Effects of ohmic heating on structural and physicochemical changes of whey proteins
Ohmic heating has received attention as a novel thermal process for foods containing protein. However, there is limited information on the effects of electric fields in addition to thermal effects on the physicochemical changes of proteins. This study investigated the electric field effects of ohmic heating on conformational changes, aggregation and the surface charge of whey proteins. Whey protein dispersion was heated at 60°C, 70°C and 80°C for 15 min under identical thermal histories for ohmic and conventional heating to study the electric field effects. The tertiary structure, aggregation and surface charge of the whey proteins were determined using intrinsic fluorescence spectroscopy, particle size distribution and the ζ-potential, respectively. Ohmic heating at ≥ 70°C induced conformational changes to the whey protein molecules (intrinsic fluorescence intensity of 23,690 ± 1,164 A.U. and 29,814 ± 1,149 A.U. at 70°C and 80°C, respectively compared to 27,457 ± 1,262 A.U. and 32,988 ± 1,213 A.U. after conventional heating), dissociated submicron-sized protein clusters and re-assembled the proteins to nano-sized particles, which were smaller than the nano-sized particles found in conventional-heated whey protein dispersions. Ohmic-heated whey protein dispersions retained greater clarity than conventional heat-treated dispersions at the respective temperature, although the ζ-potential values of the whey protein aggregates in the suspensions treated using conventional heating and ohmic heating were not different (p > 0.05). In addition to heat-induced effects, the electric fields might cause alignment, stretch and relaxation of whey protein molecules. The electric field effects and uniform heating probably accounted for the reduced tertiary structural changes and smaller particle sizes of aggregates of whey proteins. This study suggested that ohmic heating could be used as an alternative thermal process to modify the protein conformation and physicochemical properties for specific protein functionalities.