Raman Spectroscopic Characterization of Structural Changes in Heated Whey Protein Isolate upon Soluble Complex Formation with Pectin at Near Neutral pH

The mechanism leading to an alteration of heat aggregation of whey protein isolate (WPI) in the presence of pectin was investigated by assessing structural changes of proteins using Raman spectroscopy. WPI solutions were heated without or with pectin at 0.015-0.2 pectin to WPI weight ratios and pH 6.0-6.4. In the absence of pectin, thermal denaturation resulted in a loss of alpha-helical structure and an increase in beta-structure and random coils of protein. At pH 6.0 and 6.2, heat aggregation of WPI was suppressed when pectin (0.05-0.15 pectin to WPI ratios) was present as shown by a decrease in turbidity and particle size. Concomitantly, changes in the secondary structures were reduced, indicating the enhanced stability of protein structure by pectin. Raman results also revealed that alpha-helix and beta-sheet are dominant structures in heated WPI-pectin soluble complexes, and hydrogen bonding between biopolymers increased. The effect of pectin was pH dependent, indicating the involvement of electrostatic interaction.