Effects of pulsed electric fields on structural modification and rheological properties for selected food proteins

The application of pulsed electric fields (PEF) in food processing is a novel technique that requires careful study. An improved understanding of the effects of PEF treatments on structural modification, surface hydrophobicity, thermal properties and rheological properties of food proteins is needed for the application of this method to the improvement of functional properties of food proteins in food systems. Two main PEF parameters were studied: electric field intensity ranging from 12 to 25 kV cm-1, and number of pulses ranging from 10 to 120. PEF effects on the structural modification, surface hydrophobicity, thermal properties and rheological properties for selected food proteins were investigated using fluorescence spectroscopy and differential scanning calorimetry (DSC) along with rheological measurements. Whey protein isolate (WPI), soy protein isolate (SPI), soy milk and reconstituted skim milk were used in this study. The structural modification, surface hydrophobicity and thermal properties of WPI and SPI all changed with PEF treatments. The enthalpy of whey protein isolate decreased with PEF treatments. The apparent enthalpy and denaturation temperatures of WPI samples were modified by PEF treatments. The remaining native whey protein was about 56% by a PEF treatment of 20 kV cm-1 and 30 pulses. The intrinsic tryptophan fluorescence intensity and the extrinsic fluorescence intensity of whey protein and soy protein all increased with PEF treatments. The intrinsic tryptophan fluorescence of whey protein and soy protein appeared as a red shift of 2-6 nm. The extrinsic fluorescence of whey protein and soy protein appeared as a blue shift of 2-6 nm with PEF treatments. The surface hydrophobicity of whey protein and soy protein increased with PEF treatments. PEF treatments with different electric field intensities and number of pulses affected the rheological properties and color properties of reconstituted skim milk and soy milk. The apparent viscosity and shear stress of soy milk and skim milk increased with PEF treatments. For skim milk, the apparent viscosity and consistency index increased with increasing electric field intensity and number of pulses. The apparent viscosity increased from 2.12 to 4.25 (10-3 Pas) for the untreated control and 60 pulses of 20 kV cm-1. The consistency index increased from 1.42 to 1.54 (10-3 Pa.s n-1) for the control and PEF-treated at 60 pulses and 20 kV cm-1. For soy milk, the apparent viscosity of soy milk increased from 6.62 to 7.46 (16-1 Pa.s) by increasing electric field intensity from 18 to 22 kV cm-1 and number of pulses increasing from 0 (the no treated control) to 100. This study confirmed that electric field intensity and number of pulses affected the structural modification, surface hydrophobicity, thermal and rheological properties for selected food proteins.