, the results prove that the heat stability of whey protein containing products may be optimized by appropriate selection of the phospholipid composition.